The Isomeric Transition ofPb207as an Energy Standard in Beta Spectroscopy

The $K$-conversion line of the 1.06-Mev isomeric transition following the electron-capture decay of ${\mathrm{Bi}}^{207}$ is suggested as a useful energy standard for beta-spectroscopy. While the 50-year half-life and 10 percent conversion coefficient are comparable to ${\mathrm{Cs}}^{137}$, the 60 percent higher energy, larger percentage $K$ to $L$ momentum separation, and the possibility of making thin and more uniform sources may be advantageous. Precision measurements have been carried out in a double-focusing spectrometer by comparison with the 1.02-Mev electrons of RaC. The ${\mathrm{Pb}}^{207}$ $K$-line has a momentum of 4657.9±1.0 gauss-cm from which an electron energy of 975.9±0.3 kev and a transition energy of 1063.9±0.3 kev are derived. Measurements have also been made on the $\frac{K}{L}$ and $\frac{K}{\mathrm{}(L+M)\mathrm{}}$ ratios, which are 3.95±0.25 and 3.00±0.25 respectively. When compared with data on other $M4\mathrm{}$ transitions, these results indicate that deviations from the empirical $\frac{K}{L}$ curves take place at high $Z$. The usefulness of ${\mathrm{Bi}}^{207}$ as a test source for beta-gamma coincidence detectors is discussed.