Threshold photoneutron spectra Page: 5 of 20
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Fig. 3. Schematic plot of the photo-
absorption cross section in the region
above neutron threshold for a nucleus
with A = 60.
by neutron emission only through transitions to the ground state of the daughter
nucleus. In the typical exper-
iment nanosecond pulses of
bremsstrahlung are used to
excite the levels of interest
and the resonance structure
is observed with extremely
high resolution characteristic
of neutron time-of flight
measurements. A typical
time spectrum is shown in
Fig. 4 for a nucleus which
will be discussed later.
In such a spectrum the
resonance yield is
proportional to the product
r r wheree r is the
yo n Yo.
ground state radiation width.
3000g- Ni(y,nd Ni
N ~ E EeEh1, 7 IOMaV
oo 20 300 400 500 600
7f0- t L" 10. J
700 900 100 300
Fig. 4 Photoneutron time-of-flight spectrum
only a few particle channels are
open, the total resonance widths
are much less than the resonance
spacings and the structure in
the photoabsorption cross section
can be interpreted in terms of
individual highly excited nuclear
states. The key to measure-
ments in the threshold region is
the use of bremistrahlung
radiation with a precisely
defined end point energy to excite
these states. For example, if
(EY - Eth~max is lessa than the
first excited state of the daughter
nucleus, photoexcitation is
limited to a small band of excited
levels just above the neutron
binding energy, which can decay
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Jackson, H.E. Threshold photoneutron spectra, article, January 1, 1974; Illinois. (https://digital.library.unt.edu/ark:/67531/metadc1021993/m1/5/: accessed March 21, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.