Space dating using photons

Since the half-life of 187Re, 4.4 x 1010 years, is comparable to the universe age (1.37 x 1010 years), it is useful to study the initial composition ratio of 187Re to its daughter 187Os to estimate the elapsed time after the synthesis of elements. Up to the present it has been regarded that 187Re was synthesized by the pure r-process, however, the contributions from three s-processes exist. In the present experiment, the neutron capture cross sections of 185W and 186Re which are the branching point nuclei are measured and the mixing of the x-processes to the 187Re and 187Os is estimated. The capture cross section of 185Re to 186Re isomer is obtained by using JRR-4 reactor of JAERI to be σn 0.67(14) b which is about twice as large as the existing estimation. The cross section at the energy of the astronomical nuclear synthesis, kT=30keV, is estimated to be σm (kT=30keV)=0.033b using the capture cross sections from 185Re to ground state 186Re and from 185Re to 186Re isomer evaluated by JENDL-3.3. On the other hand, the cross section from 185Re to ground state 186Re at 30keV is 1.55b which is about fifty times as large as that via the isomer. Consequently the s-process contribution via 186Re isomer is small. The elapsed time based on the r-process is estimated to be 5.5 to 11.3 x 109 years using Nrad(187Os)/Nr(187Os) ratio estimated from exiting calculations as there are no available experimental photonuclear reaction cross sections of 187Re. To obtain more precise elapsed time based on the r-process, it is needed to measure the capture cross sections of the branching point nuclei 185W and 186Re experimentally and to determine the reaction cross sections of the reverse reactions. (S. Funahashi)