Isotopic Abundance Determination of Submicro Amounts of Rhenium by Neutron Activation

Neutron activation is applied to the isotopic abundance measurements of rhenium in lunar matter. This is achieved by comparing the (n, γ) induced activities of ^{1 8 6}Re and ^{1 8 8}Re by the 137 keV and the 155 keV γ-line, respectively. Radiochemical processing to a very high purity is needed in order to avoid interferences of ^{1 9 9}Au and ^{9 9 m}Tc. Additionally, the rhenium and tungsten contents were determined.

An isotopic anomaly of rhenium was established in Apollo 14 samples, exhibiting a remarkable enrichment of ^{1 8 7}Re (1.4 to 1.8 percent in soils and up to 29 percent in the breccia 14321). It is shown that this enrichment is at least partially due to neutron irradiation on the lunar surface by the reaction ^{1 8 6}W(n, γ) ^{1 8 7}W $β→$ ^{1 8 7}Re. However, the observed anomaly is not produced by lunar neutrons alone. There is also a contribution from the reaction ^{1 8 6}W(n, γ) ^{1 8 7}W $β→$ ^{1 8 7}Re (ⁿ, γ) ^{1 8 8}Re in the reactor itself which can be corrected for by the analysis of simultaneously irradiated tungsten standards. The lunar #x201C;share#x201D; of the ^{1 8 7}Re enrichment is found to be 20 to 60 percent of the observed total excess. It is shown that the ^{1 8 6}W neutron capture is mainly due to epithermal neutrons (resonances are at 18.8, 171, and 221 eV). Relationships to the recently discovered other neutron induced isotope anomalies in extraterrestrial matter are discussed, and the possibility of deriving neutron spectra and time integrated neutron fluxes is pointed up.