Precision measurement of 65Zn electron-capture decays with the KDK coincidence setup

Abstract

⁶⁵Zn is a common calibration source, moreover used as a radioactive tracer in medical and biological studies. In many cases, γ-spectroscopy is a preferred method of ⁶⁵Zn standardization, which relies directly on the branching ratio of Jπ(⁶⁵Zn) = 5/2⁻ → Jπ(⁶⁵Cu) = 5/2⁻ via electron capture (EC*). We measure the relative intensity of this branch to that proceeding directly to the ground state (EC⁰) using a novel coincidence technique, finding I_EC0/IEC* = 0.9684 ± 0.0018. Re-evaluating the decay scheme of ⁶⁵Zn by adopting the commonly evaluated branching ratio of I_β+ = 1.4271(7)% we obtain IEC* = (50.08 ± 0.06)%, and I_EC0 = (48.50 ± 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 ⁶⁵Zn, and is applicable to other isotopes.