More than 50 years ago, in 1934, Chadwick and Goldhaber (ChG 34)
published a paper entitled "A 'Nuclear Photo-effect': Disintegration of
the Diplon by -y-Rays."l in the introduction: They noted "By analogy
with the excitation and ionisation of atoms by light, one might expect
that any complex nucleus should be excited or 'ionised', that is,
disintegrated, by -y-rays of suitable energy", and furthermore: "Heavy
hydrogen was chosen as the element first to be examined, because the
diplon has a small mass defect and also because it is the simplest of
all nuclear systems and its properties are as important in nuclear
theory as the hydrogen is in atomic theory". Almost at the same time, in
1935, the first theoretical paper on the photodisinte- gration of the
deuteron entitled "Quantum theory of the diplon" by Bethe and Peierls
(BeP 35) appeared. It is not without significance that these two papers
mark the be- ginning of photonuclear physics in general and emphasize in
particular the special role the two-body system has played in nuclear
physics since then and still plays. A steady flow of experimental and
theoretical papers on deuteron photo disintegration and its inverse
reaction, n-p capture, shows the continuing interest in this fundamental
process (see fig. 1.1).