Recent absolute measurements of the ${\mathrm{Po}}^{210}$ alpha-particle energy disagree with the older value used as the standard for many nuclear reaction energy measurements. A new comparison with the ${\mathrm{Li}}^7(p, n){\mathrm{Be}}^7$ reaction threshold energy was made using the Notre Dame electrostatic accelerator and broad-range broad-range spectrograph. Four separate methods of comparison were used. In the first three the threshold was run and then protons or deuterons were scattered from appropriate targets so that the scattered group was recorded on the spectrograph plate near the alpha group from a source placed at the target position. First, the spectrograph and, second, the beam analyzer were used to compare particle momenta. Third, with both fields held constant after the threshold was run with the molecular beam, deuterons were scattered, giving particles of the same $B_{\rho }$ as the alphas. In the fourth method several reaction energies that are precisely known in terms of the ${\mathrm{Li}}^7(p, n){\mathrm{Be}}^7$ reaction threshold energy were measured in terms of the ${\mathrm{Po}}^{210}$ alpha-particle energy. These were the ${\mathrm{Mg}}^{24}(d, d^{\prime }){\mathrm{Mg}}^{24*}$ reaction to the first excited state of ${\mathrm{Mg}}^{24}$ and the ${\mathrm{N}}^{14}(d, p){\mathrm{N}}^{15}$ reaction leading to three excited states of ${\mathrm{N}}^{15}$. The four measurements agree and give 5.3086±0.003 Mev for ${\mathrm{Po}}^{210}$ alpha-particle energy based on 1.8811 Mev for the ${\mathrm{Li}}^7(p, n){\mathrm{Be}}^7$ reaction threshold energy.

• Received 17 June 1960

DOI:https://doi.org/10.1103/PhysRev.120.905