Positron Decay ofCo58

The allowed positron transition from the ${\mathrm{Co}}^{58}$ ground state (2+) to the ${\mathrm{Fe}}^{58}$ 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 ($1+\frac{b}{W}$) with $b=0.3\mathrm{}$. The Fermi-Kurie plot is linearized by this correction factor and yields and end-point energy of 474±5 keV. Similar discrepancies between the experimental and theoretically predicted shapes of Gamow-Teller beta transitions for negatrons (${\mathrm{In}}^{114}$, ${\mathrm{P}}^{32}$, and ${\mathrm{Y}}^{90}$) and positrons (${\mathrm{Na}}^{22}$) have been reported. In these cases it was found that this same shape factor with $0.2<b<\mathrm{}0.4\mathrm{}$ 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 ($1+\frac{b}{W}$) 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 excited state to the ground state of ${\mathrm{Fe}}^{58}$ yields a ${\mathrm{Co}}^{58}$-${\mathrm{Fe}}^{58}$ mass difference of 2306±5 keV.