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Authors (affiliation): 1 Yasushi Sato (NMIJ, AIST, Japan), 2 Takahiro Kikuchi (DTRI, AIST, Japan), 3 Ryan Smith (The University of Tokyo, Japan), 4 Akira Sato (DTRI, AIST, Japan), 5 Fuminori Hirayama (DTRI, AIST, Japan), 6 Hisashi Nakagawa (NMIJ, AIST, Japan), 7 Tomoya Irimatugawa (NMIJ, AIST, Japan), 8 Rio Furukawa (NMIJ, AIST, Japan), 9 Chihiro Shimodan (NMIJ, AIST, Japan), 10 Hideki Harano (NMIJ, AIST, Japan), 11 Hirotake Yamamori (DTRI, AIST, Japan), 12 Hiroyuki Takahashi (The University of Tokyo, Japan).
When lead is used as a radiation shield, the shielding itself can generate background radiation as a consequence of the presence of the radioactive Pb-210 isotope. To reduce this background radiation, it is important to selectively source the raw material based on measuring its activity. Liquid scintillation counting can provide accurate activity data but requires dissolution of the lead in a mixture of acetic acid and hydrogen peroxide. The present work devised a means of measuring the radioactivity of lead specimens without dissolution using a transition-edge sensor (TES), a cryogenic detector that responds to radiation based on increases in temperature. A TES in contact with a lead specimen was able to detect alpha particles emitted from Po-210, one of the progenies of Pb-210. The present TES, fabricated by DTRI, AIST (Device Technology Research Institute, National Institute of Advanced Industrial Science and Technology), is operated at approximately 100 mK and can be combined with a radiation absorber having a heat capacity from several to approximately 100 pJ/K. This device is also physically strengthened by incorporating a SiO2/SixNy/SiO2 tri-layer membrane for a heavy absorber.1)
Lead radiation absorbers that concurrently act as radiation sources having masses from 2 to 20 mg were fabricated by first flattening lead beads (Fujifilm Wako Pure Chemical Corporation, part number 125-05762) using a vice. Each lead specimen was subsequently cut out using an ultrasonic device (Nihon Seimitsu Kikai Kosaku Co., Ltd.) and shaped with a micro knife (Micro Support Co., Ltd.) under a microscope equipped with an LCD monitor. Each lead radiation source was then affixed to a 250 × 250 mm TES attached to a 1 × 1 mm tri-layer membrane using a micro manipulator (Micro Support Co., Ltd.). Stycast 2850 GT with a 9M catalyst (Henkel Corporation.) was used as the adhesive agent. A few lead samples having varying masses were attached to TES devices that could be operated simultaneously with a microwave-based readout. In future work, the optimum mass for the lead radiation source to allow activity measurements will be determined based on the estimated energy resolution and count rates.
1) T. Kikuchi, G. Fujii, R. Hayakawa, R. Smith, F. Hirayama, Y. Sato, S. Kohjiro, M. Ukibe, M. Ohno, A. Sato, H. Yamamori, Gamma-ray transition edge sensor with a thick SiO2/SixNy/SiO2 membrane, Applied Physics Letters (2021) 119, 222602
