Abstract
The number of K-shell vacancies per positron has been measured for the beta + decay branch of 64Cu by using a fast triple coincidence, between the annihilation quanta and the Ni X-rays, to gate the output of a high-resolution Si(Li) detector. The result is in excellent agreement with work of G. Schupp and M.S. Freedman (1980) and is some 50% higher than the prediction of the theory of J. Law and J.L. Campbell (1976).