Bakerian lecture.―The neutron

1. In an earlier paper I showed that the radiations excited in certain light elements by the bombardment of α -particles consist, at least in part, of particles which have a mass about the same as that of the proton but which have no electric charge. These particles, called neutrons, have some very interesting properties. Some of the more striking were described in the paper I have mentioned and in those of Dr. Feather and Mr. Dee which accompanied it. The most obvious properties of the neutron are its ability to set in motion the atoms of matter through which it passes and its great penetrating power. From measurements of the momenta transferred to different atoms the mass of the neutron was estimated and shown to be nearly the same as the mass of the proton, while the penetrating power shows that the neutron can have no nett electric charge. The loss of energy of a neutron in passing through matter is due to the collisions with the atomic nuclei and not with the electrons. The experiments of Dee showed that the primary ionization along the track of a neutron in air was less than 1 ion pair in 3 metres path, while Massey has calculated that it may be as low as 1 ion pair per 105 km. This behaviour is, of course, very different from that of a charged particle such as a proton, which dissipates its energy almost entirely in electron collisions. The contrast between the rate of loss of energy of a proton and a neutron of the same initial velocity is most striking. A proton of velocity 3 X 109 cm./sec. travels about 1 foot in air, while a neutron of the same initial velocity will on the average make a close collision with a nitrogen nucleus only once in 300 to 400 yards’ path and it may a distance of a few miles before losing all its energy. His collision of a neutron with an atomic nucleus, although much more frequent than with an electron, is also a rare event, tor tire electric field between a neutron and a nucleus is small except at distances of the order of 10 -12 cm. In such a close collision the neutron will be defected from its path and the struck nucleus may acquire sufficient energy to produce ions. Thus the nuclei recoiling from encounters with neutrons can be detected by ionisation measurements, using an ionisation chamber with a sensitive electrometer or with an electrical counting apparatus, or by their ionised traces when produced in an expansion chamber. Neutrons can thus be detected only in an indirect way, by the observation of the recoil atoms. For this reason, and also because they are produced as a result of a similar collision process only partly under our control, the study of their properties in detail has proved both difficult and tedious.