Half-life of ${}^{51}\mathrm{Mn}$

The half-life of ${}^{51}\mathrm{Mn}$ was measured by serial gamma spectrometry of the 511-keV annihilation photon following decay by ${\beta }^{+}$ emission. Data were collected every 100 seconds for 100,000–230,000 seconds within each measurement ($n=4$). The 511-keV incidence rate was calculated from the 511-keV spectral peak area and count duration, corrected for detector dead time and radioactive decay. Least-squares regression analysis was used to determine the half-life of ${}^{51}\mathrm{Mn}$ while accounting for the presence of background contaminants, notably ${}^{55}\mathrm{Co}$. The result was $45.59\pm 0.07$ min, which is the highest precision measurement to date and disagrees with the current Nuclear Data Sheets value by over $6\sigma$.

Figure 1

Example HPGe gamma spectrum of ${}^{51}\mathrm{Mn}$ with 3600-s. count duration.

Figure 2

Decay curve fitting for one representative measurement and associated fitting residuals. Serial spectroscopic measurements were acquired over approximately 230,000 s and the 511-keV peak fit by a two-component exponential and linear component decay curve. The right plot shows a subset of the full data set, over one decade of ${}^{51}\mathrm{Mn}$ decay, indicated by the rectangle in the left plot.