Natural Radioactivity of Rhenium

An attempt to confirm the reported K capture activity of Os¹⁸⁷, using OsO₄ mixed with argon as the counter gas, failed to reveal any activity corresponding to a half-life of less than 5×10¹¹ years. This method assumed that any K capture in Os would be followed by the emission of Auger electrons which would be recorded. The sensitivity of the system to soft Auger electrons was demonstrated by using 34.0-day A³⁷ which decays by K capture. Rhenium then was examined for beta-activity, Re¹⁸⁷ being isobaric with Os¹⁸⁷. An activity was found in two samples of different origin which resisted chemical purifications, including distillation of the perrhenic acid from concentrated sulfuric acid solution. The half-life appears to be 4±1×10¹² years calculated for Re¹⁸⁷ isotope, and the range is 3.5 mg of Al/cm² corresponding to an upper energy limit of about 43 kev.