We demonstrate a thin-film, solid-state refrigerator based on the removal of hot electrons from a metal by quantum-mechanical tunneling. We have reduced the electronic temperature in a metal film from 260 to ∼130 mK. The base temperature of the device is predicted to increase to near 140 mK under a power load of 10 pW. Both the cooling power and temperature reduction of the refrigerator are well matched to practical applications. This refrigerator will make high-performance cryogenic photon sensors more accessible to the astronomical and analytical communities.

We have built a two-stage adiabatic demagnetization refrigerator (ADR) to operate cryogenic high-resolution X-ray detectors in synchrotron-based fluorescence applications. The detector is held at the end of a 40 cm cold finger that extends into a UHV sample chamber. The ADR attains a base temperature below 100 mK with about 20 h hold time below 400 mK, and does not require pumping on the liquid He bath. We will discuss cryostat design and performance. #

Superconducting tunnel junctions can be used as high-resolution particle or photon energy spectrometers. A photon absorbed in a superconductor breaks Cooper pairs into quasiparticles. These quasiparticles tunnel through the junction barrier and are detected as a pulse of excess current. Many junction designs allow the quasiparticles to tunnel more than once, an exponentially mixed Poisson process. However, multiple tunneling increases the fluctuation in the measured charge. We calculate the significance of these fluctuations algebraically as a function of time during the current pulse. We also calculate the finite integration window that minimizes the contribution of this noise. In addition, we calculate the effects of a low-pass amplifier and a Gaussian-shaping amplifier on the tunneling noise. With certain filtering time constants, the tunneling noise can be reduced while still providing some gain.

Nb-based superconducting tunnel junctions are being developed as high energy resolution X-ray detectors. Unfortunately, loss of excess quasiparticles at the edges, combined with lateral diffusion, results in an inhomogeneous response. To study this degradation of energy resolution, we manufactured detectors with a Ta trap in the top or bottom electrode away from the tunneling barrier. Excess quasiparticles in this so-called killed electrode will be trapped effectively and thus removed from the tunneling process. The X-ray spectra of the active electrode can be fitted with a model based on classical diffusion of quasiparticles. On junctions with a killed bottom electrode also Low Temperature Scanning Electron Microscopy (LTSEM) measurements have been performed. The X-ray spectra and the LTSEM scans are consistent with each other and with the model. The energy resolution of the junctions presented here is limited by loss of quasiparticles at the edges.

In x-ray and gamma-ray spectroscopy, it is desirable to have detectors with high energy resolution and high absorption efficiency. At LLNL, we have developed superconducting tunnel junction-based single photon x-ray detectors with thin film absorbers that have achieved these goals for photon energies up to 1 keV. However, for energies above 1 keV, the absorption efficiency of these thin-film detectors decreases drasti:a fly. We are developing the use of high-purity superconducting bulk materiats as microcatorimetcr absorbers for high-energy x-rays and gamma rays. The increase in absorber temperature due to incident photons is sensed by a superconducting tramition-edge sensor (TES) composed of a MoICU multilayer lti" film. Filmr of MO and Cu are mutuafty insoluble and therefore very stable and can be annealed. The multilayer stmcture allows scaling in thickness to optimize heat capacity and normal state resistance. We measured an energy resolution of 70 eV for 60 keV incident gamma-rays with a 1 x I x 0.25 mm3 Sn absorber. We present x-ray and ganum-my results from this detector design with a Sn absorber. We also propose tie use of an active negative feedback vottage bias to improve tbe per fonmmce of our detector and show prelimimry res"ks.