Taylor, Raymond Booth
Description
In 1934 Chadwick and Goldhaber (Ch35) disintegrated the
deuteron using the 2.6 MeV gamma rays from ThC". This was
9
followed by the disintegration of Be by Szilard and Chalmers
(3z34). The early work was limited to using gamma rays from
naturally occurring radioactive sources and it was not until high
energy electron accelerators were developed that it became
possible to investigate the systematics of nuclear photodisintegrations.
The problem of accurately determining...[Show more] photonuclear
cross sections is complicated by the use of bremsstrahlung
radiation which makes the data much more difficult to interpret
than the data obtained using monochromatic gamma rays. However,
recent improvements in the energy stability of electron accelerators,
now reported as low as 5 keV (Go54), together with better
knowledge of the bremsstrahlung spectrum, have made it possible
to measure photonuclear cross sections with greater accuracy.
Also the problem of producing a monochromatic variable energy Y-ray
is being investigated (Mi59) and the results are encouraging.
A recent measurement (Se60) produced cross sections which were
very similar to other results obtained by using the conventional
bremsstrahlung radiation. With the advent of suitable monochromatic
gamma rays it will be possible to make a detailed investigation of the region above the giant resonance, where at present any
resonance structure is masked by the yield from the giant
resonance. The theoretical. approach to nuclear photodisintegration
has been analogous to that of the atomic photoeffect and it has
been possible to explain the main features (of the nuclear photoeffect)
by using non-relativistic wave functions. Although it has
been realised that meson fields must play a part in the explanations,
Siegert (Si37) has shown that they can be ignored in the giant
resonance. At high energies it is no longer possible to use nonrelativistic
wave functions and here a different approach must be
made to the problem. So far the theoretical predictions have been
limited to the gross features, such as the position of the giant
resonance, its width, magnitude and, in the case of deformed nuclei,
the splitting of the giant resonance.
The remainder of this chapter gives a brief outline of the
present state of photodisintegration theory and how it fits the
experimental facts. The discussion deals mainly with the medium
and heavy nuclei and with photon energies below the meson threshold.
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