Possible Existence of a Neutron
Nature, p. 312 (Feb. 27, 1932)
It has been shown by Bothe and others that beryllium when bombarded by α-particles of polonium emits a radiation
of great penetrating power, which has been an absorption coefficient in lead of about 0.3 (cm)¯1. Recently Mme.
Curie-Joliot and M. Joliot found, when measuring the ionisation produced by this beryllium radiation in a vessel with
a thin window, that the ionisation increased when matter containing hydrogen was placed in front of the window. The
effect appeared to be due to the ejection of protons with velocities up to a maximum of nearly 3 x 109 cm. per sec.
They suggested that the transference of energy to the proton was by a process similar to the Compton effect, and estimated
that the beryllium radiation had a quantum energy of 50 x 106 electron volts.
I have made some experiments using the valve counter to examine the properties of this radiation excited in beryllium.
The valve counter consists of a small ionisation chamber connected to an amplifier, and the sudden production
of ions by the entry of a particle, such as a proton or α-particle, is recorded by the deflexion of an oscillograph. These
experiments have shown that the radiation ejects particles from hydrogen, helium, lithium, beryllium, carbon, air, and
argon. The particles ejected from hydrogen behave, as regards range and ionising power, like protons with speeds up
to about 3.2 x 109 cm. per sec. The particles from the other elements have a large ionising power, and appear to be in
each case recoil atoms of the elements.
If we ascribe the ejection of the proton to a Compton recoil from a quantum of 52 x 106 electron volts, then the nitrogen
recoil atom arising by a similar process should have an energy not greater than about 400,000 volts, should produce
not more than about 10,000 ions, and have a range in air at N.T.P. of about 1.3 mm. Actually, some of the recoil
atoms in nitrogen produce at least 30,000 ions. In collaboration with Dr. Feather, I have observed the recoil atoms in
an expansion chamber, and their range, estimated visually, was sometimes as much as 3 mm at N.T.P.
These results, and others I have obtained in the course of the work, are very difficult to explain on the assumption that
the radiation from beryllium is a quantum radiation, if energy and momentum are to be conserved in the collisions.
The difficulties disappear, however, if it be assumed that the radiation consists of particles of mass 1 and charge 0, or
neutrons. The capture of the α-particle by the Be9 nucleus may be supposed to result in the formation of a C12
nucleus and the emission of the neutron. From the energy relations of this process the velocity of the neutron emitted
in the forward direction may well be about 3 x 109 cm. per sec. The collisions of the neutron with the atoms through
which it passes give rise to the recoil atoms, and the observed energies of the recoil atoms are in fair agreement with
this view. Moreover, I have observed that the protons ejected from hydrogen by the radiation emitted in the opposite
direction to that of the exciting α-particle appear to have a much smaller range than those ejected by the forward radiation.
This again receives a simple explanation of the neutron hypothesis.
If it be supposed that the radiation consists of quanta, then the capture of the α-particle by the Be9 nucleus will form
a C13 nucleus. The mass defect of C13 is known with sufficient accuracy to show that the energy of the quantum emitted
in this process cannot be greater than about 14 x 106 volts. It is difficult to make such a quantum responsible for
the effects observed.
It is to be expected that many of the effects of a neutron in passing through matter should resemble those of a quantum
of high energy, and it is not easy to reach the final decision between the two hypotheses. Up to the present, all the
evidence is in favour of the neutron, while the quantum hypothesis can only be upheld if the conservation of energy
and momentum be relinquished at some point.
Cambridge, Feb. 17.