Threshold photoneutron spectra Page: 4 of 20
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S...---- r-- - - - - energy spectrum by neutron
time-or-flight at 900 and 1350
0(y.nOp 3 to the incident photon beam.
- -Our results are shown in Fig. 2,
where they are compared with
b I theoretical prediction and
,. previously measured values.
- must emphasize that these are
o. a- 0.2 - '.# .4 O--_ _ our first results and that the
Er- E , (MeV) measurement is a continuing.
Fig. 2. The observed ratio of the photomagnetic The data shown is the ratio,
r /N obtainedby assuniing
to photoelectric cross section of the deutron m e y g
near threshold. Open circles are older data the elementary model
(see ref. 2). of the deuteron. The shaded
region in the figure indicates
the values implied by a reasonable choice of effective range parameters. While
the points at higher energy are in reasonable agreement with the older measure --
ments, deviations from the theoretical values are quite clear-values are high at
higher energies and too low near threshold. While it is too early to
reach any definitive conclusion, it is of interest to inquire into possible
explanations for anomalies in the angular distribution of the photoneutron. The
data suggest that the assumption of an angular distribution of the form eq. 1 is
not valid. Certainly any final state interaction leading to spin mixing would
violate this assumption and lead to an interference term which could account for
the angular distribution we observe. Alternatively, such mixing could be
produced by a momentum dependent componer in the nucleon potential, as has
been suggested recently by Breit and co-workers. In either case, the resulting
interference would explain the long standing discrepancies in the threshold region
in the reported values of the relative strength of the photomagnetic and photo-
electric disintegration and necessitate a re-analysis of the data in terms o- a
more complex angular distribution. It is clear that future measurements of
angular distribution should cover such a range as to place definite limits on the
existence of an interference asymmetry around 900.
In contrast to the deuteron, the photoneutron spectra from complex nuclei
are complicated by the existence of many exit channels leading to residual excited
states in the daughter nucleus. Figure 3 shows schematically the total absorption
cross section for a typical medium mass nucleus. Close to threshold, where
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Jackson, H.E. Threshold photoneutron spectra, article, January 1, 1974; Illinois. (digital.library.unt.edu/ark:/67531/metadc1021993/m1/4/: accessed August 14, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.