deuteron. The order of magnitude for this capture probability can be calculated, in good agreement with the experimental value, on the assumption
that the transition from a free-neutron state to the state in which the neutron
is bound in the deuteron is due to the magnetic dipole moments of the proton and the neutron. The binding energy set free in this process, is emitted
in the form of g-rays, as first observed by Lea.
All the processes of capture of slow neutrons by any nucleus are generally
accompanied by the emission of g-rays : Immediately after the capture of the
neutron, the nucleus remains in a state of high excitation and emits one or
more γ-quanta, before reaching the ground state. The γ-rays emitted by
this process were investigated by Rasetti and by Fleischmann.
Absorption anomalies
A theoretical discussion of the probability of capture of a neutron by a
nucleus, under the assumption that the energy of the neutron is small compared with the differences between neighbouring energy levels in the nucleus, leads to the result that the cross-section for the capture process should
be inversely proportional to the velocity of the neutron. While this result
is in qualitative agreement with the high efficiency of the slow-neutron bombardment observed experimentally, it fails on the other hand to account for
several features of the absorption process, that we are now going to discuss.
If the capture probability of a neutron were inversely proportional to its
velocity, one would expect two different elements to behave in exactly the
same way as absorbers of the slow neutrons, provided the thicknesses of the
two absorbers were conveniently chosen, so as to have equal absorption for
neutrons of a given energy. That the absorption obeys instead more complicated laws, was soon observed by Moon and Tillman and other authors
who showed that the absorption by a given element appears, as a rule, to be
larger when the slow neutrons are detected by means of the activity induced
in the same element. That the simple law of inverse proportionality does not
hold, was also proved by a direct mechanical experiment by Dunning, Pegram, Rasetti, and others in New York.
In the winter of 1935-1936 a systematic investigation of these phenomena
was carried out by Amaldi and myself The result was, that each absorber of
the slow neutrons has one or more characteristic absorption bands, usually
for energies below 100 volts. Besides this or these absorption bands,