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Among the radiations from 212Bi sources, Rutherford and Woods noted in 1916 a few particles whose range is markedly more than the 8.6 cm range of the well-known 8.78437-MeV alpha-particle emissions from 212Po to 208Pb. A small fraction of the excited 212Po nuclei will undergo alpha decay directly from an excited level, due to the partial half-life for alpha decay is comparable with that for gamma decay. The energies and relative intensities of these long-range alpha-particle groups of 212Po were summarized by Rytz (1953). Scarce revisions of these data have been found in the bibliography (Emery and Kane, 1960). But even in the most recent compilations (Nichols, 2011) relative intensities (including uncertainties) have been only poorly estimated.
New measurements have been made of these long-range alpha particles in our laboratory and, with the tabulated energy-emission values, relative intensities have been calculated being their uncertainties experimentally estimated for the first time. The measurements were performed by alpha-particle spectrometry with a low-resolution silicon detector inside a vacuum chamber. The source used was a 232U (in equilibrium with its daughters) collimated disc, 20 mm2 active area, made by electrodeposition, and provided by CIEMAT. A PIPS detector from Canberra, with 50 mm2 active area, was used, and 65 mm for the source-to-detector distance. Due to the short life of the 212Po levels, coincidence summing is produced between the emitted alpha particles and the beta particles coming from the 212Bi ground level, making difficult the analysis of the region above 8.78437 MeV, where the long-range particles are expected (Martín Sánchez et al., 1990). To avoid these interferences, measurements including a magnet between source and detector were also performed. Results show that the values reached for the relative intensities of the long-range alpha-particles of 212Po agree with the tabulated values, but this is the first time estimating experimentally their uncertainties.
G. T. Emery, W. R. Kane, 1960. Gamma-ray intensities in the Thorium active deposit. Phys. Rev. 118, 755.
A. Martín Sánchez, F. Vera Tomé, C. J. Bland. Recent measurements of 228Th activity by alpha-beta coincidence counting. Nucl. Instrum. Methods Phys. Res. A 295, 273.
Nichols, A. L., 2011. 212Bi-Comments on evaluation of decay data. LNE-LNHB/CEA Table de Radionuclèides.
A. Rytz, 1953. J. Recherches Centre Natl. Recherche Sci., Labs. Bellevue (Paris) 25, 254.
