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

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. 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. New calculations and new measurements with NaI(Tl) counters from Brookhaven suggest that the two photon branch is $\sim$ 2 x 10$^{-6}$. Gammasphere is the perfect tool for this investigation, having good energy resolution, high efficiency, broad coverage of angles, and sufficient granularity to minimize pile-up and count-rate difficulties. A very large data set was collected involving more than 10$^{11}$ decays. Evidence for the 2-photon branch has been found, despite the daunting background from Compton scattering. This work was supported by DOE contracts, DE-FG02-94ER40848, DE-AC02-06CH11357, DE-AC02-98CH10886 and DE-AC02-98CH10946.\\[4pt] [1] J. Kramp, et. al, Nucl. Phys. A474 (1987) 412