Abstract
Among all the nuclei, Thorium-229 has the lowest excited level at approximately 8.3 eV. This level is an isomeric state with a long radiative lifetime. Therefore, \(\mathbf {^{229}}\)Th can be excited to the isomeric state using a vacuum ultraviolet laser and is expected to have applications such as in frequency standards. Our group has been conducting experiments to excite \(\mathbf {^{229}}\)Th to the isomeric state via the second excited state using the high-intensity X-ray beam available at the SPring-8 facility. To detect vacuum ultraviolet photons from the isomeric state of \(\mathbf {^{229}}\)Th, a dedicated apparatus was constructed. We employed \(\mathbf {^{229}}\)Th-doped CaF\(_2\) crystals as the irradiation target. Because these targets emit numerous scintillation photons due to nuclear decay and X-ray beam irradiation, detectors are required to significantly reduce these background events. To achieve this, we adopted dichroic mirrors and a photomultiplier tube for detecting scintillation photons by nuclear decay, in addition to a solar-blind photomultiplier tube for detecting decay photons from the isomeric state of \(\mathbf {^{229}}\)Th. In this proceedings paper, we describe the experimental apparatus used in the beamtime in 2023.
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No datasets were generated or analysed during the current study.
Notes
The Si(880) monochromator was installed in beamtime of 2023. This is found to be important for avoiding damage to the crystal target.
The typical X-ray beam intensity between the Si(660) and Si(880) monochromator is 9.5\(\times 10^{11}\) /s.
In terms of the reduction of the radioluminescence background events, a smaller crystal is better.
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Acknowledgements
The synchrotron radiation experiments were performed at the BL19LXU line of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) and RIKEN. The author is grateful to the collaboration; Kjeld Beeks, Michael Bartokos, Hiroyuki Fujimoto, Yuta Fukunaga, Hiromitsu Haba, Yoshitaka Kasamatsu, Shinji Kitao, Adrian Leitner, Takahiko Masuda, Guan Ming, Nobumoto Nagasawa, Ryoichiro Ogake, Koichi Okai, Martin Pimon, Martin Pressler, Noboru Sasao, Fabian Schaden, Thorsten Schumm, Makoto Seto, Yudai Shigekawa, Koutaro Shimizu, Tomas Sikorsky, Kenji Tamasaku, Sayuri Takatori, Tsukasa Watanabe, Atsushi Yamaguchi, Yoshitaka Yoda, Akihiro Yoshimi, and Koji Yoshimura.
Funding
This work was supported by JSPS KAKENHI Grant Numbers JP19K14740 and JP21H01094 and Itoh Science Foundation for a grant.
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T.H. and the collaboration members conducted the experiment. T.H. wrote the script and prepared the figures.
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Appendix A: Measurement of optical components
Appendix A: Measurement of optical components
For the estimation of the expected number of detected signal photons from \(^{229{\textrm{m}}}\)Th or \(\lambda _{\textrm{isomer}}\), we measured the transmittance or reflectance data of optical components, such as dichroic mirrors and VUV band-pass filters. Figure 6 shows a schematic view of the optical property measurement system. The monochromatic VUV light beam is generated using the deuterium lamp (Heraeus, D200VUV) and the spectrometer (Shinku-kogaku, VMK-200-II). Each sample is placed on motorized stages so that position and angle dependence can be measured. The PMT (Hamamatsu, R6836) is also mounted on another motorized rotation stage so that both transmittance and reflectance can be measured.
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Hiraki, T., on behalf of the collaboration. Experimental apparatus for detection of radiative decay of \(^{229}\)Th isomer from Th-doped CaF\(_2\). Interactions 245, 14 (2024). https://doi.org/10.1007/s10751-024-01844-x
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DOI: https://doi.org/10.1007/s10751-024-01844-x