Monday, November 21, 2011, 3:00 PM, room 385E

2-gamma Decay of the 662-keV 137Ba Isomer

D.J. Millener, R.J. Sutter, and D.E. Alburger

Brookhaven National Laboratory)

2-gamma decay of the 662-keV 137Ba isomer following 137Cs beta decay has been observed using two 3"x3" NaI detectors, a 20.5-uCi source, and a Pb shielding geometry designed to minimize direct and sequential Compton scattering backgrounds. In runs totaling 296 days, a 662-keV peak has been observed in the summed gamma-ray energy providing the first convincing evidence for this second-order process in competition with a single gamma ray (the 2-gamma procees in competition with e+e- pairs has been observed previously for 0+ -> 0+ transitions). A simple closure approximation for the M2-E2 (QQ) and E3-M1 (OD) single-neutron hole h11/2 -> g7/2 -> d3/2 and h11/2 -> d5/2 -> d3/2 transitions in this N=81 nucleus gives 2-gamma/1-gamma branching ratios of order 10^{-6} for both sequences. The total experimental result is around 3 x 10^{-6} but depends on the energy sharing between the gamma rays which is different for QQ and OD. This affects the extracted branching ratio but the relative contributions can in principle be extracted from the data. Preliminary large-scale shell-model calculations suggest that the intermediate state sum is dominated by the lowest 7/2+ and 5/2+ states at 1252 and 1294 keV for which the ground-state transition strengths are measured.