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It has been demonstrated that microcalorimeter spectrometers based on superconducting transition-edge-sensors can readily achieve sub-100 eV energy resolution near 100 keV. However, the active volume of a single microcalorimeter has to be small in order to maintain good energy resolution, and pulse decay times are normally on the order of milliseconds due to slow thermal relaxation. Therefore, spectrometers are typically built with an array of microcalorimeters to increase detection efficiency and count rate. For large arrays, however, as much pulse processing as possible must be performed at the front end of readout electronics to avoid transferring large amounts of waveform data to a host computer for post-processing. In this paper, we present digital filtering algorithms for processing microcalorimeter pulses in real time at high count rates. The goal for these algorithms, which are being implemented in readout electronics that we are also currently developing, is to achieve sufficiently good energy resolution for most applications while being: a) simple enough to be implemented in the readout electronics; and, b) capable of processing overlapping pulses, and thus achieving much higher output count rates than those achieved by existing algorithms. Details of our algorithms are presented, and their performance is compared to that of the “optimal filter” that is currently the predominantly used pulse processing algorithm in the cryogenic-detector community. [ABSTRACT FROM AUTHOR]

Improving the resolution of gamma-ray detectors is important for many fields, including determinations of the Lamb shift in atoms with high atomic numbers, nuclear treaty verification, and environmental monitoring. High-purity germanium detectors are currently the tool of choice for precision gamma-ray spectroscopy. The resolution of these detectors is limited to about 500 eV full-width-at-half-maximum at 100 keV by Fano statistics. In comparison, low-temperature microcalorimeters can provide over an order of magnitude improvement in photon resolution. For instance, a gamma-ray microcalorimeter has achieved 25 eV FWHM resolution at 103 keV. These calorimeters consist of two components, a bulk absorber to stop incident gamma rays and a thermometer made from a thin film electrically biased in the superconducting-to-normal phase transition, called a Transition Edge Sensor, or TES. The standard absorber is bulk, superconducting tin. While tin has historically been the best performing absorber, pulse decays in Sn devices are much slower than predicted. We have begun a systematic study of absorber behavior in order to assess and improve response times. This study leverages two capabilities: the ability to microfabricate highly uniform arrays of gamma-ray detectors and the ability to read out many detectors in a single cool-down using SQUID multiplexer circuits. Here, we present two experiments to identify the source of thermal time constants. The first involves varying properties of the Sn absorber including purity, vendor, and crystal grain size. The second examines the role of the other elements in the microcalorimeter assembly. [ABSTRACT FROM AUTHOR]

We have measured the relation between sensor noise and the steepness (α = TR· and β = IR·dRdI) of the superconducting transition for various superconducting transition edge sensors. The measurements are performed on square Mo/Cu bilayer thermistors of three sizes (125 μm, 250 μm and 400 μm), which are fabricated for X-ray detection. We have identified the devices in two regimes: a) α>100 and b) α<100. The devices with α smaller than 100 strictly follow non-linear bolometer and micro-calorimeter noise theory while in most devices with α greater than 100, we see an unexplained noise contribution. [ABSTRACT FROM AUTHOR]