Coote, Graeme Ernest

### Description

This thesis describes four experiments on photoneutron
reactions in medium-weight nuclei, the first two carried out
with gamma radiation from (p,Y) reactions and the third and
fourth with bremsstrahlung from a 33 MeV electron synchrotron.
In chapter 1 is presented a brief review of photonuclear
reactions in the giant resonance region. Discussion is mainly
devoted to photon absorption, models for the giant resonance
and decay of the excited nucleus. Recent developments in
photonuclear...[Show more] theory are treated in more detail. Chapter 2 describes a study, by the activation method, of
the (Y,n) excitation function of Cu63 , made with gamma rays of
energies 12.2, 14.8, 16.7 and 17.6 MeV from the Li7(p,Y) and
B11 (p,y ) reactions. Residual activity and gamma ray intensity
were measured with NaI(Tl) scintillation spectrometers. The
object was to check the shape and absolute normalization of
this excitation function, which is often used as a standard
but about which conflicting reports have been made. The shape
of the curve obtained in this way is in good agreement with
bremsstrahlung measurements, but the present absolute cross
sections are about 30% lower. The discrepancy is attributed to difficulties in absolute ß-counting and in measurement
of X-ray intensities.
An extension of this work is described in chapter 3*
It consisted of measurement, with the Li (p,Y) resonance
radiation, of the (y,n) cross sections of four other nuclei
63
relative to that of Cu63. Comparison with bremsstrahlung
data is made using the experimental intensity ratio of the
14.8 and 17.6 MeV components of the radiation. It is shown
that early measurements using ß-counting of thick samples were
sometimes in error by as much as a factor of 2, but that
recent experiments utilizing 4^ ß-counting of thin samples
are much more accurate. In chapter 4 is described a measurement of competition
between the (Y,n) and (Y,2n) reactions in Ga69. From
determination of the yield curves by activation methods the
excitation functions 01 and 02 for these reactions were
obtained from their thresholds to 31 MeV. The absorption
cross section 01 + 02 has its maximum at about 16.2 MeV and
a width P of 9 MeV; both results are consistent with the
knovm systematics of the photonuclear process. The total
integrated cross section to 3O MeV is 2.2 +- 0.4 times that
of the Cu 63(Y,n) reaction. Determination of the energy at
which 01 and 02 are equal shows that for excitation by
69
gamma rays of 25 MeV the first neutron emitted from Ga69 a median energy of - 5.5 MeV, twice as high as can be
explained by evaporation from the excited nucleus. It is
deduced that at this gamma ray energy at least 40% of the
first neutrons are emitted in a direct interaction with the
photon. Further aspects of the results show tentatively
that a recent suggestion, to the effect that even in a direct
interaction an exchange energy of - 7.5 MeV is left in the
nucleus, may not always be true. The final experiment (chapter 5) consisted of the
measurement, for 15 nuclei, of the isomeric ratio in the
residual nucleus, after a (Y,n) reaction initiated by 30 MeV
bremsstrahlung. From these ratios and a recent application
of statistical theory, was deduced information on the
dispersion parameter a, which relates the density of energy
levels of spin J to that of levels with J = 0, and is closely
related to the moment of inertia of the excited nucleus.
Comparison is made with the corresponding quantity 0s derived
from shell theory. The ratio 0/0 is close to unity for A <40
and levels off at - 0.6 for A >100, with a transition region
in between. The moment of inertia of an excited nucleus (with
average excitation energy 4 - 5 MeV), is therefore close to the
shell model result for a light nucleus and falls to about 1/3
of this in a heavy element. These results are discussed in
terms of nuclear structure theory and some further experiments
are suggested.

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