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LTD-9 Abstracts

A01. Magnetic Sensors for X-Ray and Gamma-Ray Detection

C. Enss
Kirchhoff-Institut für Physik
Universität Heidelberg, D-69120 Heidelberg, Germany

In recent years metallic magnetic calorimeters have been developed for particle detection. A magnetic calorimeter consists of an absorber suited for the particles to be detected, which is weakly coupled to a thermal bath and a paramagnetic sensor located in a small magnetic field as thermometer, which should be intimately coupled to the absorber. The energy deposition of an incident particle leads to a change of the absorber temperature and thus to a change of the magnetization of the sensor. This change in magnetization can be measured with high resolution using a sensitive DC-SQUID. To meet the requirements in terms of detector speed, magnetic sensors with metallic hosts doped with rare earth ions are preferable. A material of this type, which has been studied in detail in recent years is gold containing a few hundred ppm erbium ions. The performance of metallic magnetic calorimeters based on Au:Er has improved rapidly and has now reached a level where various applications are conceivable. We discuss the principle of operation and the optimization criteria of magnetic calorimeters, the design and performance of prototype detectors for both x-ray and gamma-ray, detection and the fundamental limits of the energy resolution of such detectors.

A02. Large-format Distributed Read-Out Imaging Devices for Optical and X-ray Imaging Spectroscopy

Roland den Hartog^a, D. Martin^a, A. Kozorezov^b, P. Verhoeve^a, G. Brammertz^a, A. Peacock^a, F. Scholze^c, D.J Goldie^d
a Astrophysics Division, Space Science Dept. of the European Space Agency, ESTEC, P.O. Box 299, 2200 AG Noordwijk, The Netherlands
^b School of Chemistry and Physics, Lancaster University, Lancaster LA1 4YB, U.K.
^c Physikalisch-Technische Bundesanstalt (PTB), Abbestr. 2-12, D-10587 Berlin, Germany
^d Oxford Instruments Scientific Research Division, Newton House, Cambridge Business Park, Cambridge CB4 4WY, U.K.

We present an experimental study of the performance of Distributed Read-Out Imaging Devices (DROIDs), based on two Ta/Al-based STJs placed on either side of a Ta absorber strip. We focus our discussion on the prospects of building large-format photon-counting imaging spectrometers for applications at optical, UV and soft X-ray energies. Tunnel-limited spectroscopical resolutions have already been demonstrated for optical photons. With a 20 × 100 µm² absorber we have measured an intrinsic energy resolution of 2 eV FWHM for 500 eV photons. This demonstrates that at soft X-ray energies resolutions close to the tunnel limit are also feasible for these type of detectors. A detailed analysis of pulse-shapes with numerical models allows us to assess the main parameters that determine the performance of these detectors. In particular, we discuss the dependence of the quasiparticle diffusion constant on the temperature of the absorber. Extrapolation of these models indicates that it is possible to extend the length of the absorber to 1.5 mm, without a serious degradation of the detector's performance.

A03. Approaching Intrinsic Resolution Limits in Optical/UV Superconducting Tunnel Junction Detectors

C.M. Wilson^a, L. Frunzio^a, L. Li ^a, D.E. Prober^a, T. Stevenson^b, C. Stahle^b
aYale University, New Haven, CT
^bNASA Goddard Spaceflight Center, Greenbelt, MD

We present results of development of imaging superconducting tunnel junction detectors in the optical/UV energy range. Our detectors have a Ta strip absorber with Al tunnel junctions located on the two ends of the strip. The tunnel junction area does not overlap the absorber. Using devices designed for large backtunneling gain, we have measured an energy resolution of 0.4 eV at 4.89 eV. The resolution in these devices is limited by thermodynamic fluctuations of the thermal quasiparticle number in the Al trapping layers. We predict that this previously unconsidered noise source should be important in any device with "deep" traps. We are currently developing detectors designed to eliminate this noise and, consequently, backtunneling gain. These devices need small junctions, of order 1 square micron, for best noise performance. We are currently developing an electron beam lithography process to produce these junctions. The resolution of these devices should approach the intrinsic limits of creation and trapping statistics, and exceed the resolution of devices with backtunneling. We will present preliminary measurements of these new devices made with conventional FET amplifiers and with a new radio frequency single electron transistor amplifier (RF-SET).

A04. Factors of inhomogeneous spatial response of superconducting tunnel junction detectors

H. Pressler^a, M. Ohkubo^a, M. Koike^a, M. Ukibe^a, T. Zama^a
a National Institute of Advanced Industrial Science and Technology, Tsukuba, Japan

Spatial uniformity in response of superconducting tunnel junction detectors to photons is a main concern in the detector development. Low Temperature Scanning Synchrotron Microscopy (LTSSM) has been used to directly image spatial profiles of response of Nb-based superconducting tunnel junctions (STJ) to x-ray photons. By scanning an x-ray beam with a diameter of 5 - 10 /mum employed as a microprobe enable visualization of the spectroscopic properties of STJ x-ray detectors at an actual working temperature of about 0.4 K. We have found that the inhomogeneity of the junction response strongly depends on such parameters as junction size, bias current, the strength of applied magnetic field parallel to insulation barriers, and Josephson current density.

A05. Measurement of Quantized Thermal Conductance

K. Schwab^a, M.L. Roukes^b
aLaboratory for Physical Sciences, College Park, MD 20740
^bPhysics Department, Caltech, Pasadena, CA 91125

We have experimentally demonstrated that the thermal conductance through discrete, one-dimensional, ballistic phonon channels is quantized, G=pi²k_B²T/3h. This is the predicted, universal upper-bound for thermal conductance per channel, independent of material characteristics and particle statistics. We have developed novel, 3D nanostructures, with feature sizes of order ~100nm to enter and reveal this mesoscopic limit for phonons. We will describe the techniques and implications of this measurement to nanoscale bolometry, and describe our current work where we are exploring the limits of heat capacity in these and more advanced nanoscale devices from temperatures of ~10K to less than 100mK. Using preliminary bulk estimates and dc-SQUID-based noise thermometry, we anticipate realization of a bolometer with total heat capacity of ~ 10³ k_B, giving microsecond response time and energy sensitivity at the level of single terahertz photons.

A06. Energy resolution Delta E = 12 eV at E = 5.9 keV for lead absorber coupled to a single Al-STJ via phonons only

G. Angloher, M. Huber, J. Jochum, A. R�dig, F. von Feilitzsch, R. L. M��bauer

A superconducting lead absorber (90 x 90 x 1.3 microns³) coupled to a single Al-STJ via phonons only was read out with an energy resolution Delta E = 12 eV at E = 5.9 keV. The whole detector area was illuminated, no software cuts were applied to the ADC data output. Due to the high absorption efficiency of lead (> 50 % at E = 5.9 keV, > 99.9 % if E < 1 keV) and the detector's location on a thin Si3N4-membrane double/multi-peak structures are strongly suppressed in the pulseheight spectra. A signal to background ratio higher than 10³ was achieved for energies higher than E = 1.3 keV. At E = 1.74 keV an energy resolution lower than Delta E = 10 eV was reached, allowing to separate tungsten M-lines and silicon K-lines. The detector stability was limited by the cryostats holding time. Different from Al-STJs on massive substrates, the detector shows linear energy response between E = 1.74 keV and E = 6.49 keV. Degradation of the detector could be avoided by covering the lead absorber with a thin SiO-layer.

A07. Ultrasensitive hot-electron kinetic-inductance detectors

A. Sergeev^a, B. Karasik^b, I. Gogidze^c, V. Mitin^a
aWayne State University, Detroit, MI
^bJet Propulsion Laboratory, California Institute of Technology, Pasadena, CA
^cWinncom Technologies Corp., Solon, OH

Electron heating in thin superconducting films is an underlying physical mechanism for numerous sensors of electromagnetic radiation. Resistive sensors operating in the vicinity of superconducting transition (also known as hot-electron detectors) are already used as ultrafast detectors and broadband THz mixes. Kinetic inductance sensors can be potentially more sensitive due to lower noise, and their parameters (speed, responsivity, noise) can be adjusted in a broad temperature range below T_c. We present the results of measurements of kinetic inductance response in NbN thin films and modeling of noise characteristics for sensors based on Nb, NbN, and MgB films. Our study demonstrates that the photoresponse of NbN films is well described by the Owen-Scalapino model (change of nonequilibrium chemical potential of quasiparticles caused by radiation) with strong phonon re-trapping by condensate which significantly increases the quantum efficiency. The time constant of the sensor is given by the quasiparticle lifetime, which is about 4 ns in a 5-nm thick film at 4.2 K. The results of modeling show that the noise equivalent noise power (NEP) of the detector can be as low as 10^-17-10^-16 W/Hz^-1/2 at 4.2 K. Due to the exponential temperature dependence of the quasiparticle concentration this figure dramatically improves at lower temperature reaching 10^-20 W/Hz^-1/2 at 1 K.

A08. Sensor development for single-photon thermoelectric detectors

A. M. Gulian^a, K. S. Wood^b, G. G. Fritz^b, D. Van Vechten^c, H.D. Wu^b, J. S. Horwitz^b, G. R. Badalyantz^b, S. R. Harutyunyan^d, V. H. Vartanyan^d, V. R. Nikoghosyan^d, S. A. Petrosyan^d, A. S. Kuzanyan^d.
^aUSRA/NRL, Washington, DC
^bNRL, Washington, DC
^cONR, Arlington, VA
^dIPR, Nat'l Acad. Sci., Armenia

As we reported earlier [1], thermoelectric detectors can be competitive as non-dispersive energy resolving focal-plane instruments in X-ray/UV spectrum. The first generations of prototype devices demonstrated the viability of detector design and provided good agreement between theoretical expectations and experimental data. These devices exploited sensors made of gold with a small fraction of iron impurity. To get the projected high resolution one needs other type of material, namely, lanthanum-cerium hexaborides. We report on the first experimental tests of the feasibility of lanthanum-cerium films as sensor materials. Progress with technology of these materials argues for the success of these thermoelectric detectors. [1] A. Gulian, K. Wood, G. Fritz, A. Gyulamiryan, V. Nikogosyan, N. Giordano, T. Jacobs, and D. Van Vechten, X-ray/UV single-photon detectors with isotropic Seebeck sensors, NIMA , 441, No.3 (2000).

A09. Quasi-particle diffusion and the energy resolution of superconducting tunneling junctions as photon detectors.

Roland den Hartog^a, A.G. Kozorezov^b, J.K. Wigmore^b, D. Martin^a, P. Verhoeve^a, A. Peacock^a
a Astrophysics Division, Space Science Dept. of the European Space Agency, ESTEC, P.O. Box 299, 2200 AG Noordwijk, The Netherlands
^b School of Chemistry and Physics, Dept. of Physics, Lancaster University, Lancaster LA1 4YB, U.K.

One of the factors that limits the energy resolution of superconducting tunnel junctions (STJs) as photon detectors is the spatial dependence of the response on the photo-absorption site. To assess the role of spatial inhomogeneities we have analyzed quasiparticle diffusion processes in detail and developed a general analytical theory to describe the evolution of the quasiparticle density in an STJ. This theory accounts for (weak) non-linearity due to quasiparticle recombination, and includes the effects due to multiple quasiparticle tunneling, phonon coupling between the electrodes, diffusive losses into electrical connections and localised traps and losses at the edges. The theory does not require any assumptions about symmetry between base and counter electrode. We have derived an analytical model of the STJ response surface and the corresponding signal lineshape, which allows the influence of the above features to be treated independently. We discuss two examples in which this model is applied to experimental datasets obtained with multiple STJs, and demonstrate how this model can be used to obtain a thorough understanding of the factors that limit the energy resolution in STJs as photon detectors.

A10. Numerical Calculations of Diffusion Effects in STJ-Detectors

V.A. Andrianov^a, V.P. Gorkov^b
aInstitute of Nuclear Physics, Lomonosov Moscow State University, 119899 Moscow, Russia
^bThe Faculty of Computational Mathematics and Cybernetics, Lomonosov Moscow State University, 119899 Moscow, Russia

The numerical methods on the basis of grid functions are developed to describe the effects of diffusion of nonequilibrium quasiparticles in electrodes of STJ detectors. In addition to [1,2] numerical methods allow to describe quasiparticle motion in the electrodes of arbitrary shape with nonuniform boundary conditions. Besides different parameters of diffusion for upper and lower electrode are possible. Densities of quasiparticles in the upper and lower electrodes (n1 (x, y, t) and n2(x, y, t)) were described by a pair of the bound differential equations of Rothwarf-Taylor type (x, y are the coordinates in the plane of electrodes, t - time). The boundary and initial conditions used in the calculations were similar to those used in [1,2]. The model calculations were carried out for STJs of rhombus geometry.The pulse height spectra of the detector for electrodes with different ratio of diagonals of a rhombus were calculated. The influence of current leads and nonuniform boundary conditions were considered. The comparison of calculated spectra and experimental data was carried out. [1] O.J. Luiten, van den Berg et al., Proc.7th Int. Workshop on Low Temp. Detectors (LTD-7), ed. by S. Cooper, Munich 1997, p. 25. [2] L. Parlato, G. Ammendola et al., Nucl. Instr. Meth. A 444 (2000) 15.

A11. Fluctuations of Multitunneling in STJ Detectors

Victor V. Samedov
Moscow State Engineering Physics Institute (Technical University), 31, Kashirskoye Sh., 115409, Moscow, Russia

The theory of branching cascade processes is applied to the process of multitunneling in superconducting tunnel junctions (STJs). It is taken into account, that the process of signal formation includes multitunneling both of quasielectrons and holes. The model which takes into account the influence of the coupling of the quasielectron and phonon subsystems on multitunneling is also examined. This model takes into account the process of 2/Delta-phonon formation in recombination of two quasielectrons into a Cooper pair, 2/Delta-phonon propagation through an insulating layer, and subsequent breaking up of a Cooper pair in an opposite electrode. This process of 2/Delta-phonon transition through an insulating barrier does not transfer a charge, but contributes to the fluctuation of quasiparticle multitunneling. The probability generating functions for these processes are found. By means of probability generating functions it is possible to get expressions for the moments of distribution function of the number of quasiparticle tunnelings. In particular, the expressions for the mean number of quasiparticles multitunneling and its relative variances are obtained. In special case these expressions are reduced to expressions obtained in [1,2]. 1. D.J. Goldie et al., Appl. Phys. Lett. 64, 1994, p. 3169. 2. V. V. Samedov. Nucl. Instr. and Methods in Phys. Res., A444, 2000, p. 59.

A12. Fluctuations of STJ-Detector Signal due to Competition of Electron and Hole Tunneling Channels

V.A. Andrianov^a Victor V. Samedov^b,
^a Institute of Nuclear Physics, Lomonosov Moscow State University, 119899 Moscow, Russia
^b Moscow State Engineering Physics Institute (Technical University), 31, Kashirskoye Sh., 115409, Moscow, Russia

Statistical fluctuations of a STJ-detector collected charge under conditions of multiple tunneling of nonequilibrium quasiparticles and presence of electron and hole tunneling channels were considered. The behaviour of nonequilibrium quasiparticles in each electrode of STJ was characterised by three parameters: probabilities of tunneling by the electron channel P_e and by hole channel P_h and probability of losses P_loss. Using mathematical methods of generating functions, the expressions for mean value and relative variance of an collected charge are obtained. In a limiting case of one tunneling channel (P_h=0) the obtained expressions coincide with expressions deduced in [1]. The model calculations of signal fluctuations for asymmetric STJ-detector (Delta₁>Delta₂) as a function of bias voltage V_b were carried out. It was shown, that at voltages V_b<=(Delta₁-Delta₂)/e there is a noticeable increase of signal fluctuations due to adding the hole channel of tunneling. The minimum level of tunnel noise corresponds to high voltages V_b>(Delta₁-Delta₂)/e. 1. D.J. Goldie et al., Appl. Phys. Lett. 64, 1994, p. 3169

A13. Self-heating phenomena in superconducting tunnelling junctions

^aA.G.Kozorezov, ^bJ.K.Wigmore,A.Peacock⁺, Roland den Hartog⁺, G.Brammertz⁺ , D.Martin⁺, P.Verhoeve⁺, N.Rando⁺.
^aDepartment of Physics, Lancaster University, Lancaster, UK
^b+Astrophysics Division, European Space Agency - ESTEC, Noordwijk, Netherlands

Multiple tunnelling small gap structures are considered as an important contender for future optical photon detectors for astronomical applications. Recent experiments prove that in such STJs there occur important self-heating effects, significantly affecting their performance [1-3].
We have developed a kinetic theory of anomalous sub-gap currents in small gap multiple tunnelling STJs.
In the operating device the quasiparticle and phonon distributions are strongly non-equilibrium (non-thermalized). We predict a new class of phenomena resulting in a wealth of sub-gap current structure due to quasiparticle energy accumulation in the tunnelling - back tunnelling cycles. Among other features there occur the sub-gap current steps at voltages V=2Delta/en (n integer). We discuss the characteristics of non-equilibrium distribution of quasiparticle and phonons and show that the shape and magnitudes of steps on I-V curves as well as their dependence on bath temperature depend on junction parameters, including rates of quasiparticle tunnelling, recombination, residual losses and phonon escape from the junction. We discuss and compare the main predictions of the theory to the available experimental data. % ---------------------------------------------------------------- References 1. {A.Poelaert,A.G.Kozorezov, et.al., PRL, 82, 1257 (1999)} 2. {A.G.Kozorezov,J.K.Wigmore, et.al., APL, to appear, June (2001)} 3. {C.M.Wilson, L.Frunzio,D.E.Prober, Submitted to PRL (2001)}

A14. Vortex trapping in superconducting tunnel junction detectors

M. Ohkubo, K. Suzuki, K. Tanabe, H. Pressler, and M. Ukibe
Photonics Research Institute, National Institute of Advanced Industrial Science and Technology

Superconducting tunnel junction photon detectors, which exhibit high performance in the range between infrared and soft x-rays, are sensitive to vortices trapped in the superconducting thin films. The vortices cause the instability in detector operation and a large reduction in the signal height of detector response to photon absorption. Vortex trapping easily occurs when a residual magnetic field is present. We have investigated the details of the dependence of vortex trapping patterns on the strength of magnetic field perpendicular to the junction surface by using a scanning SQUID microscope, which enables the visualization of individual vortices, in niobium-based tunnel junctions. It has been found that the vortices are not trapped near junction edges, which can be the cause of spatial inhomogeneity of the signal height, and that there is a lower limit for vortex trapping. These experimental results may be explained by image and Meissner forces. The present results are very important for a magnetic shield design in practical applications.

A15. Strong quasiparticle trapping in a 6x6 array of Vanadium-Aluminium Superconducting Tunnel Junctions

G. Brammertz^a, A. Peacock^a, P. Verhoeve^a, A. Kozorezov^b, R. den Hartog^a, N. Rando^a, R. Venn^c
a Astrophysics Division, Space Science Department, ESA/ESTEC, P.O. Box 299, 2200AG Noordwijk, The Netherlands
^b School of Physics and Chemistry, University of Lancaster, Lancaster LA1 4YB, United Kingdom
^c Cambridge MicroFab Ltd., Trollheim Cranes Lane, Kingston, Cambridge CB3 7NJ, United Kingdom

A 6x6 array of symmetrical V/Al/AlOx/Al/V Superconducting Tunnel Junctions (STJs) was fabricated. The base electrode is a high quality epitaxial film with a residual resistance ratio (RRR) of 31. The top film is polycrystalline with an RRR of ~ 8. The leakage currents of the junctions are of the order of 0.5 pA/mm² at a bias voltage of 100 mV, which corresponds to a dynamical resistance of ~ 3 10⁵ ohm. When the array was illuminated by 6 keV X-ray photons from a Fe55 radioactive source the single photon charge output was found to be low and strongly dependent on the temperature of the devices. This temperature dependence at X-ray energies can be explained by the existence of a very large number of quasiparticle traps in the Vanadium. Quasiparticles are confined in these traps, having a lower energy gap than the surrounding material, and are therefore not available for tunnelling. The number and depth of these traps can be derived from the temperature dependence of the responsivity of the devices, (charge output per electron volt of photon input energy).

A16. Linearity investigations and pulse shape analysis of high resolution STJ X-ray detectors

M. Huber^a, G. Angloher^b, F. v. Feilitzsch^a, T. Jagemann^a, J. Jochum^a, T. Lachenmaier^a, J.-C. Lanfranchi^a, W. Potzel^a, A. Rüdig^a, J. Schnagl^a, M. Stark^a, H. Wulandari^a
aPhysik-Department E15, Technische Universität München, Munich, Germany
^bPhysik-Department E15, Technische Universität München, Munich, Germany. Current address: Oxford University, Oxford, GB

Al/Al_XO_Y/Al superconducting tunnel junctions (Al-STJs) on massive Si substrates were used as X-ray detectors. A low but detectable deviation from linearity was measured in the energy range between E = 1.74 keV and E = 6.49 keV. The nonlinearity can be explained by the escape of recombination phonons into the substrate. Varying the size of the Al-STJs shows that the nonlinearity is caused after the diffusive propagation of the quasiparticles over the STJs' electrodes. Possible "hot spot" effects could not be observed. By pulse shape analysis of the signals of Al-STJs a special event class is identified which is characterised by the distribution of the X-ray energy on the two electrodes. With these events the range of photoelectrons and Auger-electrons in the Al electrodes is estimated by MC-simulation. For a high resolution X-ray detector system based on an Al-STJ with a Pb absorber film located on a 100 nm thin Si₃N₄ membrane, pulse shape analysis allows to discriminate X-ray absorption in the Pb absorber and in the Al-STJ.

A17. Thermodynamical Model of Magnetic Calorimeters

A. Fleischmann^a, T. Danijarov^a, R. Weis^a, C. Enss^a, Y.H. Huang^b, and G.M. Seidel^b,
^aKirchhoff-Institut fuer Physik, Universität Heidelberg, D-69120 Heidelberg, Germany
^bDepartment of Physics, Brown University, Rhode Island, USA

X-ray detectors based on the concept of metallic magnetic calorimetry are well suited for high resolution spectroscopy. With prototype detectors, an energy resolution of 12 eV has been achieved. Magnetic calorimeters used for the detection of x-rays consist of a metallic paramagnetic temperature sensor, which is in good thermal contact with a suitable absorber. The sensor is placed in a small magnetic field, resulting in a magnetization roughly proportional to 1/T. Monitoring the magnetization with a low noise DC-SQUID makes the sensor a high resolution thermometer, which in turn makes the calorimeter highly sensitive to the deposition of energy. We present a model allowing for the calculation of the thermodynamic properties of the calorimeter. Within this model the parameters for optimizing the detector performance are derived. In addition, we discuss the fundamental sources of noise and the intrinsic limitation of the energy resolution of magnetic calorimeters.

A18. Thermalization of Magnetic Calorimeters

C. Enss^a, A. Fleischmann^a, T. Görlach^a, Y.H Kim^b, G.M. Seidel^b, H. Braun^c
aKirchhoff-Institut fuer Physik, Universität Heidelberg, D-69120 Heidelberg, Germany
^bDepartment of Physics, Brown University, Providence, RI 02912 USA
^cLehrstuhl für Experimentalphysik V, Universität Bayreuth, D-95440 Bayreuth, Germany

Calorimetric particle detectors based on metallic paramagnetic temperature sensors have show to be well suited devices for high resolution particle spectroscopy. Most of the work on metallic magnetic calorimeters was done using dilute alloys of erbium in gold as sensor material. In the temperature range of interest, the thermodynamic properties of erbium ions in gold all well understood, making the signal size as a function of temperature, magnetic field and concentration predictable. However, at temperatures below 50 mK the signal response of the calorimeter shows two relaxation times for the signal decay. Measurements of these time constants and the fractional amplitudes as a function of temperature and field indicate the presence of an additional thermodynamic system within the sensor material. Heat capacity measurements at temperatures as low as 100 µK suggest, that this additional contribution is arising from the quadrupole splitting of the Au nuclei (I=3/2) in the electric field gradients introduced by the presence of the Er ions. Measurements using calorimeters based on silver-erbium sensors support this assumption. In these measurements the additional, fast relaxation process was not observed. The host material silver (I=1/2) does not have a nuclear electric quadrupole moment. We discuss the origin of the two thermalization times of Au:Er-calorimeters and present the measurements on Ag:Er.

A19. Electron-phonon relaxation in hot-electron detectors below 1 K

Boris S. Karasik, Jet Propulsion Laboratory, California Institute of Technology Andrew V. Sergeev, Wayne State University Michael E. Gershenson, Rutgers University

Recently proposed submillimeter hot-electron direct detectors rely on the thermal coupling between electrons and phonons. Their sensitivity can be greatly enhanced if the coupling is made very weak. According to the theory, use of impure films should allow to achieve this goal. So far, the experimental situation has been somewhat confusing about this issue. A number of works have shown a cubic temperature dependence of the electron-phonon relaxation rate below 1 K. A conventional explanation is that this dependence is caused by direct interaction between electrons and phonons. We show that in most of these studies the pure limit was not reached. In this case, the electron scattering from vibrating impurities/boundaries dominates. The electron-phonon scattering rate varies from T⁴ × l for ql ll 1 to T² / l for 1 ll ql < 2(u_l / u_t)³ ~ 20-40 (u_l and u_t are the longitudinal and transversal sound velocities). In a wide temperature range around T ~ u_t/l the relaxation rate should have a T³ temperature dependence along with a weak l-dependence. Our recent experimental data for W and Ti films are in good agreement with the discussed interaction mechanism. The measured electron-phonon relaxation time followed the T^-4 dependence and was a record-long (25 ms) at 40 mK.

A20. Design and tests of high sensitivity NTD thermometers for the Planck-HFI instrument

M. Piat J.P. Torre J.M. Lamarre J.W. Beeman B. Leriche J.P. Crussaire F. Langlet.

The ESA satellite project Planck is dedicated to survey the sky and particularly the Cosmological Microwave Background at sub-millimeter and millimeter wavelengths. To perform this goal, the High Frequency Instrument (HFI) will use 48 bolometers cooled at 100mK by an open loop space qualified 3He/4He dilution cooler. The high sensitivity of the Planck-HFI instrument requires a challenging 100mK stability of 20nK.Hz^-0.5 down to 0.016Hz. In order to reach this requirement, a combination of a passive temperature filter with some active regulation stages is needed. High sensitivity thermometers will therefore be used for temperature stability monitoring but also for regulation. In order to optimise the design of such thermometers, we have modelled low temperature semiconducting thermometers by using a semi-analytical approach of Anderson insulators, taking into account both the electrical field and the electron/phonon decoupling effects. This leads to choose a convenient NTD Ge material with an optimal size bigger than usual. The first measurements of such devices showed important decoupling effect due to Kapitza resistance with the mounting. Nevertheless, a sensitivity of about 8nK.Hz^-0.5 down to 0.1Hz was obtained, limited at lower frequencies by the thermal fluctuations of our test bed. Recent results on these devices will be presented.

A21. Non-ideal effects in doped semiconductor thermistors

M. Galeazzi ^a, D. Liu ^a, D. McCammon ^a, W. T. Sanders ^a, P. Tan ^a, K.R. Boyce ^b, R. Brekosky ^b, J.D. Gygax ^b, R. Kelley ^b, D.B. Mott ^b, F. S. Porter ^b, C. K. Stahle ^b, C.M. Stahle ^b, and A. E. Szymkowiak ^b
aUniversity of Wisconsin, Madison, WI 53706 USA
^bNASA/Goddard Space Flight Center, Greenbelt, MD 20771 USA

Semiconductor thermistors have been used for several years and their ideal behavior is well known experimentally and theoretically. Their current performance is limited by non-ideal behaviors. These include 1/f noise and non-ohmic effects. We find that the 1/f noise appears to be a 2D effect, and can be greatly reduced by fabricating thicker thermistors. Eliminating this noise could improve the intrinsic detector resolution as much as 40%. It also allows us to study other sources of excess noise. The non-ohmic behavior can be empirically explained using a hot-electron model. Although this model seems not suitable for semiconductors in the variable range-hopping regime, where the electrons are localized, it fits well the experimental data. We measured an excess white noise at low frequencies consistent with the predicted thermodynamic fluctuations between electrons and phonons. We also measured a characteristic time of the non-ohmic behavior that is consistent with a C/G time constant in the hot electron model. Both results support the physical validity of the hot electron model. To optimize the performances of the next generation of detectors, we implemented the non-ideal behaviors in a model to predict the expected total noise and energy resolution. The comparison between the model and real data from the XQC and XRS experiments shows good agreement.

A22. Hot electron model in doped silicon thermistors

D. Liu ^a, M. Galeazzi ^a, D. McCammon ^a, W. T. Sanders ^a, B. Smith ^a, P. Tan ^a, K.R. Boyce ^b, R. Brekosky ^b, J.D. Gygax ^b, R. Kelley ^b, D.B. Mott ^b, F. S. Porter ^b, C. K. Stahle ^b, C.M. Stahle ^b, and A. E. Szymkowiak ^b
aUniversity of Wisconsin, Madison, WI 53706 USA
^bNASA/Goddard Space Flight Center, Greenbelt, MD 20771 USA

Non-ohmic behavior of doped silicon and germanium can be empirically explained using a hot-electron model, which is motivated by the hot electron effect in metals at low temperatures. This model assumes that the thermal coupling between electrons and lattice at low temperatures is weaker than the coupling between electrons, so that the electric power applied to the electrons raises them to a higher temperature than the lattice. Although this model seems not suitable for semiconductors in the variable range-hopping regime, where the electrons are localized, it fits quite well the experimental data. To determine whether the hot electron model in doped semiconductor is just an alternative way to parameterize the data or has some physical validity, we investigated the noise and frequency-dependence of the impedance of doped silicon thermistors that are used for low temperature thermal X-ray detectors. The measured excess white noise at low frequencies is consistent with the predicted thermodynamic fluctuations of energy between electron and phonon systems. The non-ohmic behavior shows a characteristic time that can be interpreted as a C/G time constant in the hot electron model. This is consistent with the assumption of a hot electron system thermally separated from the lattice system.

A23. 1/f Noise in Ion-Implanted Silicon Thermistors

D. McCammon ^a, M. Galeazzi ^a, D. Liu ^a, W. T. Sanders ^a, P. Tan ^a, K.R. Boyce ^b, R. Brekosky ^b, J.D. Gygax ^b, R. Kelley ^b, D.B. Mott ^b, F. S. Porter ^b, C. K. Stahle ^b, C.M. Stahle ^b, and A. E. Szymkowiak ^b
aUniversity of Wisconsin, Madison, WI 53706 USA
^bNASA/Goddard Space Flight Center, Greenbelt, MD 20771 USA

We have characterized the 1/f noise in standard ion-implanted silicon thermistors, which are about 250 nm thick. We find that it is associated with the bulk of the implant, and is interpretable as a dR/R fluctuation that is independent of the bias and depends only on the doping density and resistivity, or electron temperature. This excess noise is large enough that it has a significant effect on the energy resolution or NEP of a detector using these thermistors. The very steep temperature dependence of the 1/f noise suggested that it might be related to the conduction becoming two-dimensional, and we have fabricated thicker detectors to test this hypothesis. Similar doped silicon thermistors that are 1500 nm thick show negligible 1/f noise, but otherwise behave identically to the thinner thermistors of the same volume. This simple change could provide a 40% improvement in resolution for some existing X-ray detectors.

A24. Performance Modeling of Micro-calorimeter Detectors

M. Galeazzi ^a, D. Liu ^a, D. McCammon ^a, W. T. Sanders ^a, P. Tan ^a, E. Figueroa-Feliciano ^b, and C. K. Stahle ^b
aUniversity of Wisconsin, Madison, WI 53705 USA
^bNASA/Goddard Space Flight Center, Greenbelt, MD 20771 USA

In order to optimize the design of micro-calorimeters and bolometers, it is necessary to be able to predict the resolution of any given configuration. For detectors using ion-implanted silicon thermistors, we have sufficient engineering data on their non-ideal characteristics to predict the noise contributions from all currently known sources, given the basic physical parameters of the thermometer. We have constructed an analytical detector model that incorporates terms for thermistor Johnson and 1/f noise, amplifier noise, load resistor Johnson noise, and thermodynamic fluctuations between the electron and phonon systems in the thermometer as well as between the absorber, the thermistor, and the heat sink. The model has been checked by comparing its predictions to data obtained from existing detectors developed for the XRS spectrometer on Astro-E and for the XQC sounding rocket program. We are now using our model to optimize designs for new detectors employing thick implants that greatly reduce the 1/f noise term.

A25. Measurement of anomalous resistance-temperature relation for neutron transmutation doped germanium

A. L. Woodcraft^a, R. V. Sudiwala^b.
^a Department of Physics and Astronomy, University of Wales, Cardiff, 5, The Parade, Cardiff. CF24 3YB.

We present precise measurements of the resistance-temperature variation of several samples of neutron transmutation doped (NTD) germanium, at temperatures from 100~mK to 1~K. This material is used widely both for thermometry and for the thermistor element in bolometers and microcalorimeters. The resistance is generally found to follow the variable range hopping equation R(T)=R₀exp(T₀/T)ⁿ, where R₀ and T₀ are material parameters. A value of n=0.5 is supported by theory, and usually appears to allow good fits to data. However, we find that setting n=0.5 produces clear systematic errors for some of our samples. Taking n as a fitting parameter gives excellent fits over a large temperature range with n=0.53 and n=0.56 for different samples. We consider possible causes for this behaviour, and suggest that in general NTD germanium calibration data should be examined carefully for errors caused by assuming an incorrect value of n.

A26. High Performance Semiconductor-Based Bolometers

S. H. Moseley^a, D. McCammon^b
aILaboratory for Astronomy and Solar Physics, NASA/GSFC
^bUniversity of Wisconsin

Recent experiments on diffused thermometers in SOI material show great promise, having significantly less 1/f noise than their implanted counterparts. This lower noise promises better resolution for energy detectors, and offers a path to improved far infrared bolometers. Here, we present a design for infrared detectors based on micromachined structures using diffused SOI thermometers. Based on the measured thermometer properties, we show that these devices can offer significant performance improvements over the present generation of semiconductor-based bolometers in response time, sensitivity, and ease of production.

A27. The Suitability of Sapphire for Large Area Calorimeters

Y.H. Kim^a, H. Eguchi^a, C. Enss^b, A. Fleischmann^b, Y.H. Huang^a, R.E. Lanou^a, H.J. Maris^a, A.N. Mosharnuck^a, G.M. Seidel^a, B. Sethumadhavan^a, and W. Yao^a
aDepartment of Physics, Brown University, Providence, RI 02912, USA
^bKirchhoff-Institut für Physik, Universität Heidelberg, D-69120 Heidelberg, Germany

Large wafers having an area of at least 100~cm², to serve as calorimeters, are required in a proposed detector of low energy solar neutrinos using liquid helium below 50~mK as the target material. The calorimeters must have a threshold energy below 16~eV to be able to detect single photons of the UV scintillation from helium. In the development of such calorimeters we have investigated the properties of sapphire and the thermal coupling of sapphire to a sensor in the temperature range from 30 to 100~mK. Energy is deposited in a 1~cm³ sapphire crystal by an x-ray and the resulting response of a magnetic sensor is monitored. The sensor is a 40 micron diameter piece of erbium doped gold, the temperature- dependent magnetization of which is measured using a SQUID. Thermal contact between the sapphire and sensor is made through an Au film. The magnitude and response time of the signal are found to depend upon the properties of the gold film, in particular its thickness. The rise and decay times of the signal provide information on the thermalization processes involving the athermal phonons. The return of the calorimeter to thermal equilibrium appears, in part, to be influenced by the presence of two-level tunneling systems in the sapphire.

A28. A Constant Temperature TES Micro-calorimeter with an External Electronic Feedback System

N. Moeckel ^a, M. Galeazzi ^a, Mark Lindeman ^b, and C. K. Stahle ^b
aUniversity of Wisconsin, Madison, WI 53706 USA
^bNASA/Goddard Space Flight Center, Greenbelt, MD 20771 USA

One major problem with TES micro-calorimeters is the very narrow temperature range in which they work. To detect high energy events the heat capacity of the detector must be sufficiently big to avoid running the sensor outside its sensitivity region. The bigger heat capacity limits the best energy resolution that can be obtained. One way to avoid a saturation effect keeping the heat capacity small is to run the detector at constant temperature reducing the power dissipated into it by an amount equal to the power conducted into the thermometer from the energy deposition. We studied the possibility of such a detector using an external electronic feedback system that reduces the bias voltage on the sensor. Using a traditional electro-thermal feedback to run a constant temperature detector, the signal amplitude can be reduced, but the noise of the readout electronics does not change and this may worsen the energy resolution. Moreover the gain of the feedback, determined by the sensitivity alpha may not be big enough to keep the temperature constant. With an external feedback, the noise of the readout electronics is reduced proportionally to the signal and the feedback gain can be much bigger. In this paper we outline the characteristics of such a system, we study its optimization and we investigate its performances.

A29. Materials Considerations in the Operation of Geometrically-metastable Superconducting Detectors

M.R. Gomes^a, TA Girard^a, P.Valko^a,
^aCentro de F�sica Nuclear,Universidade de Lisboa 1649-003 Lisbon, Portugal

Investigations of the signal response of a variety of superconducting, of both Type I and II, are reported. The results are interpreted in terms of the flux motion sensitivity of the fast-pulse acquisition electronic readout, and suitability for use in geometrically-metastable devices indicated.

A30. Response Dynamics of Geometrically-metastable Superconducting Detectors under Irradiation

TA Girard^a, M.R. Gomes^a, P.Valko^a,
^aCentro de F�sica Nuclear,Universidade de Lisboa 1649-003 Lisbon, Portugal

We report investigations of the response of geometrically-metastable superconducting devices under irradiation, demonstrating that the behavior differs significantly from that under normal variation of external magnetic field at constant temperature. The results are interpreted within the context of hotspot formation and the "explosive nucleation" model of Shapiro, and the possible relationship between measurements and the Kibble-Zurek mechanism indicated.

A31. Weakly expressed effects in HTS detected by a single-layer flat coil-based Tunnel Diode oscillator technique demonstrating its wide possibilities for high-resolution detection

S.G. Gevorgyan^a,b, T. Kiss^a, T. Nishizaki^c, H.G. Shirinyan^b, A.A. MovsisyanV^b, V.S. Gevorgyan^b, M. Inoue^a, T. Harayama^a, T. Matsushita^a, N. Kobayashi^c, and M. Takeo^a
a Graduate School of ISEE, Kyushu University, Fukuoka, 812-8581, Japan
^b Institute for Physical Research, National Academy of Sciences, Ashtarak, 378410, Armenia
^c Institute for Materials Research, Tohoku University, Sendai, 980-8577, Japan

We developed flat coil-based TD-oscillator technique for high-resolution tests of penetration depth of MHz-range magnetic field and detection of absorption inside HTS. The method is based on simultaneous detection of the inductance and Q-factor changes of single-layer coil, which leads to frequency and amplitude changes of the oscillator. It enables detecting the changes ~ 10^-12H of film's magnetic inductance (changes ~1 Angstrom of penetration depth). It detects also the changes ~ 10^-9W of testing field's power absorption by a film, originated from variation of its internal flux distribution. It operates in high magnetic fields, measures in wide dynamic ranges of testing parameters, and is sensitive especially near T_c. These allow revealing of fine peculiarities of superconductive transition and flux flow in plate-like HTS at beginning of "Cooper" pairs' formation. We detected new "paramagnetic" peculiarity on temperature-transition curves of HTS and LTS materials before their diamagnetic repulsion. We revealed an anomalous absorption of testing field's power by HTS film close to end of its magnetic-transition. Simultaneous frequency and amplitude tests enabled also constructing the vortex magnetic phase diagram in HTS. What is important, the method enables all above in small-volume films and crystals without disturbing them near T_c, and during the simple measurement.

A32. NIS Tunnel Junction Detector with Segmented Absorbers

F. Yoshihara^a, I. Kanno^a, M. Katagiri^b, M. Ukibe^c, M. Ohkubo^c
aKyoto University, Sakyo, Kyoto 606-8501, Japan
^bJapan Atomic Energy Research Institute, Tokai, Ibaraki 319-1195, Japan
^cNational Institute of Advanced Industrial Science and Technology, Tsukuba,Ibaraki, 305-8568, Japan

An NIS tunnel junction detector is a calorimeter, which energy resolution is proportional to the square root of the heat capacity of the normal metal absorber. It is not possible to enlarge the absorber area of this detector without deteriorating the energy resolution, in general. In this paper, we suggest a method to enlarge the absorber area of an NIS tunnel junction detector without deteriorating its energy resolution: we divide the absorber area into several parts and connect them by superconductor leads. The heat capacity of each segmented absorber remains small, but the summed area of absorbers is large. By applying this method, we estimated better energy resolution in case of dividing absorber into segments, comparing to the one-body absorber with the same area. Considering the case dividing the absorber into two, the heat capacity of each segmented absorber becomes a half, its heat response becomes twice as fast in case its tunnel current is the same. Moreover, one may have detectors with thicker absorbers, which is sensitive to higher energy X-rays, with dividing the normal metal into some segments and keeping the heat capacity small. So far, we fabricate an NIS tunnel junction detector by dc magnetron sputtering. Each layer pattern was defined by using shadow mask. We measured I-V characteristics of the detector at 270mK.

B01. Physics and Applications of Normal-Insulator-Superconductor Tunnel Junctions

J. N. Ullom^a,
^aLawrence Livermore Laboratory, Livermore, CA

In this talk, the physics and applications of Normal-Insulator-Superconductor (NIS) tunnel junctions will be reviewed. The current-voltage properties of NIS junctions are diode-like with a strong temperature dependence. Hence, these structures can be used as sensitive thermometers at temperatures well below the energy gap, delta, of the superconducting electrode. Calorimeters and bolometers for the detection of X-rays and millimeter-wave radiation, respectively, have successfully been built from NIS junctions. For junction voltages comparable to delta/e, current flow removes energy from the normal electrode. This property has been exploited to build microrefrigerators capable of cooling thin-film circuits from 0.3 K to temperatures near 0.1 K. Recent efforts to increase the size of these refrigerators and prospects for their integration with other low-temperature electronics will be discussed. Finally, it has been demonstrated that structures built from two NIS junctions in series can act as superconducting three-terminal devices. Recent results from these devices will also be described. This work was performed under the auspices of the U.S. Department of Energy by the University of California, Lawrence Livermore National Laboratory under contract No. W-7405-ENG-48.

B02. Annular Superconducting Tunnel Junction Detectors: experimental results under X-ray illumination

L. Frunzio^a,c, L.Li^a, D.E. Prober^a, I. Vernik^b, M. Lissitski^c,d, C. Nappi^c, R. Cristiano^c
aDepartment of Applied Physics and Physics, Yale University, P.O. Box 208284, New Haven, CT 06520-8284, USA
^bHypres Inc., 175 Clearbrook Rd., Elmsford, NY 10523, USA
^cIstituto di Cibernetica "E. Caianiello", Via Campi Flegrei 34, I-80078 Pozzuoli, Napoli, Italy and INFN Sezione di Napoli
^dINFM Sezione di Napoli

We report on the first experiment detecting 55Fe X-ray photons by an annular Superconducting Tunnel Junction (STJ) detector. The experiments have been performed on Nb/Al-AlOx/Nb annular STJ detectors fabricated at HYPRES Inc. The radiation, impinging on an STJ detector, breaks Cooper pairs and creates excess quasiparticles. They are collected and counted by tunneling in order to measure the incoming energy. The stability of the bias point of the detector has been obtained in two different configuration. In the first one, as usual, we applied, an external magnetic field parallel to the barrier to suppress the STJ critical current and resonances. In the second configuration, we trapped a single fluxon through the anular structure in the STJ barrier during the transition to the superconducting state. This is the innovative configuration which allows to obtain the same suppression without any externally applied parallel magnetic field. In both configurations we observed current pulses produced by the interaction of the STJ electrodes with single X-ray photons and they appear to be identical.

B03. Energy, position and time resolving photon spectrometers with Ta-based STJ devices

Ph. Lerch, E. C. Kirk, J. Olsen, A. Mchedlischvili, A. Zehnder H. R. Ott ^a
Paul Scherrer Institute CH - 5232 Villigen, Switzerland
^a ETH - Hönggerberg, CH - 8093 Zürich, Switzerland

We are developing superconducting tunneling junction (STJ) devices as photon counting imagers. In an attempt to increase the number of pixels and the collection area without increasing the complexity of the wiring in the same proportion we suggest to use a distributed readout scheme. A Ta absorber strip is readout by 2 STJs placed at both ends. Our devices are made out of 8 strips and 16 STJs. Their readout will thus require 16 channels for an effective area of 64 pixels. We compare the behavior of Ta/Al₁/AlO_x/Al₂/Nb devices with results obtained from numerical simulations of the proximity effect. We have performed irradiation experiments on single strips with visible and soft x-ray photons. The effects of the absorber/trap interface and the thickness of the Al trapping layer on the charge collection are presently investigated and shall be presented.

B04. Single Photon 1-D Imaging X-ray Spectrometers using Superconducting Tunnel Junctions

L. Li, L. Frunzio, C. Wilson, D. E. Prober
Yale University, New Haven, CT

We are developing superconducting, single photon, 1-D imaging x-ray detectors for applications in x-ray astronomy and astrophysics. The detectors use a Ta absorber and Al-AlOx-Al tunnel junctions for readout. Photons absorbed in the Ta create excess quasiparticles which are trapped in the Al tunnel junctions on each end of the absorber and cause an increase in their subgap current. The integrated charge is proportional to the photon energy. The charge division between the two junctions gives 1-D position information. Present devices have achieved an energy resolution of 13 eV FWHM for 6 keV x-rays over an 20 �m x 100 �m absorber area near the end of the absorber. The energy resolution is within a factor of two of the statistical limit. The broadening of the energy resolution in the center is caused by the niobium contact. A device with Ta ground contact to the trap of one junction instead of Nb in the center was tested. The energy resolution of this device is uniform over a large area in the center. We will discuss the performance and physics of these devices and scaling to larger absorber sizes with more pixels.

B05. Two-dimentional position resolution and correction on incidence position dependency of signal heights of series-junction detectors.

S. Kamihirata, M. Kurakado, A. Kagamihata, K. Hirota, H. Hashimoto and K. Taniguchi [a] H. Sato, Y. Takizawa, C. Otani and H. M. Shimizu [b] [a] Division of Electronics and Applied Physics, Osaka Electro-Communication University [b] RIKEN (The Institute of Physical and Chemical Research)

Superconducting tunnel junction (STJ) detectors have the possibility of ultrahigh energy resolution[1]. Series-junction consists of STJs connected in series. The series-junction detector absorbs incident energy of a radiation in the single-crystal substrate[2]. The energy of radiation is converted to non-thermal phonons[1].The detection efficiencies of the superconducting series-junction detectors are much higher than those of superconducting single junction detectors. Present series-junction detector has four series-junctions on a surface of the substrate. We can obtain two-dimensional position resolution by making use of the heights of signal from the four series-junctions. Total signal heights depend on the incidence position in the series-junction detector. We developed a method to correct the incidence position dependency of pulse height. Correction factors were determined for each incidence region, making use of the position resolution, and thereafter pulse heights of signals were modified by the correction factors. By means of measurements of radiations, i.e., X-rays and ��-particles, and through simulations of radiation detection, it was confirmed that the correction of the incidence position dependency by the method is useful to improve the energy resolutions of the series-junction detectors. References [1] M. Kurakado, Nucl. Instrum. Methods 196, 275 (1982) [2] M. Kurakado, X-Ray Spectrometry, 29, 137 (2000)

B06. Ta Superconducting Tunnel Junctions fabrication process for photon counting detectors

Philippe Feautrier Laboratoire d'AstrOphysique de Grenoble, 414 rue de la piscine, Domaine Universitaire, 38040 Grenoble Cedex 9, France Jean-Claude Villegier CEA-Grenoble SPSMS/LCP, F38054 Grenoble Cedex 9, France Corentin Jorel Laboratoire d'AstrOphysique de Grenoble, 414 rue de la piscine, Domaine Universitaire, 38040 Grenoble Cedex 9, France Alain Benoit CRTBT, CNRS BP 166-38042 Grenoble Cedex 9, France. Bertrand Delaet LAOG \& CEA-G; present adress: JPL Jet Propulsion Laboratory California Institute of Technology M/S 168-314 4800 Oak Grove Drive Pasadena, CA 91109

The properties of Superconducting Tunnel Junctions (STJ) make them very suitable for low light level astronomical observations: they are able to count photons from the visible light to the near-infrared with a rather good energy resolution. We have developed a modified version of the "SNEP" fabrication process for making Ta/Al-AlOx-Al/Ta/Nb STJ's. These junctions show a very low sub-gap leakage current at 0.1 K. A double thin aluminum trapping layer and a Ta absorber grown epitaxially by magnetron sputtering on a R-plane sapphire substrate maintained at 600�C are achieved. High quality epitaxial tantalum films has been obtained with RRR = 30 and sharp X-Ray diffraction peaks. Some recent improvements in the fabrication process will be detailed. The experimental junction parameters will be analyzed: current density, subgap leakage current, Ta mean free path, barrier thickness and intrinsic junction capacitance. An optical set-up has been completed to achieve photon counting performances. An original, room temperature, charge sensitive preamplifier has been built for this purpose. Photon counting ability in the near infrared at 0.78 micron has been demonstrated with Nb junctions. First results with the new tantalum junctions will also be detailed.

B07. Aluminum Superconducting Tunnel Junction as X-Ray Detector: Technological Aspects and Phonon Decoupling from the Substrate

M. Lissitski^a,b, E. Esposito^a,c, L. Frunzio^a,f, D. Perez de Lara^a,c, R. Cristiano^a,c, G. Angloher^d, M. Salvato^e, G. Carbone^e, L.Li^f, D.E. Prober^f
aIstituto di Cibernetica "E. Caianiello", Via Campi Flegrei 34, I-80078 Pozzuoli, Napoli, Italy
^bINFM Sezione di Napoli
^cINFN Sezione di Napoli
^dTechnische Universitat Munchen, Physik Department E15, James-Franck-Str., D-85748 Garching, Germany
^eDipartimento di Fisica and INFM Universit� di Salerno, Via S. Allende, I-84081 Baronissi, Salerno, Italy
^fDepartment of Applied Physics and Physics, Yale University, P.O. Box 208284, New Haven, CT 06520-8284, USA

We have investigated Al/AlOx/Al Superconducting Tunnel Junctions (STJs) for their use as X-ray detector. The junctions have been fabricated on sapphire substrates, by using different underlayers (Nb, SiO) and barrier transparencies. It was found that the transparency and the quality of the AlOx tunnel barrier strongly depend on the microstructure of the Al base electrode (grain size, texture) which, in turn, is determined by the substrate/underlayer materials. The deposition parameters were optimized in order to fabricate junctions capable to resolve in energy the spectrum emitted by an 55Fe X-ray source. Experiments under X-ray irradiation on STJs with and without a SiO buffer layer are reported. The spectra show that the SiO layer succeeds in reducing the events coming from the phonons generated into the substrate. Besides, it is interesting to remark that the yield is about 85% for STJs with the buffer layer, quite high in comparison with the 5% of STJs without the buffer layer. Another feature is that the junction with the buffer layer shows one single peak instead of the four peaks corresponding to the two MnK lines of the 55Fe source for each electrode, as it was observed in a similar STJ without the SiO buffer layer.

B08. STJ X-Ray Detectors with Killed Electrode

V.A. Andrianov^a, P.N. Dmitriev^b, V.P. Koshelets^b, M.G. Kozin^a, I.L. Romashkina^a, S.A. Sergeev^a
aInst. of Nuclear Phys., Lomonosov Moscow State Univ., 119899 Moscow, Russia ^bInst. of Radio Engineering and Electronics RAS, 103907 Moscow, Russia

STJ detectors with killed electrode[1] have a number of potential advantages. They enable a) to cut substantially the signal from the killed electrode as compared with that from the main absorbing electrode, b) to reduce the tunneling noise, c) to reduce the influence of the boundary conditions due to decrease of the quasiparticle diffusion length. In the present work the junctions with Al/Nb/Al/AlO_x/Al/Nb/NbN layer sequence were studied. The base electrode was killed by adjacent to the substrate Al-layer. Pulse height spectra arising under STJ irradiation with X-rays from Fe-55 source were measured at T=1.35 K. Signal amplitude dependence on the bias voltage was studied. Various mechanisms of signal formation are considered. Insufficient attenuation of the signal from the killed electrode can be connected with the phonon exchange between electrodes. Variation of the spectra shape with detector area is in accord with quasiparticle diffusion model. [1] O.J. Luiten , van den Berg et al., Proc.7th Int. Workshop on Low Temp. Detectors (LTD-7), ed. by S. Cooper, Munich 1997, p. 25.

B09. Development of Superconducting Tunnel Junctions as an Energy-Dispersive Detector for EUV and UV Radiation

H. Sato, T. Ikeda, T. Oku, C. Otani, K Kawai, H. M. Shimizu, Y. Takizawa, H. Miyasaka, H. Watanabe
RIKEN (The Institute of Physical and Chemical Research), Wako, Japan

We are developing an energy-dispersive detector for EUV and UV radiation using superconducting tunnel junctions with Al trapping layers. We have evaluated the performance of the detector for EUV photons using the Synchrotron Facility (KEK-PF) at High Energy Accelerator Research Organization in Tsukuba.We have achieved the energy resolution of FWHM=18 eV (including the external noise of 17 eV) for 55 eV EUV photons with a 100 x 100 um² STJ. In this paper, we will present and discuss the details of the junction design,our experiments and the results.

B10. Once More on the Energy Resolution of STJ Detectors

Victor V. Samedov
Moscow State Engineering Physics Institute (Technical University), 31, Kashirskoye Sh., 115409, Moscow, Russia

The paper[1] presents an experiments with Ta based STJ (200 nm Ta/5 nm Al/1 nm AlOx/5 nm Al/30 nm Ta/20 nm NbN), which show measured energy resolutions of 4.6 eV, 8.1 eV and 20.5 eV at 525 eV, 1.5 keV and 6 keV respectively. This experimental data are analyzed on the basis of the expression for energy resolution obtained in [2]. In this report it is shown that after subtraction of the electronic noise from the energy resolution, it is necessary to plot the relative variance of output signal as a function of inverse number of quasielectrons created by incident photons. The interception of this linear dependence with the axis of ordinates gives the contribution to the broadening of the line caused by spatial fluctuation of the photon interaction point. This contribution to the energy resolution can be considered as the criterion of STJ quality and can be used for comparing STJs. The noticed discrepancies between the experimental data and the calculated energy resolution in [1] are explained by incorrect accounting for quasiparticle multitunneling. 1. G. Brammertz et al. Preprint ESLAB 2000/107/CA. 2. V. Samedov. Proc.7th Int. Workshop on Low Temp. Detectors (LTD-7), ed. by S. Cooper, Munich 1997, p. 29.

B11. Periodical distribution of charge output in superconducting tunnel junction having a structure of Nb/Al/AlOx/Al/Nb

T. Nakamura^a, M. Katagiri^a, M. Ohkubo^b, H. Pressler^b, H. Takahashi^c and M. Nakazawa^c
aJapan Atomic Energy Research Institute
^bNational Institute of Advanced Industrial Science and Technology
^cUniversity of Tokyo

Periodical distribution of charge output in superconducting tunnel junction (STJ) having a structure of Nb/Al/AlOx/Al/Nb with size of 200 x 200 um2 was first observed by Low Temperature Scanning Synchrotron Microscope experiments. Two swelled parts were appeared in the distribution of charge output and the period of the swelled parts was about 100um. It is found that this distribution was occurred under the conditions; (i) the size of the STJ is more than twice of Josephson characteristic length and (ii) a bias voltage applied to the device is more than 0.2mV of about the energy gap of the trapping layers. It was confirmed that this anomalous distribution of charge output significantly distorts the shape of the photo peak and degrades the energy resolution of the STJ detector.

B12. High counting rate X-ray detector using a superconducting tunnel junction with current readout method

M. Katagiri^a, T. Nakamura^a, M. Ohkubo^b, H. Pressler^b, H. Takahashi^c and M. Nakazawa^c
aJapan Atomic Energy Research Institute
^bNational Institute of Advanced Industrial Science and Technology
^cUniversity of Tokyo

We developed a high counting rate X-ray detector using a superconducting tunnel junction (STJ) having a structure of Nb/Al/AlOx/Al/Nb with a fast current readout system. The STJ has a current rise time of 100ns, a current decay time of 260nsec and resistance of 20ohm at 0.4K. A fast current readout system consists of a superconducting coil of about 1mH for current load, a fast current amplifier and a fast peak hold circuit. High counting rate characteristics of the X-ray detector are measured by synchrotron radiation. The energy resolution for counting rate between 10cps and 100kcps is about 270eV and the energy resolution for 300kcp is about 300eV at X-rays of 4keV. It is confirmed that this X-ray detector has an excellent performance for high counting rate X-ray detection.

B13. The response function of superconducting tunnel junction detectors

S. Friedrich^a, B. Beckhoff^b, M. Frank^a, R. Fliegauf^b, S. E. Labov^a, G. Ulm^b,
^aLawrence Livermore National Laboratory, L-418, Livermore, CA 94551, U.S.A.
^bPhysikalisch-Technische Bundesanstalt, Abbestrasse 2-12, 10587 Berlin, Germany

We have characterized the response function, the spectral artifacts and the absolute efficiency of superconducting tunnel junction (STJ) x-ray detectors using monochromatic synchrotron radiation. The experiments were performed at the plane grating monochromator beamlines for bending magnet and for undulator radiation in the PTB radiometry laboratory at the electron storage ring BESSY II. Both beamlines provide monochromatic radiation of high spectral purity. Two STJ detectors were characterized in the energy range from 100 eV to 1900 eV with respect to their energy resolution ranging from about 12 eV to 28 eV FWHM and their count-rate capability. We have investigated the spatial origins and spectral distribution of artifacts by systematically scanning a collimator across the detector area. In addition, we have determined the absolute detection efficiency of one STJ detector in special operation shifts of BESSY II by adapting the stored electron beam current to the count-rate capability of the STJ. We will present x-ray fluorescence spectra taken with STJ detectors at the ALS synchrotron, discuss the consequences of spectral artifacts for X-ray fluorescence analysis of dilute samples and propose some improvements for future detector lay-outs.

B14. Characterisation of Titanium films for low temperature detectors

E. Monticone^a, V. Lacquaniti^a, M. Rajteri^a, M. L. Rastello^a, C. Gandini^a, E. Pasca^b, G. Ventura^b,
^aIstituto Elettrotecnico Nazionale Galileo Ferraris, Torino, Italy
^bDept. of Physics, University of Florence, Florence, Italy

Transition edge sensors (TES) are promising devices as low temperature light detectors due to their high energy resolution and broadband response. One of the suitable materials for TES is titanium that shows transition temperatures up to 0.5 K. Titanium films have already been investigated by different authors, but the correlation between the electrical properties at different temperatures in the normal state and the transition temperatures is lacking. In this work we study Ti films, with thickness between 10 nm and 1000 nm, deposited by e-beam on silicon and silicon nitride. Critical temperatures and electrical resistivities of these films have been measured as function of thickness, deposition pressure, deposition rate and substrate temperature. The behaviour of the critical temperatures versus the residual resistivities is discussed in the frame of the Testardi and Mattheiss theory.

B15. Rectifying Characteristics of InSb Device at Low Temperature

I. Kanno^a, F. Yoshihara^a, O. Sugiura^b, M. Katagiri^c
aKyoto University, Sakyo, Kyoto 606-8501, Japan
^bTokyo Institute of Technology, Ookayama, Meguro, Tokyo 152-8552, Japan
^cJapan Atomic Energy Research Institute, Tokai, Ibaraki 319-1195, Japan

McHarris predicted the possible high energy resolution of InSb semiconductor detector. InSb has the band gap of 0.165eV, and the mobilities of electrons and holes 78000 and 750cm²/(Vs), respectively. This band gap energy is less than 1/6 of that of Si, and gives the energy resolution of 60eV for 6keV X-rays, without taking the Fano factor into account. High atomic number and high density of InSb also make the detector more attractive. The impurity in InSb, however, makes its resistivity very small, even at low temperature. No successful attempt was reported on InSb as a high energy resolution detector so far. Infrared sensors and Hall devices have been the main application of InSb. We report here the rectifying characteristics of InSb device, at the temperature below 4.2K. The employed InSb was p-type one with Ge dopant of 3.5x10¹⁵cm^-3 concentration. The resistivity of the substrate was 0.29ɏcm at 77K. The Schottky elctrode was made by Al evaporation and Mo was deposited on Al to make better electric contact. The Ohmic contact was simply taken by soldering InSb wafer to Cu plate. This rectifying characteristics shows the possibility of InSb device as X-ray detector.

C01. Superconducting transition edge sensors for X-ray microcalorimetry

H.F.C. Hoevers, Space Research Organization Netherlands

The current work on superconducting Transition Edge Sensors (TESs) for X-ray microcalorimetry can be separated into the research involving the optimization of a single pixel detector and the development of an imaging array of microcalorimeters. The research is motivated by astronomical applications, such as Constellation-X or XEUS, and microanalysis. During the past years, research groups have achieved promising energy resolutions of 2 eV at 1.5 keV and 5 eV at 5.89 keV with single pixel microcalorimeters. The paper discusses the measured resolutions in relation with the noise levels and the theoretical predictions. The current performance of microcalorimeters and routes for further optimization of single pixel detectors are discussed, based on the physics involved. Recently, the development of imaging microcalorimeter arrays using TES microcalorimeters has started. The feasibility and expected performance of an imaging array is determined by the geometry of the array and the design of the individual pixels. Starting from the requirements for energy resolution, sensor speed, absorption efficiency and packing density the design of the individual pixels is discussed along with the expected performance of a complete array, including the issue of thermal and electrical crosstalk between pixels.

C02. Performance of X-ray microcalorimeters with an energy resolution below 4.5~eV and 100~µs response time

W.M. Bergmann Tiest^a, H.F.C. Hoevers^a, W.A. Mels^a, M. Ridder^a, M.P. Bruijn^a, P.A.J. de Korte^a, and M.E. Huber^b
aSpace Research Organization Netherlands,
Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
^bDepartment of Physics, University of Colorado at Denver
P. O. Box 173364, Denver, CO 80217-3364, USA

The performance of X-ray microcalorimeters with different geometries, and under different illumination conditions, has been investigated. These microcalorimeters consist of a TiAu Transition Edge Sensor (TES) and a Cu absorber. With these microcalorimeters we regularly achieve energy resolutions below 4.5 eV FWHM for 5.89 keV X-rays in combination with a response time of about 100 µs. The measured energy resolution is analysed and discussed based on current-voltage characteristics and noise spectra. The variation in geometry did not influence the detector performance to a large extend, and spatial non-uniformities are considered to be small. In order to increase the absorption efficiency and packing density (mushroom-shaped) Bi absorbers are being produced and tested with our microcalorimeters. In addition, the performance of several digital filtering schemes is investigated using both experimental as well as simulated data.

C03. Performance of Mo/Au TES microcalorimeters

Mark A. Lindeman, Regis P. Brekosky, Enectali Figueroa-Feliciano, Fred M. Finkbeiner, Mary Li, Caroline K. Stahle, Carl M. Stahle, Nilesh Tralshawala (NASA GSFC)

We are developing X-ray calorimeters to meet the specifications of the Constellation-X mission. Each calorimeter consists of a transition-edge-sensor (TES) thermometer, which is suspended on a silicon-nitride membrane. Our TES thermometers are Mo/Au bilayer films that are biased in the sharp phase transition between the superconducting and normal-metal states. These calorimeters have demonstrated very good energy resolutions: 2.4 eV at 1.5 keV and 3.7 eV at 3.3 keV. The energy resolutions are limited by thermal noise and Johnson noise (which are intrinsic to any resistive calorimeter) plus excess noise. The excess noise, which is several times larger than the Johnson noise, is consistent with frequency-independent voltage noise in the TES. Detailed measurements of one Mo/Au TES demonstrate that the excess noise is independent of the voltage applied to the TES over a range of biases at the same TES resistance. The magnitude of the excess noise is smallest at the high-resistance end of the phase transition. We also compared noise in square Mo/Au TES's ranging in size from 150 microns to 600 microns to learn how the excess noise is affected by the geometry of the TES.

C04. Fabrication of bridge-type microcalorimeter arrays with Ti-Au Transition-edge-sensors

M. Ukibe^a, T. Kimura^b, T. Nagaoka^b, H. Pressler^a, M. Ohkubo^a,etc.
^aNational Institute of Advanced Industrial Science and Technology, 1-1-1, Umezono Tsukuba, Ibaraki 305-8568 JAPAN
^bUniversity of Meiji, 1-1-1 Higashisanda Tama-ku Kawasaki, Kanagawa 214-8571 JAPAN

The present stage of the cryogenic detectors is in the development of arrays. The arrays of TES microcalorimeters have generally been fabricated on SiN_X membrane structure. In conventional membrane structure fabricated by backside etching, it is difficult to realize arrays having a mechanical toughness and a large filling factor. It is possible to overcome the difficulty by using a bridge-type membrane structure, of which SiN_X membranes are floating with a gap of 10-50 µm on Si substrates. For checking etching and other processes, we have fabricated a small scale array with the bridge-type membrane structure. The preliminary version of the TES microcalorimeter array has a dimension of 10 x 6 mm² and 5 x 5 pixels. Each pixel consists of a 0.7 mm x 2 mm SiN_X membrane with a thickness of 1 µm and a Ti/Au bilayer TES of 0.5 x 0.5 mm². This array has a filling factor of about 0.1. However, it is possible to obtain a high filling factor by changing a mask design or attaching mushroom-shaped absorbers.

C05. The Constellation-X Focal Plane Microcalorimeter Array: An NTD-Germanium Solution

J. Beeman(a), E. Silver(b), S. Bandlerb(b), H. Schnopper(b), S. Murray(b), N. Madden(a), D. Landis(a), E. E. Haller(a), and M. Barbera(c) (a)Lawrence Berkeley National Laboratory (b)Harvard-Smithsonian Center For Astrophysics (c)Osservatorio Astronomico G.S. Vaiana

The hallmarks of Neutron Transmutation Doped (NTD) Germanium cryogenic thermistors include high reliability, reproducibility, and long term stability of bulk carrier transport properties. The dopant concentration in this material is easily controlled by adjusting the exposure time of the germanium to a known neutron fluence. Since the neutron absorption cross section in germanium is relatively small, the resulting dopant distribution in this material is extremely uniform. This uniformity is especially important when building large area arrays. Using micro-machined NTD Ge thermistors with integral "flying" leads, we can now fabricate two-dimensional arrays that are built up from a series of stacked linear arrays. We believe that this modular approach of building, assembling, and perhaps replacing individual modules of detectors is essential to the successful fabrication and testing of large multi-element instruments. Details of construction are presented.

C06. Fabrication of Close-packed TES Microcalorimeter Arrays Using Superconducting Molybdenum/Gold Transition-Edge Sensors

F. M. Finkbeiner^a,b, R. P. Brekosky^a,c, J. A. Chervenak^a, E. Figueroa-Feliciano^a,d, M. J. Li^a,e, M. A. Lindeman^a,f, N. Tralshawala^a,e, C. K. Stahle^a, C. M. Stahle^a
aNASA/Goddard Space Flight Center, Greenbelt, MD 20771
^bDepartment of Astronomy, University of Maryland
^cSwales Corporation, Beltsville, MD
^dPhysics Department, Stanford University
^eRaytheon ITSS, Lanham, MD
^fNAS/NRC Resident Research Associate

We present an overview of our efforts in fabricating TES microcalorimeter arrays for use in astronomical x-ray spectroscopy. Two distinct types of array schemes are currently pursued: A 5x5 single pixel TES array, where each pixel is a TES microcalorimeter, and the position-sensitive TES (PoST) array. In the latter, a row of 15 thermally-linked absorber pixels is read out by two transition-edge sensors (TES) at its ends. Both schemes employ 150x150 square micron large superconducting Mo/Au bilayers as the TES which are produced by electron-beam deposition under ultra-high vacuum conditions. The TES are placed on silicon nitride membranes for thermal isolation from the structural frame. The silicon nitride membranes are prepared by a state-of-the-art Deep Reactive Ion Etch process into a silicon wafer before or after the TES deposition. The absorbers are 2 to 10 microns thick bismuth layers thermally deposited onto the TES or onto a patterned gold film in the case of the PoST array. In order to closely pack the arrays without decreasing its structural and functional integrity, we have already produced arrays of cantilevered pixel-sized absorbers and slit membranes. Furthermore, we have started to investigate ultra-low resistance through-wafer micro-vias to bring the electrical contact out to the back of a wafer.

C07. Detailed characterization of Mo/Au TES microcalorimeters

Mark A. Lindeman, Regis P. Brekosky, Enectali Figueroa-Feliciano, Fred M. Finkbeiner, Mary Li, Caroline K. Stahle, Carl M. Stahle, Nilesh Tralshawala (NASA GSFC)

We are optimizing Mo/Au transition-edge-sensor (TES) calorimeters to meet the specifications of NASA's Constellation-X mission. Our calorimeters have already demonstrated very good energy resolution of X rays (2.4 eV at 1.5 keV). We wish to further improve the energy resolution by reducing excess noise in the calorimeters. Development of a detailed model and understanding of the noise is instrumental to reaching this goal. Towards that end, we employ a linear model that describes the response of a calorimeter to signal and various sources of noise. The model is based on detailed measurements of the parameters that affect the calorimeter's performance, such as current-voltage characteristics of the TES, thermal conductance of our silicon-nitride membranes, and inductance in the electronic circuit used to bias the TES. We determine the sharpness of the superconducting phase transition by fitting the model to the measured responsivity of the calorimeter. The model relates sources of noise, such as phonon noise, Johnson noise, and hypothetical sources of excess noise, to measurements of noise in the TES. Based on this analysis, we find that the excess noise is consistent with frequency-independent voltage noise in the TES.

C08. Arraying Compact Pixels of Transition-edge Microcalorimeters for Imaging X-ray Spectroscopy

C. K. Stahle, M. A. Lindeman, E. Figueroa-Feliciano, M. J. Li, N. Tralshawala, F. M. Finkbeiner, R. P. Brekosky, J. A. Chervenak
NASA Goddard Space Flight Center, Greenbelt, MD 20771 USA

We are developing superconducting transition-edge sensor (TES) based x-ray microcalorimeters for astronomical x-ray spectroscopy. We have obtained very high energy resolution (2.4 eV at 1.5 keV and 3.7 eV at 3.3 keV) in large, isolated TES pixels using Mo/Au proximity-effect bilayers on silicon nitride membranes several mm wide. In order to be truly suitable for use behind an x-ray telescope, however, such devices need to be arrayed with a pixel size and focal-plane coverage matched to the telescope focal length and spatial resolution. For the Constellation-X mission, this requires fitting the TES, its thermal link, and contact wiring into a 0.25 mm square, a far more compact geometry than has previously been investigated. We have demonstrated that the weak link can be restricted to a narrow (~10 micron) perimeter of membrane around the TES and still provide a thermal conductance in the acceptable range. Varying the size and placement of slits in that nitride perimeter, we can tune that value. We are also investigating the impact of restricting the sensor area to a 0.15 mm square. We will present the results of these characterizations and the spectral performance of individual devices.

C09. Fabrication of TES X-Ray Detector Arrays

Deiker, S., Bergren, N., Hilton, G., Irwin, K., Nam, S.W., Rudman, R., Wollman, D., NIST-Boulder

Transition Edge Sensor (TES) microcalorimeters have demonstrated record-breaking energy resolution over a large range of energies (2 eV at 1.5 keV, 4.5 eV at 6 keV), and because of their low impedance couple well to SQUID amplifiers. These factors make them ideal candidates for the large format detector arrays needed in both imaging x-ray astronomy missions and high count-rate materials microanalysis applications. NIST is fabricating multiple pixel arrays of transition edge sensor (TES) microcalorimeter detectors with these uses in mind. With the eventual goal of large, close-packed arrays, we have begun with small (2x2) arrays to investigate the fabrication and operation challenges involved. Our latest fabrication techniques and results will be presented.

C10. Development of high resolution X-ray detectors using tungsten phase transition thermometers (SPT)

G. Angloher¹, A. Bento¹, H. Kraus¹, F. Pröbst², W. Seidel^{2 1}University of Oxford, Department of Physics, Keble Road, Oxford OX1 3RH, UK ²Max-Planck-Institut fuer Physik, F�hringer Ring 6, 80805 M�nchen, Germany

The next generation of X-ray detectors in astrophysics is required to exhibit high resolving power over >mm2 detector areas. This can be achieved with tungsten as thermometer material as its bulk transition temperature is as low as 15 mK. We investigate detector designs where superconducting and normal conducting absorbers are read out by tungsten-SPTs. In addition, we deposit tungsten films on a range of substrates and by varying the growth conditions we can choose the transition temperature of the W-SPTs between 15 mK and 65 mK. This may be of interest for applications where an operating temperature as low as 15mK is impractical. Sharp phase transitions are obtained with tungsten grown on epitactic Al2O3, and also on amorphous buffer layers of Si3N4, SiO2 and Al2O3. This allows testing of various detector geometries and absorber coupling schemes (use of membranes, electron or phonon coupling, etc). So far, an energy resolution of Delta E = 60 eV (E = 5.9 keV) is achieved for a lead absorber of 1 mm diameter and 6 microns thickness.

C11. Status of X-ray Microcalorimeter Development at ISAS

R. Fujimoto^a, K. Mitsuda^a, N. Iyomoto^a, M. D. Audley^a, T. Miyazaki^a, T. Oshima^a, M. Yamazaki^a, K. Futamoto^a, Y. Takei^a, Y. Ishisaki^b, T. Kagei^b, T. Hiroike^b, T. Ohashi^b, N. Y. Yamasaki^b, A. Kushino^b, H. Kudo^c, H. Sato^c, T. Nakamura^c, E. Goto^c, S. Shoji^c, T. Homma^c, T. Osaka^c, Y. Kuroda^d, M. Onishi^d, M. Goto^d
aInstitute of Space and Astronautical Science, Sagamihara, Japan
^bTokyo Metropolitan University, Hachioji, Japan
^cWaseda University, Tokyo, Japan
^dMitsubishi Heavy Industries, Nagoya, Japan

At ISAS/TMU/Waseda, we are developing a superconducting phase-transition microcalorimeter array for the next Japanese X-ray astronomy satellite. Our goal is to achieve an energy resolution of ~ 3~eV, and imaging capability with a large covering area, using ~ 30× 30 pixels. In our design, a thin-film Ti-Au bilayer is fabricated on a thermally isolated silicon pixel, and an absorber with a large covering factor is attached on it. The readouts of a large-format array are multiplexed either in the time domain or in the frequency domain. So far, we glued a tin film as an absorber, operated at 100--200~mK, and achieved the energy resolution of 100~eV (FWHM) at 5.9~keV. We are now working on a calorimeter with a new design, where an absorber is electrodeposited on the silicon pixel. As readouts of our calorimeter system, a SQUID array amplifier and a 4-input SQUID array amplifier are used. Both achieve low readout noise. The latter is a device that has recently been developed for multiplexing calorimeter signals. In this presentation, we summarize the outline of our development, and report the performance of our new calorimeter system. The details of the fabrication processes, and the multiplexing technique in the frequency domain are presented in separate papers.

C12. Fabrication of an x-ray microcalorimeter with an electrodeposited x-ray microabsorber

H. Kudo^a, H. Sato^a, T. Nakamura^a, E. Goto^a, S. Shoji^a, T. Homma^a, T. Osaka^a, K. Mitsuda^b, R. Fujimoto^b, N. Iyomoto^b, M. D. Audley^b, T. Miyazaki^b, T. Oshima^b, M. Yamazaki^b, K. Futamoto^b, Y. Takei^b, Y. Ishisaki^c, T. Kagei^c, T. Hiroike^c, T. Ohashi^c, N. Y. Yamasaki^c, A. Kushino^c Y. Kuroda^d, M. Onishi^d, M. Goto^d
aWaseda University, Tokyo Japan
^bInstitute of Space and Astronautical Science, Sagamihara, Japan
^cTokyo Metropolitan University, Hachioji, Japan
^dMitsubishi Heavy Industries, Nagoya, Japan

A superconducting phase-transition microcalorimeter is a promising detector for high resolution X-ray spectroscopy. For the astronomical use, a large format array (~1000 pixels) is now strongly requested. We are studying micromachining process to achieve such a large format array. In our process, the absorber is fabricated as a so-called `mushroom' shape by electrodeposition, to attain a high covering fraction. In our design, the calorimeter pixel of 1 mm x 1 mm is supported by a fine long silicon beam. First, a 200 um thick silicon wafer is processed from the surface by reactive ion etching (RIE). After patterning a temperature sensor (TES) on the silicon pixel, a sacrificial photoresist structure is fabricated. It consists of two layers, a negative photoresist as a mold of the supporting point of the absorber, and a thick positive photoresist as a mold of the absorber. Then, the tin absorber is fabricated by electrodeposition. To obtain good purity and uniformity of tin, the optimized condition was chosen using the SnSO4 and H2SO4 bath with additives of cresol sulfonic acid and polyethylenglycol (PEG). The absorber is then polished by CMP (chemical mechanical polishing), and finally the wafer is etched by RIE from the back side to produce the pixel and the beam. We will present our fabrication process and the results in detail.

C13. First Results from a Position-Sensing TES Imaging Calorimeter

E. Figueroa-Feliciano^1,2, J. Chervenak¹ F. M. Finkbeiner^1,3, M. Li^1,4, M. A. Lindeman^1,5, C. K. Stahle¹, C. M. Stahle¹ , N. Tralshawala^1,4
1NASA Goddard Space Flight Center, Greenbelt, MD 20771 USA
²Stanford University, Department of Physics, Stanford, CA 94305 USA
³University of Maryland, Department of Astronomy, College Park, MD 20742 USA
⁴Raytheon ITSS, Lanham, MD 20706 USA
⁵NAS/NRC Resident Research Associate

We are developing a high energy resolution, imaging spectrometer called a Position Sensing Transition Edge Sensor (PoST). A PoST is a calorimeter consisting of two Transition Edge Sensors (TESs) on the ends of a long absorber to do one dimensional imaging spectroscopy. Comparing rise time and energy information, the position of the event in the PoST is determined. Energy is inferred from the sum of the two pulses. We have fabricated 7, 15, and 32 pixel PoSTs using our Mo-Au TESs on a silicon nitride suppot structure. We show our first results from this promising technology, where we have successfully read out 7 pixels with two TESs and achieved a resolution of 30 eV at 1.5 keV. We also discuss the theoretical limits for the resolution of these devices.

C14. Integrated Focal Plane Arrays for Millimeter-wave Astrophysics

J. Bock^a, A. Goldin^a, C. Hunt^b, A. Lange^b, H. LeDuc^a, J. Zmuidzinas^b,
^aJet Propulsion Laboratory, Pasadena, CA
^bCalifornia Institute of Technology, Pasadena, CA

We are developing focal plane arrays of bolometric detectors for sub-millimeter and millimeter-wave astrophysics. We propose a flexible array architecture using arrays of slot antennae coupled via low-loss superconducting Nb tranmission line to microstrip filters and antenna-coupled bolometers. By combining imaging and filtering functions with tranmission line, we are able to realize unique architectures such as a multi-band polarimeter and a planar, dispersive spectrometer. Micro-strip bolometers have significantly smaller active volume than standard detectors with extended absorbers, and can realize higher sensitivity and speed of response. The integrated array has natural immunity to stray infrared radiation or spectral leaks, and minimizes the suspended mass operating at 0.1 - 0.3 K. Calculations and performance of the slot antennae and filtering is presented in a parallel poster presentation. We discuss development of an instrument that realizes the planar spectrometer concept in waveguide. We also describe future space-borne spectroscopy and polarimetery applications.

C15. Focal Plane Arrays of Voltage-Biased Superconducting Bolometers

Michael J. Myers^a, John Clarke^a, J. M. Gildemeister^a, Adrian T. Lee^a, P. L. Richards^a, Dan Schwan^a, J. T. Skidmore^a, Helmuth Spieler^b, Jongsoo Yoon^a
aDepartment of Physics, University of California at Berkeley
^bPhysics Division, Lawrence Berkeley National Laboratory

The 200µm to 3mm wavelength range has great astronomical and cosmological significance. Science goals include characterization of the cosmic microwave background, measurement of the Sunyaev-Zel'dovich effect in galaxy clusters, and observations of forming galaxies. Cryogenic bolometers are the most sensitive broadband detectors in this frequency range and the development of large arrays will be critical for future science progress. We report on two types of voltage-biased superconducting bolometer arrays being developed by our group. We have developed an absorber coupled bolometer suitable for large-format filled arrays. This detector is produced on a silicon nitride suspension using standard planar lithography. We have produced a 1024 element array of fully released and suspended 1.5mm x 1.5mm nitride micro-meshes with a filling factor of 88%. We also built and tested a voltage-biased superconducting bolometer on a similar nitride micromesh as a prototype for one array element. Our group is also developing planar antenna coupled bolometers. Antenna coupled detectors do not require horns and the band defining filters can be fabricated in superconducting microstrip on the same substrate as the bolometer. This highly integrated design should facilitate scaling to a very high pixel count. We will report on our current progress in the fabrication and testing of these devices.

C16. SAMBA: Superconducting Antenna-coupled, Multi-frequency, Bolometric Array.

Alexey Goldin^a,c, James J. Bock^a, Cyntia Hunt^a Andrew E. Lang^b, Henry Leduc^a Tasos Vayonakis^b Jonas Zmuidzinas^b
a JPL
^b California Institute of Technology ^c NRC fellow

We present a design for multipixel, multiband submillimeter instrument: SAMBA (Superconducting Antenna-coupled, Multi-frequency, Bolometric Array). Unlike traditional designs like SCUBA and BOLOCAM, SAMBA uses slot antenna coupled bolometers and microstrip filters. The concept allows for a much more compact, multiband imager compared to a comparable feedhorn-coupled bolometric system. The concept has some advantages compared to horn coupled bolometric systems. SAMBA incorporates array of slot antennas, superconducting transmission lines, wide band multiplexor and superconducting transition edge bolometers. The transition-edge film measures the millimeter-wave power deposited in the resistor that terminates the tranmission line. By combining imaging and filtering functions with tranmission line, we are able to realize unique architectures such as a multi-band polarimeter, which are discussed in parallel oral presentation. We present a numerical analysis of the wide-band microstrip filters and the slot antenna array.

C17. Transition Edge Sensor Bolometers for Submillimeter Astronomy

J. A. Chervenak^a, D. Benford^a, T. Chen^a, B. DiCamillo^a, R. Shafer^a, J. Staguhn^a, S. H. Moseley^a, K. D. Irwin^b, S. W. Nam^b, N. Bergren^b, John M. Martinis^b E. N. Grossman^b, C. D.Reintsema^b, T. E. Harvey^b,
^aNASA Goddard Space Flight Center, Greenbelt, MD
^bNIST, Boulder, CO

We describe our effort to fabricate and test transition edge sensor bolometers (TESs) that are suitable for sub-millimeter and millimeter astronomy. In particular, we are exploring the integration of a robust low T_c TES bilayer process with low thermal conductance structures for both filled focal plane arrays and feedhorn-coupled arrays. A comparison of our fabrication methods for Mo/Au and Mo/Cu bilayers will be presented.

C18. Superconductive Hot-Electron Direct Detectors for Submillimeter Space Telescopes

Boris S. Karasik, Bertrand Delaet, and William R. McGrath Jet Propulsion Laboratory, California Institute of Technology Michael E. Gershenson Rutgers University Andrew V. Sergeev Wayne State University

We develop a hot-electron direct detector (HEDD) capable of counting single millimeter-wave photons. Such a detector will meet the needs of future space far-infrared missions and can be used for background-limited detector arrays on future space telescopes. The HEDD is based on a microbridge (1-�m-size) transition edge sensor fabricated from an ultra-thin film of a superconductor with T_c = 0.1-0.3 K. A very strong temperature dependence of the electron-phonon coupling in superconductors with small electron-mean-free-path allows to adjust the electron-phonon scattering time to the desired value. The Nb contacts block the thermal diffusion of hot carriers out of the bridge because of the Andreev reflection. The measurements of the electron-phonon relaxation time in hafnium and titanium have demonstrated that the bolometer response time of ~ 1 ms at T = 0.1 K is possible without using any high-thermal-resistance suspension of the detector. For a device with lateral dimensions 1x1 �m², this would result in a NEP = 10^-20 W/sqrt(Hz). The frequency response of the prototype antenna-coupled devices have been measured to be flat over the range 250-1000 GHz. First test results of a multiplexing scheme based on the Hadamard Transform encoding and using a single SQUID amplifier will be discussed.

C19. Development of high-resolution gamma-ray spectrometers

A. Loshak^a, D.T. Chow^a,b, M.F. Cunningham^a,b, O. Drury^a,b, M.L. van den Berg^a, J.N.Ullom^a, T.W. Barbee Jr.^a, M. Frank^a, S.E. Labov^a
aLawrence Livermore National Laboratory, Livermore, CA, 94550
^bDepartment of Applied Science, University of California, Davis

We are developing arrays of gamma-ray microcalorimeters based on a Mo/Cu multilayer superconducting transition edge sensor (TES) thermally coupled to a bulk absorber. Our goal is to build a high resolution and good quantum efficiency gamma-ray spectrometer in the energy range of 60 keV-200 keV. This energy range is particularly interesting for nuclear weapons materials inspection and measurement of nuclear reaction cross-sections for U and Pu. We need to achieve better than DeltaE/E<=10^-3 relative energy resolution which is more than a factor of 4 better than theoretical resolution of modern Ge detectors. Previously, using electrothermal feedback, we have achieved energy resolution of 70 eV FWHM for 60 keV gamma-rays using 1 mm² × 0.25 mm Sn absorber which had 70% absorption efficiency at 60 keV. In order to linearize the response of the detector and extend the detection energy range we have implemented an external active feedback bias circuit. Here we describe recent results from single-pixel detectors and discuss future plans. This work was performed under the auspices of the U.S. Department of Energy by the University of California, Lawrence Livermore National Laboratory under contract No. W-7405-ENG-48.

C20. Mo/Au microcalorimeters

J. E. Olsen ^a, E. C. Kirk ^a, Ph. Lerch ^a, A. Zehnder ^a, H. R. Ott ^b, M. Huber ^c, G. C. Hilton ^c, J. M. Martinis ^c
aPaul Scherrer Institute CH - 5232 Villigen, Switzerland
^b ETH - Hönggerberg, CH - 8093 Zürich, Switzerland
^b NIST, 80303 Boulder, CO, USA

We fabricated Mo/Au bilayers on silicon nitride membranes as transition edge sensors (TES). The critical temperature of the bilayers is adjusted, by proximity effect, to be around 100 mK. TES devices are mounted on the cold stage of a dilution refrigerator and are voltage biased with the use of thin film Au shunts. The input coil of a one stage series array of {em dc} SQUIDs, kept at 2.2 K, is connected in series with the device. Results on 6 keV irradiation experiments will be presented.

C21. Mo-Cu bilayers as Transition Edge Sensors for X-ray astrophysics

P. Tan, L. D. Cooley, M. Galeazzi, D. Liu, D. McCammon, K. L. Nelms, and W. T. Sanders
University of Wisconsin, Madison, WI 53706 USA

We are developing Transition Edge Sensor (TES) thermometers for X-ray astrophysics applications. We are using Mo-Cu bilayers to obtain a working temperature around 60 mK. It is known that the shape of the device edges affects the transition properties. We have developed a process for undercutting the top Cu layer that appears to adequately control the edge effect. Since we plan to fly these detectors in a magnetic refrigerator, we studied the effect of small magnetic fields on the bilayer transition. We are also studying the effect of the bias power density on the sensitivity and excess noise of the TES.

C22. Development of the X-ray microcalorimeter with a superconductive Ir layer

M. Ohno*a, Y. Noguchi*a, D. Fukuda*a, H. Takahashi*b, M. Nakazawa*a, M. Ataka*c, M. Ukibe,*d, F. Hirayama*d, M. Ohkubo*d *a Department of Quantum Eng. and System Sci., The University of Tokyo,Tokyo *b RACE, The University of Tokyo,Tokyo *c Institute of Industrial Science, The University of Tokyo,Tokyo *d National Institute of Advanced Industrial Science and Technology, Ibaraki

We are developing an Ir TES X-ray microcalorimeter. Ir has good properties such as chemical stability and low Tc of 112mK, therefore by using Ir TES we can expect good quality of high energy resolution X-ray detector. First we fabricated Ir film of 0.5mmX0.5mm using a dc magnetron sputtering with heating and then the film is etched to form a TES by a reactive ion etching method. The weak thermal link to the heat sink is created by fabricating the Ir TES on a silicon nitride membrane. Our TES sensor is voltage biased with an on-chip shunt resistor, and current is measured by a dc-SQUID array. The measured energy resolution was 194eV for 5.9keV X-ray but this value is extremely worse than a theoretical resolution. We think the curent limitation in energy resolutuion of our Ir TES arises from non-uniformity in thermalization process. Thermalization process in Ir is relatively slow as compared with other materials therefore the localization in electrothermal feed-back and resulting fluctuation in temperature might propagate through the sensor. To improve the thermalization property of TES device, now we are trying to employ a new geometry that has shorter length of electrodes and some thin slits that separate the thermalization area of the Ir TES device.

C23. TES x-ray microcalorimeters with single element superconductors

D. Fukuda^a, Y. Noguchi^a, H. Takahashi^b, M. Ohno^a M. Ataka^c, M. Ukibe^d, F. Hirayama^d, M. Ohkubo^d, M. Nakazawa^a.
^aDep. of Quantum Eng. and Systems Sci., The University of Tokyo, Tokyo.
^bRACE, The University of Tokyo, Tokyo.
^cInstitute of Industrial Science, The University of Tokyo, Tokyo.
^dNational Institute of Advanced Industrial Science and Technology, Ibaraki.

In chemical state microanalysis with PIXE spectrometry, it is necessary to employ detectors with high-energy resolution, large detection efficiency and high counting rate. We are now developing TES x-ray microcalorimeters using single element superconductor for this application. One of the adavantages of the single superconductor TES is its very stable and reproducible property because of no inter-diffusion process and simple structure compared to the bilayered TES. Tungsten and Iridium are promising superconducting elements because they have very low T_c that is necessary to achieve a high-energy resolution detector. Prototype TES microcalorimeters were fabricated with these films and the x-ray responses are measured. Observed signals with a W-TES are saturated for 5.9keV x-ray energy because of the small heat capacity of our device, however, we have obtained the energy spectrum by integrating the current changes of the signal. The currently achieved energy resolution with this method is 37eV (FWHM) @ 5.9keV. We have observed excess noise from the x-ray measurement with the TES of a large detection area. We think the noise comes from non-uniform response depending on the position of the TES where x-rays are absorbed because of the slow thermalization property. We will present these results and discuss the characteristics of the single superconducting TES microcalorimeters.

C24. Development of a distributed read-out imaging TES X-ray microcalorimeter

S. Trowell A. D. Holland G.W.F. Fraser University of Leicester, University Road, Leicester, LE1 7RH, UK D. Goldie Oxford Instruments Scientific Research Division, Newton House, Cambridge Business Park, Cowley Road, Cambridge, CB4 4WZ, UK

The requirement for high resolution imaging X-ray spectroscopy for astrophysics is well documented. We report on the development of a linear absorber detector for one-dimensional imaging spectroscopy, read-out by two Transition Edge Sensors (TESs). The current design, details of which are discussed, incorporates a composite gold/bismuth absorber to maximise the thermal conductivity for a given heat capacity. The TESs used are based on a single layer of iridium and have been developed under our single pixel microcalorimeter programme. These demonstrate stable and controllable superconducting-to-normal transitions in the region of 100 mK. Results from Monte Carlo simulations are presented indicating that the device configuration is capable of detecting photon positions to better than 200 microns, thereby meeting the spatial resolution specification for missions such as XEUS of ~250 microns.

D01. Photon counting detectors for the far infrared

O. Astafiev, V. Antonov, T. Kutsuwa and S. Komiyama
Department of Basic Science, University of Tokyo
Japan Science and Technology Corporation (JST)

We study quantum dots (QDs) as photon detectors of the far-infrared (FIR). The QDs are formed by laterally confining two-dimensional electron gas (2DEG) by negatively biased metal gates, which are deposited on the top of GaAs/Al_xGa_1-xAs heterstructures with the high mobility 2DEG. Transport through the QDs is measured in the single electron transistor (SET) regime at temperature of 70 mK. Two mechanisms of the photo-excitations are explored: (i) inter Landau level photo-excitation in high magnetic fields (B = 3.4 - 4.15 T) and (ii) collective plasma excitations at a characteristic frequency of the parabolic bare confinement potential. The single photon counting is achieved by the QDs in high magnetic fields at nearly cyclotron frequencies in a wavelength range 0.17 mm < lambda < 0.21 mm. Oppositely, in the second mechanism, double-QDs are studied in the absence of magnetic fields. With these double-QDs, the high sensitive detection approaching to the single photon detection level is realized at wavelength of 0.6 mm.

D02. Radio-Frequency Single-Electron Transistors Readouts for UV thru Submillimeter Single-Photon Counters

R.J. Schoelkopf^a, K. Segall^a, J.D. Teufel^a, and K. Aidala^a T.R. Stevenson^b, C.M. Stahle^b, and S.H. Moseley^b, A. Aasime^c, P. Wahlgren^c, and P. Delsing^c.
^aYale University, New Haven, CT
^bNASA/Goddard Space Flight Center, Greenbelt, MD
^cChalmers University of Technology, Goteborg, Sweden

We describe progress on the use of Radio-Frequency Single-Electron Transistors (RF-SETs) as high-performance readout amplifiers for cryogenic single-photon detectors based on superconducting tunnel junctions (STJs). The high charge sensitivity and large bandwidth of the RF-SET, along with low power dissipation, low capacitance and on-chip integrability make it a good candidate for high impedance detectors. We recently demonstrated an RF-SET with a voltage noise of 25 nV/rt(Hz), configured as a transimpedance amplifier in a charge-locked loop feedback configuration with a bandwidth of about 100 kHz. The initial performance is sufficient to readout single-photon pulses from UV/optical STJ detectors at their intrinsic energy sensitivities. A proof-of-concept demonstration of wavelength-division multiplexing of the RF-SET amplifiers has also been performed. Finally, we will describe preliminary results on very high-sensitivity direct detectors (SQPCs) for the submillimeter waveband. This device, essentially an antenna-coupled STJ, offers the possibility of true single-photon counting for milli-eV photons. Dark current measurements of prototype devices suggest that noise-equivalent powers of 10^-18 Watts/rt(Hz) or better could be attained at 250 mK. Predicted sensitivities for an optimized device could approach {10^-20} Watts/rt(Hz), and would be background-limited for even the most demanding applications in space astrophysics.

D03. Superconducting electronic device with transistor-like properties

N.E. Booth^a, L. Parlato^b, G.P. Pepe^c, G. Ammendola^c, E. Esposito^d, G. Peluso^c, A. Barone^c and R. Scaldaferri^c
aDepartment of Physics, University of Oxford, Keble Road, Oxford OX1 3RH, UK
^bDip. Ingegneria dell'Informazione, Via Roma 16, Aversa (CE), Italy
^cDip. Scienze Fisiche, I.N.F.M., Universita di Napoli Federico II, I-80125 Naples, Italy
^dIstituto di Cibernetica del C.N.R., Via Toiano 6, Arco Felice (NA), Naples, Italy

Particle and photon detectors based both on superconducting tunnel junctions and on superconducting transition edge sensors are becoming widely used. A superconducting transistor with large current gain and bandwidth that can operate in close proximity to the detectors could open up new possibilities. Recently the Naples-Oxford collaboration has fabricated and tested a three-terminal superconducting device with transistor-like properties at an operating temperature of 4.2 K. It consists of a stacked double tunnel junction structure where the intermediate film is a bilayer of superconducting Nb and an Al quasiparticle trap which can work either in the superconducting or in the normal metal state. Current amplification factors of more than 50 are observed at 4.2 K when the Al is in the normal state. The device also has power gain of more than 1000 and a high degree of unidirectionality. The results can be explained on the basis of the recently proposed QUAsiparticle TRApping TRANsistor, which should have wide applications in detection systems operating at low temperatures. Results of device design studies will also be presented.

D04. Electron Multiplier with a Cascade of NIS Tunnel Junctions

V.S. Shpinel, V.A. Andrianov
Institute of Nuclear Physics, Lomonosov Moscow State University, 119899 Moscow, Russia

It is shown that in a cryogenic detector with hot electron microcalorimeter a signal can be recorded and amplified by a device which consists of a series of normal metal-insulator-superconductor (NIS) tunnel junctions. In the sequence S₀N₁IS₁N₂IS₂�N_nIS_nN_n+1, the normal metal N acts as a microcalorimeter and simultaneously plays a role of tunnel junction normal electrode. Absorption of the energy E in an absorber S₀ gives rise to production of hot electrons in the microcalorimeter SN₁S and to increase of the tunnel current through the first and next NIS junctions. Each transfer of an electron through the tunnel barrier to the neighboring microcalorimeter brings the energy ~Delta and raises its electron temperature. Thus the proposed structure has the properties of electron multiplier. The temperature rise in each SN microcalorimeter is a solution of the system of differential equations. Model calculations of a signal of this device were carried out for a wide range of physical parameters. Signal amplification by means of two NIS junctions in series was mentioned in [1]. The same principle of amplification was put forward and tested in [2]. 1. V.S. Shpinel. Izvestiya RAN, ser. Fiz. 64 (2000) 2216. 2. N.E. Booth, P.A. Fisher, M. Nahum, J.N. Ullom. Nuclear Instr. Meth. Phys. Res. A 444 (2000) 33.

D05. Dark Currents of Prototype Submillimeter-Wave Photon Counting Detectors

T.R. Stevenson^a, M. Li^a, R.J. Schoelkopf^b, C.M. Stahle^a, and J. Teufel^b
aNASA Goddard Space Flight Center, Code 553, Greenbelt, MD 20771 USA
^bDepartment of Applied Physics, Yale University, PO Box 208284, New Haven, CT 06520-8284 USA

A combination of single electron and superconducting tunnel junction devices with antenna coupling has been proposed as a fast submillimeter-wave detector with photon counting sensitivity. We have fabricated series of detector elements with a range of design parameters. We describe the corresponding measured dependence of dark currents on temperature and bias voltage.

D06. Multiplexing of Radio-Frequency Single Electron Transistors

T.R. Stevenson^a, K. Aidala^c, F. Pellerano^b, R.J. Schoelkopf^c, and C.M. Stahle^a
aNASA Goddard Space Flight Center, Code 553, Greenbelt, MD 20771 USA
^bNASA Goddard Space Flight Center, Code 555, Greenbelt, MD 20771 USA
^cDepartment of Applied Physics, Yale University, PO Box 208284, New Haven, CT 06520-8284 USA

We describe application of wavelength division multiplexing to combine the outputs of Radio-Frequency Single-Electron Transistors (RF-SETs). Using multiple rf carrier frequencies permits simultaneous readout of many amplifiers with a common electrical connection. Multiplexed RF-SET amplifiers could provide fast and sensitive on-chip readout of arrays of high impedance cryogenic photodetectors.

E01. SQUID-based readout schemes for microcalorimeter arrays

M. Kiviranta VTT Automation / Measurement Technology Espoo FINLAND

Microcalorimeter arrays with superconducting phase-transition thermometers are an attractive alternative for construction of imaging X-ray spectrometers. The low source impedance and low operating temperatures make a dc SQUID a good candidate for preamplifier. In large arrays, the circuit complexity as well as heat leakage through wiring make some sort of a multiplexing scheme desirable. Some circuit topologies for time-domain and frequency-domain multiplexing as well as correlation-based schemes are compared. One particular frequency-domain multiplexing circuit, being developed by VTT/Finland and SRON/Netherlands for the XEUS space mission, is reviewed in more detail. The design issues related with scalability, dynamic range, bandwidth and noise folding are discussed.

E02. SQUID Multiplexers for Large Format Arrays of Transition-Edge Sensors

K.D. Irwin^a, L.R. Vale^a, S.W. Nam^a, G.C. Hilton^a, M.E. Huber^b
aNational Institute of Standards and Technology, Boulder CO
^bUniversity of Colorado, Denver, CO

SQUID multiplexers make it possible to build arrays of thousands of low-temperature bolometers and microcalorimeters based on superconducting transition-edge sensors (TES) with a manageable number of readout channels. We discuss the constraints on the performance of the SQUID multiplexer for both x-ray and infrared applications. Our first-generation, 8-channel SQUID multiplexer (MUX) is now being deployed in one-dimensional TES arrays for submillimeter astronomy. We present our second-generation SQUID multiplexer, based on a new architecture which significantly reduces the power dissipation at the first stage, allowing thousands of SQUIDs to be operated at the base temperature of the cryostat. This performance allows the integration of the multiplexer into the focal plane, which is necessary for truly large-format bolometer arrays of many thousands of pixels. This SQUID MUX design will be used in the SCUBA-2 instrument at the James Clerk Maxwell Telescope (JCMT), which will have more than 10,000 TES submillimeter bolometer pixels. It is also a leading candidate for the multiplexer for the Constellation-X x-ray microcalorimeter instrument.

E03. Single SQUID multiplexer for arrays of voltage-biased superconducting bolometers

Jongsoo Yoon, John Clarke, Adrian T. Lee, M. J. Myers, P. L. Richards, H. G. Spieler^a
University of California, Berkeley, CA
^aLawrence Berkeley National Laboratory, Berkeley, CA

We have demonstrated a frequency-domain superconducting quantum interference device (SQUID) multiplexer for a row of low-temperature sensors [1]. Each sensor is biased with an alternating current (AC) at a distinct frequency significantly above the roll-off freqnency o