A Precise Measurement of the Energy Change in the Transmutation of Beryllium into Lithium by Proton Bombardment

Abstract

The very high yield of alpha-particles from the reaction ${}_4{}^{}{}_{}{}^{}\mathrm{Be}_{}^9{}_{}{}^{}(p, \alpha ){}_3{}^{}{}_{}{}^{}\mathrm{Li}_{}^6{}_{}{}^{}$ has made it possible to measure their energies by deviating a beam through 90° in an electrostatic field. A steady source of high voltage, which could be measured to 0.1 percent up to 40,000 volts, was used to deflect the alpha-particles. Two experiments are reported in which the beryllium was bombarded by 320 kv and 383 kv protons, respectively. The energy distribution curves of the alphas show a sharp high energy limit, and this limit increases with bombarding energy of the protons. The experiment shows that the energy balance of the reaction is 2.152±0.04 Mev, corresponding to 2.310±0.04×${10}^{-3\mathrm{}}$ atomic weight unit. This establishes the diffference in mass between ${\mathrm{Be}}^9$ and ${\mathrm{Li}}^6$ as 2.99804±0.00009. If we assume that of the two, the mass of ${\mathrm{Li}}^6$ is more accurately known, and adopt Livingston and Bethe's value of 6.01686±0.00020, the experiment shows that the mass of ${\mathrm{Be}}^9$ is 9.01491±0.00025.