Half-Life ofMn53

Abstract

A value of 3.7 × ${10}^6$ yr (± 10%) for the half-life of ${\mathrm{Mn}}^{53}$ was determined by a direct method involving measurement of both the specific activity [(dis/min ${\mathrm{Mn}}^{53}$)/(μg Mn)] and the isotope ratio (${\mathrm{Mn}}^{53}$/${\mathrm{Mn}}^{55}$). The ${\mathrm{Mn}}^{53}$ specific activities were determined by neutron activation, and the ${\mathrm{Mn}}^{55}$/${\mathrm{Mn}}^{53}$ isotopic ratios by mass spectrometry. A surface-ionization method was applied using a rhenium single filament with the sample mounted in a silica-gel layer. This technique gave a stable and enhanced ion beam of ${\mathrm{Mn}}^{+}$. One source of ${\mathrm{Mn}}^{53}$ was an electroplated Cr target bombarded by 15-MeV $\alpha$ particles in a cyclotron. The other source of ${\mathrm{Mn}}^{53}$ was the Mn fractions extracted from iron meteorites by carrier-free processes. Activities of 10 dis/min ${\mathrm{Mn}}^{53}$ and 2000 dis/min ${\mathrm{Mn}}^{54}$ were obtained with about 2 μg of ${\mathrm{Mn}}^{55}$ from the Cr target. The isotopic ratio ${\mathrm{Mn}}^{55}$/${\mathrm{Mn}}^{53}$ was 750. Activities of about 1 (dis/min ${\mathrm{Mn}}^{53}$)/(μg Mn) and isotopic ratios ${\mathrm{Mn}}^{55}$/${\mathrm{Mn}}^{53}$ of 3000-4000 were obtained from the iron-meteorite extracts.