Natural Radioactivity of Rhenium

An attempt to confirm the reported $K$ capture activity of ${\mathrm{Os}}^{187}$, using Os${\mathrm{O}}_4$ mixed with argon as the counter gas, failed to reveal any activity corresponding to a half-life of less than ${5\times 10}^{11}$ 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 ${\mathrm{A}}^{37}$ which decays by $K$ capture. Rhenium then was examined for beta-activity, ${\mathrm{Re}}^{187}$ being isobaric with ${\mathrm{Os}}^{187}$. 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 ${4\pm 1\times 10}^{12}$ years calculated for ${\mathrm{Re}}^{187}$ isotope, and the range is 3.5 mg of ${\mathrm{A}\mathrm{l}/\mathrm{c}\mathrm{m}}^2$ corresponding to an upper energy limit of about 43 kev.