Search for $\beta^+$EC and ECEC processes in $^{112}$Sn and $\beta^-\beta^-$ decay of $^{124}$Sn to the excited states of $^{124}$Te

TL;DR

This study sets new experimental limits on rare double beta decay processes in tin isotopes using high-purity germanium detectors, exploring potential enhancements and future detection prospects.

Contribution

The paper provides the first experimental limits on $eta^+$EC, ECEC, and $eta^-eta^-$ decay modes in tin isotopes, with discussions on possible decay rate enhancements.

Findings

Limit of $0.92 imes 10^{20}$ years on ECEC(0$ u$) in $^{112}$Sn

Decay half-life limits of $(0.8-1.2) imes 10^{21}$ years for $^{124}$Sn

Discussion on near-degenerate state enhancement effects

Abstract

Limits on $\beta^+$EC and ECEC processes in $^{112}$Sn and on $\beta^-\beta^-$ decay of $^{124}$Sn to the excited states of $^{124}$Te have been obtained using a 380 cm$^3$ HPGe detector and an external source consisting of natural tin. A limit with 90% C.L. on the $^{112}$Sn half-life of $0.92\times 10^{20}$ y for the ECEC(0$\nu$) transition to the $0^+_3$ excited state in $^{112}$Cd (1871.0 keV) has been established. This transition is discussed in the context of a possible enhancement of the decay rate by several orders of magnitude given that the ECEC$(0\nu)$ process is nearly degenerate with an excited state in the daughter nuclide. Prospects for investigating such a process in future experiments are discussed. The $\beta^-\beta^-$ decay limits for $^{124}$Sn to the excited states of $^{124}$Te were obtained on the level of $(0.8-1.2)\times 10^{21}$ y at the 90% C.L.