Determination of transuranium elements produced by the Castle Bravo explosion

Abstract

The production of transuranium elements was investigated in the thermonuclear tests “Mike” (Marshall Islands), “Barbel” and “Par” (Nevada Test site, USA). However, little data has been reported for important tests such as the “Castle Bravo” explosion (Bikini Atoll, Marshall Islands). This test produced the largest yield (15 MT) of the entire series in the Pacific Proving Grounds. We use surface soils collected at Bikar Atoll (Marshall Islands), known to have been contaminated with debris from the “Castle Bravo” explosion, to investigate the production of transuranium elements. The activities and isotope ratios of several transuranium radionuclides were analysed by mass spectrometry techniques after radiochemical separation. The activity concentrations of 239Pu, 240Pu, 241Pu and 241Am radionuclides were found to be in the order of a few Bq kg−1. 242Pu, 243Am and 245Cm radionuclides showed much lower activity concentrations in the order of a few mBq kg−1. 244Pu was also detected, in the range of a few µBq kg−1. The isotope ratios, normalized to mass 239 (239Pu), showed a consistent decreasing trend with increasing isotope mass, which was approximated by an exponential function. Taking into account that 241Pu has a relatively short half-life, the exponential function is used for estimating the age of the Pu contamination. The result obtained by this approach is consistent with the age of the contamination expected at this atoll. Radionuclide abundances also showed the even-odd anomaly of heavy nuclei as it has been observed in other thermonuclear explosions.