Measurements have been made of the cross-sections for the reactions ⁷Li(γ, p₀)⁶He and ⁷Li(γ, n₀+n₂²)⁶Li, by detection of ⁶Li and
⁶He ions ejected from thin lithium foils irradiated with electron
bremsstrahlung. This technique ensures that only events from two-body
breakup are recorded and enables the energy of the photon responsible for a particular event to be determined from the reaction kinematics. Cross -sections are presented for photon energies from 40
to 140 MeV and for nucleon emission angles of 24°, 50°, 78°, 110° and
144° in the centre of mass frame,and the ratios of the cross-sections
for the two reactions are obtained free from systematic errors. The
general features of the data are: i) angular distributions which are forward peaked for both reactions, this being more pronounced for
(γ, p₀), ii) cross -sections which decrease exponentially with photon
energy,with a little structure in the (γ, n₀ +n₂) energy spectra at
central angles, iii) cross-section ratios of the order of unity, with
the (γ, n₀+n₂) cross -sections enhanced over the (
γ, P₀) cross-sections at backward angles and higher photon energies.
Comparison with the predictions of the direct knockout model
indicate that the (γ, p₀) cross-sections may be explicable assuming
no other process contributes; a consistent 'nucleon momentum distribution' is obtained from the data and the shape of the angular distributions and the energy spectrum at 50° are reproduced by the available
calculations. There is considerable disagreement, however, between
the various calculations over the absolute magnitude of the cross-sections
predicted by the model. Comparison with the data suggests that the
direct knockout model is not able to account for the observed (γ, n)
cross-sections; in particular the forward peaking of the angular distributions. However,a simple PWIA calculation of the (γ, n) / (γ, p)
ratio indicates that the disagreement between this model and such data
is not as great as has previously been suggested.
A modified quasi-deuteron model is found to correctly predict the
general shape of both the energy spectra and the angular distributions
of the two reactions and the magnitude and variation with angle of the cross -section ratios. It fails to reproduce the variation of this
ratio with photon energy and the structure in the (y,n) energy spectra.
It is shown that these failures are, at least, partly due to approximations made in the model to simplify computation.
Shortcomings and problems which exist in both of the above models
and the applicability of other models which include nucleon-nucleon
correlation effects are discussed. Areas requiring further theoretical
and experimental study are highlighted.