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

Zn-65 is a common calibration source, moreover used as a radioactive tracer in medical and bi-ological studies. In many cases, gamma-spectroscopy is a preferred method of Zn-65 standardization, which relies directly on the branching ratio of J pi(Zn-65) = 5/2(-) -> J pi(Cu-65) = 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 Zn-65 by adopting the commonly evaluated branching ratio of I-beta+ = 1.4271(7)% we obtain I-EC* = (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 similar to 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 Zn-65, and is applicable to other isotopes.