Neutrinoless Double-Beta Decay of 136Xe and Related Nuclear Structure Studies

The search for neutrinoless double-beta decay (0nbb) explores new physics by directly probing the unknown mass scale and possible Majorana nature of the neutrino. The nEXO experiment, one of the two leading proposed ton-scale projects in the US, has a projected sensitivity to the 136Xe 0nbb half-life of 1028 years. The interpretation of possible signals in this next generation of experiments would be complicated by significant variations in theoretical calculations of the nuclear matrix elements (NMEs) of the decay, and nuclear structure measurements testing those theories can help address that uncertainty. This proposal supports the involvement of undergraduate students at UNC Wilmington in this exciting field through collaboration with the Triangle Universities Nuclear Laboratory (TUNL) on two nuclear structure experiments and R&D toward nEXO's cryostat cooling strategy. Measurement of 134Xe(3He,n)136Ba transitions to low-lying 0+ states in the residual probes the BCS assumption for QRPA NME calculations, while charge-exchange reactions on 136Xe are used to explore the excited states of the virtual intermediate 136Cs nucleus as well as their gamma de-excitations. Decays of the lowest 1+ state are of particular interest in estimating the sensitivity of large liquid xenon experiments like nEXO to charge-current processes including low-energy solar neutrino interactions. The UNCW group will continue to be responsible for target preparation and operation for both efforts and carry out data analysis on both. The UNCW group has also constructed a thermosyphon test system as part of the nEXO R&D program. The use of thermosyphons for cryostat temperature maintenance would constitute an alternative to the baseline strategy of recirculating the HFE heat transfer fluid out to an external heat exchanger. A static thermal bath would provide a contingency in the case of a larger than expected 222Rn decay rate in recirculated HFE. Toward this end, the group will design and test feedthrough and immersed heat-exchanger concepts in a small HFE test system coupled to the thermosyphon.