K-capture branch in the decay of ^113mIn

Abstract

^113mIn is known to decay with a 99-min half-life predominantly by an M4 transition of 393 keV to ¹¹³In. We find that it also decays by a weak K-capture branch of (0.07±0.01)% to ¹¹³Cd. Using a high-resolution Ge(Li) detector (600 eV for the ⁵⁵Fe K X ray) and ^113mIn sources we have carried out careful experiments which reveal the presence of Cd K X rays with an intensity of 0.0024 relative to In K X rays. Supporting evidence comes from the (CdKα)/(CdKβ) ratio of 4±2, the half-life, and the observation that chemical treatment to remove Cd impurity that might be fluorescence-excited by the In Kβ X rays did not affect the Cd K X-ray intensity. The measured intensity corresponds to a logft of 5.1±0.3. This is rather low but not unusual for a first-forbidden transition. Analysis of the effects of backscattering and collimation was crucial to the success of the experiment. The significance of the result lies in the fact that the inverse reaction ¹¹³Cd+ν→^113mIn+e⁻→¹¹³In+393 keV+e⁻ could serve as a possible low-threshold detector for low-energy neutrinos (<100 keV).