Mitigation of Backgrounds from Cosmogenic $^{137}$Xe in Xenon Gas Experiments using $^{3}$He Neutron Capture

Abstract

136 Xe is used as the target medium for many experiments searching for 0νββ. Despite underground operation, cosmic muons that reach the laboratory can produce spallation neutrons causing activation of detector materials. A potential background that is difficult to veto using muon tagging comes in the form of 137 Xe created by the capture of neutrons on 136 Xe. This isotope decays via beta decay with a half-life of 3.8 minutes and a Q β of ∼4.16 MeV. This work proposes and explores the concept of adding a small percentage of 3 He to xenon as a means to capture thermal neutrons and reduce the number of activations in the detector volume. When using this technique we find the contamination from 137 Xe activation can be reduced to negligible levels in tonne and multi-tonne scale high pressure gas xenon neutrinoless double beta decay experiments running at any depth in an underground laboratory.

Sensitivity of a tonne-scale NEXT detector for neutrinoless double beta decay searches

“…A particularly interesting development in the siting that shows the flexibility of a NEXT-like detector, is the suppression of neutron activations on 136 Xe by addition of a small amount of 3 He [53]. Reducing the yield 137 Xe inherently reduces the cosmogenic background index.…”

“…Combining simulated expectations of 137 Xe production in the detector (implemented as in Ref. [43]) with the predicted muon flux yields an estimate for the yield of 137 Xe per year is obtained. This can be multiplied by the simulated acceptance factor to yield a background index contribution from cosmogenic 137 Xe.…”