M.L. van den Berg

High-Resolution Gamma-Ray Spectrometers using Bulk Absorbers Coupled to Mo/Cu Multilayer Superconducting Transition-Edge Sensors

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 drastically. We are developing the use of high-purity superconducting bulk materials as microcalorimeter absorbers for high-energy x-rays and gamma rays. The increase in absorber temperature due to incident photons is sensed by a superconducting transition- edge sensor (TES) composed of a Mo/Cu multilayer thin film. Films of Mo and Cu are mutually insoluble and therefore very stable and can be annealed. The multilayer structure 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 1 X 0.25 mm3 Sn absorber. We present x-ray and gamma-ray results from this detector design with an Sn absorber. We also propose the use of an active negative feedback voltage bias to improve the performance of our detector and show preliminary results.

Inhomogeneous response of superconducting tunnel junctions with a killed electrode for X-ray spectroscopy

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.

Multiple-tunneling noise in superconducting tunnel junctions from partial current integration

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.

High quality superconducting tunnel junctions on Nb and Ta single crystals for radiation detection

High quality Nb/Al-based tunnel junctions, fabricated with a superconducting interface onto thick single crystalx-ray absorbers of Nb and Ta are discussed. Current-voltage characteristics, recorded at 0.5 K, show a subgap current which is still dominated by thermally excited quasiparticles. The quality parameter Rsubgap/Rnormal reaches a value of several million, which is unequalled for nonepitaxially sputtered tunnel junctions. The fabrication process and some development steps, such as preparation of ultrasmooth crystal surfaces are described. Observations of x-ray photons absorbed in Nb and Tasingle crystals detected by the superconducting tunnel junctions are also presented.

Back-illuminated CCDs made by gas immersion laser doping

Abstract Back-illuminated CCDs with high quantum efficiency in the soft X-ray range have been developed by EEV in collaboration with the Space Research Organisation Netherlands. By using Gas Immersion Laser Doping (GILD) for producing the backside accumulation layer very shallow doping profiles can easily be achieved. Additionally the GILD process does not affect the silicon behind the p+ layer in contrast to the commonly used ion implantation process. This implies that only the electrons generated in, or reaching the very small accumulation layer will have a probability to recombine at the surface or in the accumulation layer itself. Therefore only a small fraction of the electron clouds produced by the absorbed soft X-rays will suffer charge loss, resulting in a high quantum efficiency. X-ray measurements of back-illuminated CCDs with doping profiles of 50 and 100 nm depth are presented and shown to be consistent with calculations based on minority carrier transport.

Gamma-ray spectrometers using superconducting transition edge sensors with external active feedback bias

We are developing X-ray and gamma-ray spectrometers with high absorption efficiency and high energy-resolution for X-ray and gamma-ray spectroscopy. They are microcalorimeters consisting of a bulk Sn absorber coupled to a Mo/Cu multilayer superconducting transition edge sensor (TES). We have operated these microcalorimeters with an external active feedback bias to linearize the detector response, improve the count rate performance, and extend the detection energy range. We measured an energy resolution of 120 eV FWHM for 60 keV incident gamma-rays with no degradation of resolution from active bias. We present X-ray and gamma-ray results and operation of this detector design in both bias modes.

Diffusion of quasiparticles in SIS tunnel junctions for X-ray spectroscopy

X-ray detectors based on Superconductor-Insulator-Superconductor (SIS) tunnel junctions have the potential of a very high energy resolution. We investigate, limitations on the attainable energy resolution due to the combined effect of lateral diffusion of the excess quasiparticles, generated after absorption of an X-ray photon, and loss of the quasiparticles at the edges. We present an exactly solvable model, which we apply to recent X-ray measurements on a set of tunnel junctions which only differ in size. This clearly shows the detrimental influence of loss at the edges. The model gives quantitative criteria which have to be met to reach the desired 10 eV energy resolution for 6 keV photons.

DC and AC biasing of a transition edge sensor microcalorimeter

We are developing AC-biased transition edge sensor (TES) microcalorimeters for use in large arrays with frequency-domain multiplexing. Using DC bias, we have achieved a resolution of 17 eV FWHM at 2.6 keV with a decay time of 90 μs and an effective detector diameter of 300 μm. We have successfully measured thermal pulses with a TES microcalorimeter operated with an AC bias. We present here preliminary results from a single pixel detector operated under DC and AC bias conditions.

A multichannel cryogenic detector system for synchrotron-based x-ray spectroscopy

Fluorescence-detected x-ray absorption spectroscopy probes the fine structure of electronic energy levels with sub-eV resolution by scanning a monochromatic synchrotron beam through the corresponding absorption edge and measuring the intensity of the resulting x-ray fluorescence. For dilute samples, grating spectrometers lack the detection efficiency and conventional Si(Li) or Ge detectors often lack the energy resolution to separate the weak fluorescence signal from strong nearby emission lines. We have built a high-resolution, high-efficiency cryogenic detector system for synchrotron-based soft x-ray spectroscopy. The sensor is a 3×3 array of 200 μm×200 μm superconducting Nb-Al-AlOx-Al-Nb tunnel junctions with an energy resolution of ≈15 eV below 1 keV and a total count rate capability of ≈100,000 counts/second. This sensor array is cooled to below 0.4 K by a two-stage adiabatic demagnetization refrigerator while held at the end of a 40-cm-long cold finger that can be inserted into a UHV sample chamber ...

A new idea for a solid-state microrefrigerator operating near 100 mK

We propose a new design for a solid-state microrefrigerator based on Normal-Insulator-Superconductor (NIS) tunnel junctions. These devices are a promising means of providing continuous refrigeration from 0.3 to 0.1 K without vibration or moving parts. Previously, the area and cooling power of NIS refrigerators have been limited by heating of the superconducting electrode. This problem can be overcome by using a superconducting single crystal as both the substrate and superconducting electrode of the NIS junction. In this paper, we briefly explain the benefits of our new design and describe experimental progress towards building such a device.