Gamma-ray spectroscopy is suitable for non-destructive analysis of nuclear materials important for nuclear fuel cycles and global security. Transition-edge sensors (TESs) exhibit typically 10 times higher energy resolution (ΔE) than that of popular High Purity Germanium (HPGe) detectors. In low-energy range (20 keV< E < 220 keV), TESs with 236 pixels show 40 eV< ΔE < 60 eV suitable for measuring isotopic composition of nuclear materials [1,2]. On the other hand, there is a demand in high-energy range (E > 220 keV), such as detection of cesium (137Cs) or positron-annihilation spectroscopy [3,4]. However, to our knowledge, there are no reports on multi-pixel TESs operating in this energy range.
We have been developing multi-pixel TESs applicable for both energy range. A unique improvement in our fabrication is a 7-μm thick SiO2/SixNy/SiO2 membrane  with the same thermal conductance (≈1.5 nW/K) as conventional 1-μm thick silicon-nitride (SixNy) membranes [1,6]. We expect that this thick membrane is more robust to support the heavier-metal absorber required for stopping higher energy photons and converts them into heat. In the low-energy range, a TES with a 0.5-mm-cubic Sn absorber shows 40 eV < ΔE < 47 eV at 86.5 keV for 4 pixels read out by our microwave SQUID multiplexer (MW-Mux) [7, 8]. It clearly separates two lines of neptunium (237Np) at 86.5 keV and protoactinium (233Pa) at 86.6 keV that HPGe cannot distinguish . Moreover, their peak ratio agrees with the expectation based on the radio-active equilibrium between the two nuclides. In the high-energy range, another TES with a 0.8-mm-cubic Sn absorber shows 182 eV < ΔE < 290 eV at 320 keV for 6 pixels. Their ΔE are expected to be high enough for applications introduced above. These results are a milestone for gamma-ray spectrometers equipped with the large number of pixels. We will realize it, incorporating several TES and MW-Mux chips in the same measurement module .
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Keywords: Transition Edge Sensor, Gamma ray, Trilayer Membrane, Spectroscopy