Positron Decay ofCo58

Abstract

The allowed positron transition from the Co/sup 58/ ground state (2+) to the Fe/sup 56/ first excited state (2+, 810 kev) was studied using a 4 pi positron-scintillation spectrometer. The experimental shape factor exhibits a small systematic deviation from that theoretically predicted for an allowed transition and corresponds to an excess of low-energy positrons. The experimental shape factor can be fit by a curve proportional to (1 + b/W) with b = 0.3. The FermiKurie plot is linearized by this correction factor and yields an end-point energy of 474 plus or minus 5 kev. Similar discrepancies between the experimental and theoretically predicted shapes of Gamow-Te-ller beta transitions for negatrons (In/sup 114/, P/sup 32/, and Y/sup 90/) and positrons (Na/sup 22/) have been reported. In these cases it was found that this same shape factor with 0.2 < b < 0.4 would linearize the theoretically-corrected Fermi-Kurie plots. No satisfactory explanation for this effect has been offered. As previously suggested by other investigators, the shape factor (1 + b/W) must, for the present, be regarded as a purely empirical correction. The end-point energy from the present investigation combined with the recent high-precision measurement of the gamma-ray transition energy from the first excitedmore » state to the ground state of Fe/sup 58/ yields a Co/sup 58/-Fe/sup 58/ mass difference of 2306 plus or minus 5 kev. (auth)« less