K. Segall

Experimental quasiparticle dynamics in a superconducting, imaging x-ray spectrometer

We present an experimental study of the time scales for various quasiparticle processes in a superconducting single photon spectrometer. Processes studied include quasiparticle recombination, diffusion, trapping, tunneling, and energy redistribution. Experiments were performed with a double junction, imaging x-ray detector whose charge output provides a measure of the photon energy. Time scales are extracted with a simple model and the values of several parameters, including the diffusion constant and recombination time, are found to differ from theoretical predictions. These results provide guidelines for performance analysis, device scaling, and future designs.

Single photon imaging X-ray spectrometers using low noise current preamplifiers with dc voltage bias

We have developed superconducting single-photon imaging X-ray detectors with an energy resolution of 54 eV at 6 keV and a spatial resolution of 1 /spl mu/m over an effective length of 40 /spl mu/m. They utilize a current-sensitive low-noise preamplifier with a dc voltage bias. It has a signal bandwidth of 300 kHz, current noise of i/sub n/=0.26 pA//spl radic/(Hz) and voltage noise of e/sub n/=0.5 nV//spl radic/(Hz) with an input capacitance of 200 pF under operating conditions. Injected pulses with a charge Q=3.7/spl middot/10/sup 6/ electrons have been measured with a standard deviation /spl sigma/Q=3400 electrons, corresponding to an electronic noise of 13 eV at 6 keV.

Noise mechanisms in superconducting tunnel-junction detectors

We present a theory and measurements of noise mechanisms in superconducting tunnel-junction detectors used as single-photon spectrometers. These mechanisms result from incomplete cooling of the excited quasiparticles in the tunnel-junction electrode. Due to the incomplete cooling, only a fraction of the initially created charge is collected by tunneling. Additional effects include reduced dynamic resistance, voltage dependence of the integrated charge, and increased statistical broadening of the signal. We demonstrate these noise mechanisms in our device, and show that they explain the measured energy resolution of 25 eV at 5.9 keV. We also suggest ways to reduce their contribution in future devices.

A high-performance cryogenic amplifier based on a radio-frequency single electron transistor

We demonstrate a high-performance cryogenic amplifier based on a radio-frequency single-electron- transistor (rf-SET). 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 a general-purpose cryogenic amplifier for high impedance sources. We measure a large-gate rf-SET with an open-loop voltage noise of 30 nV/(Hz), among the lowest reported voltage noise figures for a SET. Using a closed-loop transimpedance configuration, the amplifier shows almost 2 orders of magnitude increase in dynamic range, a 3 dB bandwidth of 30 kHz, and a transimpedance gain of 50 V/μA for a cryogenic 1 MΩ load resistor. The performance of this amplifier is already sufficient for use as an integrated readout with some types of high-performance cryogenic detectors for astrophysics.

X-ray single photon 1-D imaging spectrometers

We have developed superconducting single-photon 1-D imaging X-ray detectors with an energy resolution of 13 eV FWHM at 6 keV and 1-D spatial resolution of 0.25 /spl mu/m over a length of 20 /spl mu/m in an effective area of 20/spl times/100 /spl mu/m/sup 2/. The energy resolution along the 200 /spl mu/m long absorber is 36 eV. The energy resolution of 13 eV is among the best reported for this kind of detectors and is within factor of 2 of its theoretical limit. The signals are read out by low-noise current amplifier with a dc voltage bias. The electronic noise measured by injecting pulses is 8 eV FWHM at 6 keV. By cooling the feedback resistor the current noise is reduced from 160 fA/Hz/sup 1/2/ to 90 fA/Hz/sup 1/2/.

Dynamics and energy distribution of nonequilibrium quasiparticles in superconducting tunnel junctions

We present a full theoretical and experimental study of the dynamics and energy distribution of nonequilibrium quasiparticles in superconducting tunnel junctions (STJ’s ). STJ’s are often used for single-photon spectrometers, where the numbers of quasiparticles excited by a photon provide a measure of the photon energy. The magnitude and fluctuations of the signal current in STJ detectors are in large part determined by the quasiparticle dynamics and energy distribution during the detection process. We use this as motivation to study the transport and energy distribution of nonequilibrium quasiparticles excited by x-ray photons in a lateral, imaging junction configuration. We present a full numerical model for the tunneling current of the major physical processes which determine the signal. We find that a diffusion framework models the quasiparticle dynamics well and that excited quasiparticles do not equilibrate to the lattice temperature during the time scales for tunneling. We extract physical time scales from the measured data, make comparisons with existing theories, and comment on implications for superconducting mesoscopic systems and single-photon detectors.

Superconducting NbTaAlAlOxAl X-ray detectors with spatial resolution

Abstract Superconducting X-ray detectors based on SIS tunnel junctions offer potential for high spectral resolution and single photon efficiency. When used in a double junction geometry, they also offer spatial resolution needed for focal plane imaging. We are developing NbTaAlAlOxAl detectors for space-based X-ray astronomy applications. The design employs superconductor bandgap engineering for improved charge collection and adaptability for double junction geometries. For irradiation of a single junction device with 6 keV X-rays at a single location, the detectors have an energy resolution of 87 eV at 0.27 K. Initial studies of double junction detectors show a spectral resolution of 178 eV and an inferred spatial resolution of 4 μm over an effective length of 60 μm.

Single photon imaging X-ray spectrometers

We have developed superconducting, single-photon imaging X-ray detectors with an energy resolution of 26 eV FWHM at 6 keV and a spatial resolution of 0.5 /spl mu/m over an effective area of 18 /spl mu/m/spl times/100 /spl mu/m. The energy resolution is among the best reported for this kind of detector and is within a factor of /spl ap/4 of its theoretical limit. The calculated absorption efficiency of the detector is 28%. Scaling to larger areas and higher quantum efficiency appear possible. We discuss the device design and readout along with possible sources of resolution broadening.

RF single electron transistor readout amplifiers for superconducting astronomical detectors of X-ray to sub-mm wavelengths

We have made Radio-Frequency Single-Electron Transistors (RF-SETs) with large input gates, and tested performance and modes of operation with the goal of using such devices as on-chip amplifiers for a variety of high impedance cryogenic photodetectors. We achieved /spl ap/100 kHz of closed-loop bandwidth for charge-locked-loop and transimpedance amplifier feedback configurations, and have combined amplifier outputs using a form of wavelength division multiplexing. With our choice of SET junction resistance, a 0.5 fF input gate capacitance gave a cotunneling-degraded charge noise of 1/spl times/10/sup -4/ e//spl radic/Hz, but a fairly low input voltage noise of 30 nV//spl radic/Hz.

Diamond film optical X-ray and particle detectors

Synthetic diamond film diodes have been fabricated and tested with electromagnetic and particle radiation (above and below bandgap). It is shown that drifted ionization properties and yields in synthetic diamond film diodes within a factor of 2-4 of predicted values are achievable with minimal effort in films with thicknesses less than 50 mu . Potential diamond film applications in high-energy and nuclear physics are briefly described. It is concluded that, while synthetic high-resistivity diamond film ionization detector technology has interesting features for calorimetry and tracking radiation hardness and readout speed, it also carries a high cost per area (at present) and has modest predicted calorimetric performance, relative to other techniques, based on low electron yields. >