Doctor of Philosophy
Department of Physics
Company, Farhang Zamani, The neutron capture mechanism in some closed shell nuclei, Doctor of Philosophy thesis, Department of Physics, University of Wollongong, 1983. http://ro.uow.edu.au/theses/1639
An investigation of the neutron capture mechanisms in magic nuclei (8 8Sr, 1 3 9La and 1 4 1Pr) and in 3 2 S is reported. Valence capture in 3 2S and 8 8Sr is expected to dominate the capture mechanism in some resonances and partial radiation width measurements are needed for a direct comparison with valence capture calculations. For 1 3 9La and 1 4 1Pr neither valence nor statistical mechanisms are applicable, and a 2p-lh mechanism has been proposed. It is decided to further investigate the capture mechanism in these nuclei through neutron capture gamma-ray measurements.
A well-shielded 20 x 15 cm Nal detector is used with time of flight discrimination of scattered neutron events to obtain gamma-ray spectra for average neutron energies up to 1000 keV. Neutron capture theory is reviewed in Chapter 1, with emphasis on the statistical model of the giant dipole resonance, and the nonstatistical valence and doorway models.
The neutron capture cross sections of the magic nuclei are relatively small. To obtain accurate reduced gamma-ray spectra, a high degree of sophistication is needed in the detector, associated electronics and pulsed Van de Graaff accelerator performance. In Chapter 2 the accelerator is calibrated, special shielding is designed for the Nal detector, the gamma-ray line shape is explained and the effect of self-shielding, multiple scattering and gamma-ray selfattenuation are calculated for the samples.
The absolute number of neutrons in the solid angle subtended by the sample at the source, i.e. the number of neutrons per keV at En, are obtained in Chapter 3. Since the neutron angular distribution is not isotropic, a theoretical expression based on the 7Li(p,n)7Be reaction kinematics is used to calculate the total number of neutrons incident on the sample as a function of total number of neutrons incident on the 6Li neutron monitor.
The valence contribution to the total radiation width of some resonances in 3 2S are strong. The correlation between experimental and calculated partial widths is needed to provide definite evidence for valence capture in both s- and p-wave resonances in 3 2S. In Chapter 4 the partial radiative widths of resonances with large Γ2n and (ΓVγ/Γγ) values atandkeV neutron energies in 3 2S are measured. The observed data are compared with valence calculations and the width correlations are calculated.
The ground and low-lying states of 8 8Sr have been found to be almost pure single particle states in (d,p) studies and strong positive correlations have been observed between the reduced neutron and radiation widths for both p3⁄2 and p½ wave resonances. Hence the valence effect is expected to dominate the neutron capture processes in 8 8Sr, and in Chapter 5 neutron capture gamma-ray spectra are measured at eleven average neutron energies for resonances at 12.4, 29.5, 54.6, 122, 150, 287, 321, 336, 399, 423 and 508 keV in 8 8Sr. The intensity of the primary gamma-rays per neutron capture are calculated. The partial radiation widths and the calculated partial valence widths are compared for the p-wave resonances and the correlation between these widths is calculated.
The total valence radiation width for 1 3 9La and 1 4 1Pr is about sixty times smaller than the average radiation widths of both nuclei, yet their neutron capture gamma-ray spectra show anomalous gamma-ray transitions to the low-lying states. Neither statistical nor valence models alone could account for the anomalous gamma-ray intensities. Since the energies of the anomalous gamma-rays are similar to the energies of unperturbed p-h states for El transitions, a 2p-1h mechanism was suggested to explain the observed data. To further investigate the capture mechanism for these nuclei, detailed gamma-ray spectra are needed.
In Chapter 6 neutron capture gamma-ray spectra are measured at average neutron energies and keV in 1 3 9La and at , , and keV in 1 4 1Pr. The corrected energy level densities are obtained and the reduced intensities are investigated. The data fit well within the framework of the Lorentzian model, and the non-statistical capture mechanism hypothesis is ruled out as a dominant mechanism in these nuclei.
The nuclei used in this work were found to have different neutron capture mechanisms and conclusions are therefore given at the end of the last three chapters.