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We are developing position-sensitive transition-edge sensor (TES) microcalorimeters (PoSTs) to detect gamma rays up to a few MeV. Each gamma-ray PoST consists of a long absorber with a TES on each end and works as a 1D imaging spectrometer. We fabricated PoSTs with 0.5 × 0.5 × 18.8 mm lead absorbers. We irradiated two PoST devices with gamma rays from a Cs-137 source. Gamma-ray pulses of the PoSTs show a strong correlation between the pulse height and the rise time. We divided the pulses in the 662 keV line into 12 groups after sorting them by rise time to determine effective pixels. We modeled the PoST device and compared the average pulses of the 12 effective pixels to numerical simulations. The actual pulses and simulated pulses were in good agreement. [ABSTRACT FROM AUTHOR]

An x-ray microcalorimeter consists of an absorber and a thermometer connected to each other, and to a heat sink, via well defined thermal conductances. The standard theoretical derivation of energy resolution treats the absorber and thermometer as point elements that are internally isothermal. In reality, the finite size and internal diffusivity of the absorber and thermometer prevents these elements from instantly achieving a uniform temperature, leading to a variation in observed pulse shapes as a function of the interaction’s position within the absorber. These variations result in a distortion of the detector response and a subsequent degradation of the energy resolution. This paper presents diffusivity measurements for x-ray microcalorimeters fabricated at the NASA/GSFC. Using a diffusion model we developed, we show quantitatively how a 2 eV Gaussian response is distorted into a non-Gaussian profile roughly 12 eV wide at an energy of 6 keV for an absorber diffusivity of 104 μm2/μs. We then present a method for eliminating the effect of pulse shape variation on the detector energy response with a modified optimal filter approach. [ABSTRACT FROM AUTHOR]

This paper proposes a novel waveform multiplexing method to realize high channel-density front-end application specific integrated circuit (ASIC) design. In the proposed method, multiplexing is achieved through connecting multiple preamplifiers to a single analog-to-digital converter (ADC). These preamplifiers have different decay time (90%-10%) which will be used as the tag information. The channel information can be identified by analyzing the decay time using digital signal processing. Based on this method, two prototype ASICs have been designed with 2-to-1 multiplexing. They are a 36-channel multiplexing charge sensitive preamplifier ASIC (MCSP) and a 24-channel multiplexing waveform sampling front-end ASIC (MWSFE). [ABSTRACT FROM AUTHOR]

We are developing bulk lead absorber coupled transition edge sensor gamma-ray detectors that can be applied to positron annihilation spectroscopy at 511 keV. We observe a double exponential decay in the signal response curves of our detectors. Here, we characterize this signal response using three gamma-ray detectors with different thermal conductance values. [ABSTRACT FROM AUTHOR]

Properties of Transition-Edge Sensor (TES) microcalorimeters operated with AC bias are studied utilizing the calorimeter Wheatstone bridge circuit called Calorimeter Bridge Biased by an AC Generator (CABBAGE). The CABBAGE eliminates the AC carrier significantly, thus enables us to study the AC responses of the TES with high sensitivity. We tested two kinds of TES devices operating at 110 and 440 mK, respectively. With the 110 mK device biased with 25 kHz, an energy resolution of 28 eV is obtained for Mn Kα line. On the other hand, we multiplexed the signals from two 440 mK device biased with 50 and 20 kHz, respectively, and obtained 167 and 271 eV energy resolutions. Even at the balance point of the bridge, AC signal did not disappear and odd-order harmonics were observed. They are considered to arise from the current dependence of the TES resistance, which is characterized by β≡d log R/d log I. Numerical solution for the CABBAGE response can reproduce the experimental results well if β=0.24±0.02. Since the harmonics may cause severe problem in the SQUID operation even after attenuated by a band-pass filter, especially at high bias frequency operation such as several hundred kHz, it is important to make β small. [Copyright &y& Elsevier]

Evaluates the performance and design of the hard X-ray detector constructed for the ASTRO-E satellite. Components of the satellite; Analysis of the analog signals of pre-amplifiers; Algorithms for the program.

Presents a study on the resolved response of p-i-n diode to determine the origin of ASTRO-E hard X-ray detector. Description of p-i-n diodes; Details of the experiment; Results and conclusion.

Abstract: We report the latest results from our development of Position-Sensitive Transition-edge sensors (PoSTs), which are one-dimensional imaging spectrometers. In PoSTs with segmented Au absorbers, we obtained 8eV energy resolution on K lines, which is consistent to the baseline energy resolution and the design values, on all of the nine pixels, by choosing the best combination of the thermal conductance in absorbers and in links that connects the absorbers. The pulse decay time of is fast enough for our purpose. In a PoST with a continuous Bi/Cu absorber, by dividing the events into 63 effective pixels, we obtained energy resolutions of 16eV at the center “pixel”, which is comparable to the baseline energy resolution, and 33eV at the outer “pixel”. The degradation of the energy resolution in the outer “pixel” is due to position dependence, which we can cancel out by dividing the events into smaller “pixels” when we have sufficient X-ray events. [Copyright &y& Elsevier]

Abstract: In a microcalorimeter that uses a transition-edge sensor to detect energy depositions, the small signal energy resolution improves with decreasing heat capacity. This improvement remains true up to the point where non-linear and saturation effects become significant. This happens when the energy deposition causes a significant change in the sensor resistance. Not only does the signal size become a non-linear function of the energy deposited, but also the noise becomes non-stationary over the duration of the pulse. Algorithms have been developed that can calculate the optimal performance given this non-linear behavior that typically requires significant processing and calibration work—both of which are impractical for space missions. We have investigated the relative importance of the various non-linear effects, with the hope that a computationally simple transformation can overcome the largest of the non-linear and non-stationary effects, producing a highly linear “gain” for pulse-height versus energy, and close to the best energy resolution at all energies when using a Wiener filter. [Copyright &y& Elsevier]

Abstract: An ideal microcalorimeter is characterized by a constant energy resolution across the sensor''s dynamic range. Any dependence of pulse shape on the position within the absorber where an event occurs leads to a degradation in resolution that is linear with event''s energy (excess broadening). In this paper we present a numerical simulation that was developed to model the variation in pulse shape with position based on the thermal conductivity within the absorber and between the absorber, sensor, and heat bath, for arbitrarily shaped absorbers and sensors. All the parameters required for the simulation can be measured from actual devices. We describe how the thermal conductivity of the absorber material is determined by comparing the results of this model with data taken from a position sensitive detector in which any position dependent effect is purposely emphasized by constructing a long, narrow absorber that is readout by sensors on both ends. Finally, we present the implications for excess broadening given the measured parameters of our X-ray microcalorimeters. [Copyright &y& Elsevier]

Abstract: We are developing a gamma-ray microcalorimeter for Positron Annihilation Spectroscopy. Our detector composed of a transition edge sensor (TES) thermometer and an epoxy coupled bulk Pb absorber has been reported an energy resolution of 4.7keV FWHM at 662keV with a tens of milliseconds long response time. To enhance the detector properties, we designed a new attachment method in the focus of improving the thermal coupling between the absorber and the TES. In the new design, we created epoxy-posts both on the TES film and on the absorber, and then they were connected together using an epoxy-dot put on the epoxy-post on the absorber. Measured current signal of this epoxy-post TES gamma-ray detector showed an ultra fast response with 80μs fall time. [Copyright &y& Elsevier]

We are developing transition edge sensor (TES) microcalorimeter arrays for future Japanese astronomy missions. Although 6.6 eV energy resolution for 5.9 keV and 74 μs decay time are achieved, our energy resolution was limited by the excess noise of unknown origin. The dominant noise showed 1/R dependence and it only appeared when the current through the TES, I, is larger than 10 μA. This is explained if we assume a constant voltage noise source with a level of ∼2(4kTCRn)1/2. We also investigated the influence of I on the TES performance and found the TES sensitivity α is reduced when the ratio of current to the critical current I/IC is large. It can explain the reduction of α observed when operated without a magnetic shield. We also found a strong correlation between IC and α or the baseline width. Thus, we conclude that IC is an essential parameter for the TES performance. [Copyright &y& Elsevier]

Abstract: We present a geometrical model to describe excess electrical noise in transition-edge sensors (TESs). In this model, a network of fluctuating resistors represents the complex dynamics inside a TES. The fluctuations can cause several resistors in series to become superconducting. Such events short out part of the TES and generate noise because much of the current percolates through low resistance paths. The model predicts that excess white noise increases with decreasing TES bias resistance (R/R N) and that perpendicular zebra stripes reduce noise and alpha of the TES by reducing percolation. [Copyright &y& Elsevier]

Abstract: Transition-edge sensors (TESs) exhibit two major types of excess noise above the expected and unavoidable thermodynamic fluctuation noise (TFN) to the heat sink and Johnson noise. High-resistance TESs such as those made by the Netherlands Institute for Space Research (SRON) show excess noise consistent with internal TFN (ITFN) caused by random energy transport within the TES itself while low resistance TESs show an excess voltage noise of unknown origin seemingly unrelated to temperature fluctuations. Running a high-resistance TES on a high thermal conductivity substrate should suppress ITFN and allow detection of any excess voltage noise. We tested two TESs on a solid silicon substrate fabricated by SRON of a relatively high normal state resistance of ∼200mΩ. After determining a linear model of the TES response to noise for the devices, we found little excess TFN and little excess voltage noise for bias currents of up to ∼20μA. [Copyright &y& Elsevier]