We report on new measurements establishing the existence of low-lying
isomeric states in 136Cs using γ rays produced in
136Xe(p,n)136Cs reactions. Two states with O(100)~ns
lifetimes are placed in the decay sequence of the 136Cs levels that are
populated in charged-current interactions of solar neutrinos and fermionic dark
matter with 136Xe. 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.Comment: Supplemental material available upon request. Version accepted by
Phys.Rev.Let