Photoneutron studies in medium-weight elements

Abstract

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 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. Show more