Threshold photoneutron spectra Page: 3 of 20
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established through measurements of the inverse n-p capture cross section,
and above energies of 2.5 MeV measurements using a radioactive monochromatic
photon sources have been made. The energy region between these points continues
to be of interest because of an apparent enhancement of the n-p cross section by
about 10% at thermal energies and unresolved discrepancies between the observed
and theoretical angular distributions of the photonucleons. Riska and Browni)
have recently demonstrated that the 10% discrepancy in the thermal value can be
accounted for in terms of the effect of meson exchange currents.
In the elementary theory of the deuteron, the photodisintegration process
is assumed to proceed solely through 3S I magnetic dipole and the 3 3
electric dipole transitions. In the threshold region, the amplitudes for these
processes can be calculated accurately using the zero range central force
approximation. Typical results are shown in Fig. 1. Very close to threshold
2.2 2.4 2.6
Fig. 1. Photoelectric and photomagnet
cross sections of the deutron near threat
electric cross sections in the ration a-
the photomagnetic process
dominates, but very quickly
the photoelectric reaction
becomes the major reaction
mode. Because the singlet
and triplet states do not
interfere in this approximation,
the angular distribution of the
photoneutron is given by:
= a i b sin2
where 0 is the angle between
the photoneutron and the incident
photon in the center of mass
ic system. The constants a and b
;hold. are directly related to the total
total photomagnetic and photo-
/a = 3a/2b. Deviations from the simple
model which result in departures from the angular distribution of eq. 1 would be
expected to be most pronounced in the region where a and e are comparable,
i.e. about 100-200 keV above threshold. We are currently studying the photo-
disintegration reaction at ANL in the threshold region by irradiating a deuterium
target with low energy pulsed bremsstrahlung and measuring the photoneutron
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Jackson, H.E. Threshold photoneutron spectra, article, January 1, 1974; Illinois. (https://digital.library.unt.edu/ark:/67531/metadc1021993/m1/3/: accessed March 21, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.