Ba Double Gamma Decay Measurement with Gammasphere

The study of the electromagnetic moments, and decay probability, provides detailed information about nuclear wave functions. The well-know structure of the E M interactions are good for extracting information about the properties of the individual nucleons. However, the lowest multipoles are not always the only ones included in the decay process. It has been observed previously in the case of a 0+ →0+ transitions, where a single gamma transition is forbidden, the simultaneous emission of two γ rays. A great opportunity to further investigate this phenomena is by using the standard 137Cs source populating via β-decay the isomeric state at 662 keV in 137Ba. In this case two photon process can have contributions from quadrupole-quadrupole or dipole-octupole multipolarities due to the high multipolarity of the decay, M4. Since the yield of the double gamma decay is around 10-6 orders of magnitude less than the first order transition very good statistics are needed in order to observe the phenomena. Using Gamma sphere is ideal since its configuration allows a good coverage of the angular distribution and the Compton events can be suppressed. Nevertheless the process to understand and eliminate the Compton background is a challenge. GEANT4 simulations were carried out to help correct for those factors. A new direct cascade via the low-lying Jπ = 1/2+ has been found