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/Si_xN_y/SiO₂ membrane [5] with the same thermal conductance (≈1.5 nW/K) as conventional 1-μm thick silicon-nitride (Si_xN_y) 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 [9]. 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 [10].

[1] D. A. Bennet et al., Rev. Sci. Instrum, 83, 093113, 2012
[2] J. A. Ullom et al., Supercond. Sci. Technol. 28, 084003, 2015
[3] M. Ohno et al., IEICE Trans. Electron., 100, 283, 2017
[4] S. W. Leman et al., J. Low Temp. Phys., 151, 784, 2008
[5] T. Kikuchi et al., Appl. Phys. Lett., 119, 222602, 2021
[6] H. F. C. Hoevers et al., Appl. Phys. Lett., 86, 251903, 2005
[7] Y. Nakashima et al., IEEE Trans. Appl. Supercond. 29, 2100705, 2019
[8] Y. Nakashima et al., Appl. Phys. Lett., 117, 122601, 2020
[9] T. Kikuchi et al., J. Low Temp. Phys., Accepted.
[10] J. A. B. Mates et al., Appl. Phys. Lett., 111, 062601, 2017

Keywords: Transition Edge Sensor, Gamma ray, Trilayer Membrane, Spectroscopy