A search for 2-photon emission from the 662 keV state in $^{137}$Ba using Gammasphere

Two photon decays from excited nuclear states provide an interesting test both of QED and nuclear structure. It has been extensively studied for cases where one photon decay is forbidden [1]. Two photon decay in direct competition with the first order process has never been convincingly demonstrated. Nonetheless, observation of this decay will provide additional challenging tests for experiment and theory. The $^{137}$Ba case is particularly interesting as the decay has high multipolarity, M4, so the 2-photon process can have contributions from both quadrupole-quadrupole and dipole-octupole multipolarities. Gammasphere is the perfect tool for this investigation, having good energy resolution, good efficiency, good coverage of angles, and sufficient granularity to minimize pile-up and count-rate difficulties. A short test experiment showed the power of Gammasphere and the dauntingly high Compton scattering background that need suppression. However, new calculations and new measurements from Brookhaven suggest that the two photon branch is $\sim $2 x 10$^{-6}$ and should be measurable. This work was supported by DOE contracts, DE-FG02-94ER40848, DE-AC02-06CH11357 and ~DE-AC02-98CH10946.\\[4pt] [1] J. Kramp, et. al, Nucl. Phys. A474 (1987) 412