The Energy Spectra of the Neutrons from the Disintegration of Fluorine, Boron and Beryllium by Alpha-Particles

By using an expansion chamber filled with hydrogen or methane at a pressure of about 15 atmospheres, range-distribution curves have been obtained of the recoil protons from Po-F, Po-B, and Po-Be neutrons. These curves revealed the presence of numerous groups of neutrons in the radiation from the three sources. In the case of Po-F five neutron groups were found. These were fitted into a disintegration scheme, made by assuming that the $\alpha$-particle enters the nucleus through resonance levels and by using the energies of these levels, which are known from the disintegration of fluorine with proton emission. For Po-B eight groups were detected, but it was not possible to arrange a disintegration scheme. The maximum energy of the neutrons from Po-B was found to be 4.2 M.E.V., a value considerably higher than the previously accepted figure, 3.3 M.E.V. This value decreases the mass of the neutron calculated from the disintegration of ${\mathrm{B}}^{11}$ from 1.0067 to 1.0058. It is suggested that it is possible to explain the high value, 1.0098, obtained by Curie and Joliot from the disintegration of ${\mathrm{B}}^{10}$, by assuming that both ${\mathrm{B}}^{11}$ and ${\mathrm{B}}^{10}$ emit neutrons. If the maximum energy of the neutrons from ${\mathrm{B}}^{10}$ were about 7.5 M.E.V. it would bring this mass into agreement. Reasons are given for believing that at least 10 times as many neutrons come from ${\mathrm{B}}^{11}$ as from ${\mathrm{B}}^{10}$ so that there may be a few as yet undetected neutrons from ${\mathrm{B}}^{10}$ extending up to this energy. About 22 groups were found in the neutron emission of Po-Be. A possible scheme of disintegration is given.