Beta Decay and Rhenium Cosmochronology

Among the problems which limit the use of the ^{187}Re/^ {187}Os isobaric pair as a cosmochronometer for the age of the galaxy and the universe are the uncertainties in the partial half-lives of the continuum and bound state decays of ^{187}Re. While the total half-life of the decay is well established, the partial half-life for the continuum decay is uncertain, and several measurements are not compatible. A high temperature quartz proportional counter was used in this work to remeasure the continuum beta^- decay of ^{187 }Re. The beta end-point energy for the decay of neutral ^{187 }Re to singly ionized ^{187 }Os of 2.75 +/- 0.06 keV agrees with the earlier results. The corresponding half -life of (45 +/- 3) times 10^9 years improves and agrees with the earlier measurement of Payne and Drever and refutes other measurements. Based on the new half -life for the continuum decay and a total half-life of (43.5 +/- 1.3) times 10^9 years reported by Linder et al., the branching ratio for the bound state decay into discrete atomic states is estimated to be (3 +/- 6)% in agreement with the most recent calculated theoretical branching ratio of approximately 1%. Anomalies in beta spectra reported by J. J. Simpson and others have been attributed to a 17 keV heavy-neutrino admixture. If confirmed, the implications from the existence of such a neutrino for particle and astrophysics would be significant. A multiwire "wall-less" stainless steel proportional counter has been used in the present work to investigate the spectral shape of the beta decay of ^{63}Ni. No anomalies in the spectral shape were observed which could be attributed to the presence of 17 keV heavy neutrino.