Prevalence of technical mesothorium in self-luminous compounds used by New Jersey radium dial workers

摘要

Forty-five sealed glass ampoules containing samples of radium dial paint prepared by the US Radium Corporation (USRC) and used by New Jersey dial workers in the period 1915--1928, were analyzed for radium-226 and radium-228 activity by high-resolution gamma-ray spectrometry. Radium-228 was found to be the dominant activating agent at the probable time of use in most of the sampled paints in which the ratio of radium-228 to radium-226 activity was determinable, the calculated radium-228 to radium-226 activity ratio in 1920 in these ranging from 7.2 to 10 (median 8.4), indicating that radium element chemically separated from commercialthorium ores (technical mesothorium) was used as the activator. Published isotopic activity ratios in USRC dial-paint samples that were appreciably in excess of those we found are shown invariably to be due to errors in calculation. Our results and information in the early literature suggested the hypothesis that dial paints used at USRC before July 1919 were activated with isotopically pure radium-226 whereas compounds used thereafter until the year 1925 were activated with technical mesothorium- Isotopic activity ratios predicted by the hypothesis compared well with median ratios observed in two groups of former workers. We conclude that inaccuracies in dates of hire and termination at USRC might well be the principal source of uncertainty in estimates of skeletal dose for former workers in whom the isotopic activity ratio has not been measured.