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The position determination capability of the series-connected superconducting tunnel junctions (STJs) with Nb/Al/AlOx/Al/Nb junctions as a radiation detector is reported. Four groups of series-connected STJs are fabricated on a single crystal of sapphire substrate surrounding 3.5×3.5 mm2 detection area. The signals detected by two STJs with 1.5 mm separation give a time delay caused by the propagation of phonons, and this enables us to measure the irradiated position. Using α-particles of 5.5 MeV, we clearly distinguished the incident positions with a potential resolution of 0.16±0.07 mm (FWHM). Directional dependence in the phonon propagation in the sapphire substrate is not detected. [Copyright &y& Elsevier]

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]

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 succeeded in fabricating 256-pixel arrays of Ti/Au transition edge sensor (TES) microcalorimeters with electrodeposited microabsorbers made of Bi, and report on their test results. We confirmed all pixels tested so far were alive, however most of the microabsorbers were stripped off during cooling cycles. We investigated some pixels by irradiating with 5.9keV X-rays, and confirmed that they actually worked as microcalorimeters. The X-ray pulses are represented by a sum of two components with different decay times. The ratio of the two components fluctuates from pulse to pulse, which limits the energy resolution to . We discuss the pulse shape of two decay-time components considering thermal conductivities, i.e., inside microabsorbers, between microabsorbers and TES. [Copyright &y& Elsevier]