The only naturally occurring isotope of bismuth, 209Bi, is commonly regarded as the heaviest stable isotope. But like most other heavy nuclei abundant in nature and characterized by an exceptionally long lifetime, it is metastable with respect to α-decay1. However, the decay usually evades observation because the nuclear structure2,3 of 209Bi gives rise to an extremely low decay probability and, moreover, generates low-energy α-particles difficult to detect. Indeed, dedicated experiments2,3,4,5,6 attempting to record the α-decay of 209Bi in nuclear emulsions failed. However, scintillating bolometers7,8,9 operated at temperatures below 100 mK offer improved detection efficiency and sensitivity, whereas a broad palette of targets could be available10. Here we report the successful use of this method for the unambiguous detection of 209Bi α-decay in bismuth germanate detectors cooled to 20 mK. We measure an energy release of 3,137 ± 1 (statistical) ± 2 (systematic) keV and a half-life of (1.9 ± 0.2) × 1019 yr, which are in agreement with expected values.
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This work is supported by the R&D programme from CNRS/INSU, and by BNM for the high-resolution α-spectrometry part. The design of the optical detectors results from years of support by CNES. P. Pari from CEA/SPEC designed our dilution refrigerator. We thank the SEMIRAMIS members of CSNSM for their accurate implantation, as well as F. Jomard for SIMS analysis, G. Audi for discussions, E. Leblanc from LNHB/BNM for independent calibration source measurements, and I. Rameau and Y. Bouvÿ for their support and comments.
The authors declare that they have no competing financial interests.
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de Marcillac, P., Coron, N., Dambier, G. et al. Experimental detection of α-particles from the radioactive decay of natural bismuth. Nature 422, 876–878 (2003). https://doi.org/10.1038/nature01541
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