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Challenge to improve the $$^{229}$$Th isomer energy measurement with TES calorimeters

田中 圭太*; 平野 航亮*; 山口 敦史*; 村松 はるか*; 林 佑*; 湯浅 直樹*; 中村 圭佑 ; 滝本 美咲 ; 羽場 宏光*; 白崎 謙次*; 菊永 英寿*; 前畑 京介*; 山崎 典子*; 満田 和久*

Tanaka, Keita*; Hirano, Kosuke*; Yamaguchi, Atsushi*; Muramatsu, Haruka*; Hayashi, Tasuku*; Yuasa, Naoki*; Nakamura, Keisuke; Takimoto, Misaki; Haba, Hiromitsu*; Shirasaki, Kenji*; Kikunaga, Hidetoshi*; Maehata, Keisuke*; Yamasaki, Noriko*; Mitsuda, Kazuhisa*

Determination of the isomer energy of $$^{229}$$Th is essential to realize a nuclear clock by an UV laser excitation. Several experiments such as spectroscopy of internal conversion electrons or $$gamma$$-ray spectroscopy using TES or magnetic calorimeters have been made and the isomer energy seems to converge to around 8 eV. In 2019, we have measured the energy of the 29.2-keV $$gamma$$-ray with a single-pixel TES, and determined the isomer energy to be 8.30$$pm$$0.92 eV (A. Yamaguchi et al. 2019). We continued our experiments at Tohoku University to reduce the statistical errors and determination of the isomer energy with smaller uncertainty. We optimized a TES design and achieved an improved energy resolution of about 20 eV (H. Muramatsu et al. 2020). By using 3 pixels of this new TES elements with a 5 times stronger $$^{233}$$U source, the new measurements we reperformed. We are also trying to resolve two emission lines separating by the isomer energy, which were successfully demonstrated with magnetic calorimeters (T. Sikorskyet al. 2020). We will report the results obtained in our new measurement campaigns.

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