Measurement of the isomer production ratio for the Cd 112 (n,γ) Cd 113 reaction using neutron beams at J-PARC

The astrophysical origin of a rare isotope Sn115 remains an open question. An isomer (T1/2=14.1y) in Cd113 is an s-process branching point from which a nucleosynthesis flow reaches Sn115. The s-process abundance of Sn115 depends on the isomer production ratio in the Cd112(n,γ)Cd113 reaction. However, the ratio has not been measured in an energy region higher than the thermal energy. We have measured γ rays following neutron capture reactions on Cd112 using two cluster high-purity germanium (HPGe) detectors in conjunction with a time-of-flight method at J-PARC. We have obtained the result that the relative γ-ray intensity ratio of the isomer is almost constant in an energy region of up to 5 keV. This result suggests that the s-process contribution to the solar abundance of Sn115 is minor. We have found that the ratio of a resonance at 737 eV is about 1.5 times higher than other ratios. This enhancement can be explained by a p-wave neutron capture. This result suggests measurements of decay γ rays to isomers are effective to assign the spin and parity for neutron capture resonances.