Observation of Low-Lying Isomeric States in ${}^{136}\mathrm{Cs}$: A New Avenue for Dark Matter and Solar Neutrino Detection in Xenon Detectors

We report on new measurements establishing the existence of low-lying isomeric states in ${}^{136}\mathrm{Cs}$ using $\gamma$ rays produced in ${}^{136}\mathrm{Xe}(p,n){}^{136}\mathrm{Cs}$ reactions. Two states with $\mathcal{O}(100)\mathrm{ns}$ lifetimes are placed in the decay sequence of the ${}^{136}\mathrm{Cs}$ levels that are populated in charged-current interactions of solar neutrinos and fermionic dark matter with ${}^{136}\mathrm{Xe}$. Xenon-based experiments can therefore exploit a delayed-coincidence tag of these interactions, greatly suppressing backgrounds to enable spectroscopic studies of solar neutrinos and dark matter.

Figure 1

Energy spectrum of events in the near LEPS detector. The black curve shows the spectrum with no selection on arrival time. The blue curve displays the delayed spectrum defined as events arriving $>40\mathrm{ns}$ after the beam pulse. Gray arrows indicate background lines from ${}^{136}\mathrm{Cs}$ ${\beta }^{-}$ decay and x rays from tungsten and lead shielding.

Figure 2

The rightmost column shows our proposed level scheme for ${}^{136}\mathrm{Cs}$ up to 590 keV, with energies given in keV and measured state lifetimes. A scheme using all observed coincidences is given in the Supplemental Material [28]. Blue lines show measured $\gamma$-ray transitions, where those drawn as dashed lines have weak coincidence intensity but are observed in the singles spectrum. The left column shows the spectrum of states predicted by the shell model of Ref. [17]. The central column shows levels measured using ${}^{138}\mathrm{Ba}(d,\alpha )$ [24] and ${}^{136}\mathrm{Xe}({}^3\mathrm{He},t)$ [18, 19] reactions with their assigned $J^{\pi }$ values shown in green and red, respectively.

Figure 3

Background-subtracted $\gamma$-ray arrival times of the three delayed transitions in our level scheme. The 105 and 67 keV $\gamma$’s are fit with exponential and constant background terms only, while the fit to the 74 keV transition contains a term accounting for the additional delay caused by preceding 67 keV transitions, in accordance with the proposed level scheme. X rays from lead produce the prompt peak observed in the 74 keV distribution.