New Limits on Naturally Occurring Electron Capture of 123Te

TL;DR

This study sets new experimental lower limits on the half-life of electron capture in 123Te, using underground thermal detectors, and clarifies previous ambiguous signals, indicating a strong suppression of nuclear matrix elements.

Contribution

It provides the first stringent lower bounds on the half-life of 123Te electron capture from the K shell based on underground detection methods.

Findings

No evidence for E.C. of 123Te from the K shell with a half-life > 5×10^19 years.

Previous signals attributed to 123Te E.C. are likely due to neutron activation of 121Te.

Theoretical interpretation suggests a strong suppression of nuclear matrix elements.

Abstract

Electron capture of 123Te from the K shell has been investigated in a new underground search with an array of 340 g TeO_2 thermal detectors. We find that some previous indication of this decay could be attributed to E.C. of 121Te resulting from neutron activation of natural Tellurium. There is therefore so far no evidence for E.C. of 123Te from the K shell with a 90 % c.l. lower limit t_{1/2}^K > 5.E19 years on the half lifetime. Taking into account the predicted K E.C. branching ratio, the corresponding lower limit on the 123Te EC half lifetime is t_{1/2} > 9.2E16 y, which can be theoretically interpreted only on the basis of a strong suppression of the nuclear matrix elements. A complementary analysis based on the expected fraction of E.C. accompanied by internal bremsstrahlung is discussed.