Threshold photoneutron spectra Page: 2 of 20
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THR ESHOLD PHOTONEUTRON SPECTRA
H. E. Jackson. Argonne National Laboratory
Photoneutron spectra in the resolved resonance region near threshold
are reviewed as a tool for the study of the neutron capture reaction mechanism.
Recent measurements of the photodisintegration of the deuteron near threshold
suggest discrepancies with the elementary model of the reaction. Data for
nuclei in the mass region A Z 50-60 contain partial width correlations and
non-resonant reactior components characteristic of channel capture. Other
nuclear structure effects in the form of intermediate structure and partial
width correlations for heavier nuclei are also discussed. Photon strength
functions for El and MI radiation resulting from measurement of threshold
photoneutron spectra are compared with data from other sources and theoretical
predictions. Intense p-wave photoneutron emission by the Pb isotopes indicates
the presence of a magnetic dipole giant resonance whoez integrated strength is
close to the theoretical limit.
To a considerable extent, the continuing interest in threshold photoneutron
reactions is a result of their value as a tool in the spectroscopy of highly excited
nuclear states. However, most of our knowledge in this area has come from
studies of particle induced reactions, and so we quite naturally discuss the
radiative process as a capture reaction and cast our theoretical models in terms
of capture mechanisms. For this reason it is convenient to view photoneutron
emission as "inverse capture" and relate the results to our understanding of the
capture reaction. I would like to discuss this field from that perspective.
Inasmuch as the me-t basic capture reaction is neutron capture in hydrogen, it
is appropriate .o introduce the subject with a brief discussion of the inverse
process, i.e. the photodisintegration of the deuteron in the threshold region.
The main body of the paper will be a review of our present experimental
understanding of threshold photoneutron spectra, with emphasis placed
successively on three topics - reaction mechanisms, El radiative strength,
and the Mi giant dipole resonance.
The simplest and the most basic threshold measurement one can make
is the photodisintegration of the deuteron. At threshold, the cross section is well
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Jackson, H.E. Threshold photoneutron spectra, article, January 1, 1974; Illinois. (https://digital.library.unt.edu/ark:/67531/metadc1021993/m1/2/: accessed March 22, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.