Observation of De-Excitation Gamma-Rays from Excited States of 136Cs.

Excited states in the isotope 136Cs are of interest both as intermediate states in the double beta decay of 136Xe and as final-state products in neutrino charged-current interactions in next-generation liquid-xenon-based particle physics experiments. In the latter case, the presence of long-lived isomeric states may enable background-free detection of solar neutrinos with sensitivity down to ~700 keV, which would enable new measurements of CNO neutrinos in upcoming experiments such as nEXO or DARWIN. Measurements of the 136Cs excited states are sparse, though recent QRPA and shell model theoretical calculations have been made. In this talk, we will describe an experimental program to characterize the nuclear levels of 136Cs and search for long-lived isomeric states. We will show new results from experiments performed at the Triangle Universities Nuclear Laboratory using (p,n) reactions on a 136Xe target, including the first observation of 31 de-excitation gamma-rays in 136Cs. Ongoing gamma-gamma coincidence measurements to elucidate the decay schemes of particular excited states will also be discussed.