Precision measurement of ${}^{65}$Zn electron-capture decays with the KDK coincidence setup

Abstract

${}^{65}$Zn is a common calibration source, moreover used as a radioactive tracer in medical and biological studies. In many cases, ${\hat{I}}^3$-spectroscopy is a preferred method of ${}^{65}$Zn standardization, which relies directly on the branching ratio of $J\ddot{I}€{(}^{65}\text{Zn})=5/2^{-}\to J\ddot{I}€{(}^{65}\text{Cu})=5/2^{-}$ via electron capture (EC*). We measure the relative intensity of this branch to that proceeding directly to the ground state (EC${}^0$) using a novel coincidence technique, finding $I_{{\text{EC}}^0}/I_{\text{EC*}}=0.9684\pm 0.0018$. Re-evaluating the decay scheme of ${}^{65}$Zn by adopting the commonly evaluated branching ratio of $I_{β^+}= 1.4271(7)\%$ we obtain $I_{\text{EC*}}=(50.08\pm 0.06)\%$, and $I_\text{EC^0} = (48.50 \pm 0.06) \%$. The associated 1115 keV gamma intensity agrees with the previously reported NNDC value, and is now accessible with a factor of ~2 increase in precision. Our re-evaluation removes reliance on the deduction of this gamma intensity from numerous measurements, some of which disagree and depend directly on total activity determination. The KDK experimental technique provides a new avenue for verification or updates to the decay scheme of ${}^{65}$Zn, and is applicable to other isotopes.

Uses similar methodology to the 40K measurement by the KDK Collaboration (Stukel et al PRL 2023, arXiv:2211.10319; Hariasz et al PRC 2023, arXiv:2211.10343), as such there may be some similarity in figures and text