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Title: Aspects of neutron emission following the charged particle bombardment of nuclei
Author: Wakefield, Bruce
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 1967
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A study of the nuclei Be6 , Be9 and Cl33 has been made. These nuclei hare been investigated by observing the neutrons emitted during or after their formation by charged induced reactions. The neutron energies have been measured by the "time of flight" technique, using both the pulsed beams and associated particle methods. The first chapter contains an outline of the Distorted Wave Born Approximation and the application of this theory to the calculation of the angular distributions of neutrons emitted from a (p,n) reaction, which is treated as a special case of inelastic scattering, and also from the single and double stripping reactions (d,n) and (He3, n). In Chapter II is a description of the pulsed beam and associated particle time of flight techniques. A detailed account of the design, construction and mode of operation of the beam pulsing device used for the (p,n) experiments is given. Also included is a description of the neutron detector used together with an account of a method of pulse shape discrimination against gamma rays. The reaction L16(p,n)Be6 is reported in Chapter III. The reaction has been studied using the pulsed beam method with proton energies ranging from 7 MeV to 18 MeV. At all incident energies the ground state of Be was excited. For incident energies between 10.0 arid 11.3 MeV there was evidence for an excited state in Be6 at 1.7 MeV. Measurements with proton energies between 15.0 and 18.0 MeV confirmed this. The excitation and width of the first excited state has been found to be :-Ex = 1.67 ± 0.05 MeV; Γ = 1.18 ± O.07 MeV. At incident energies of 16.55 and 18.0 MeV evidence has also been found for a second excited state in Be6 at an excitation of about 3 MeV. Angular distributions for the ground state neutrons have been measured at incident energies of 8.3, 9.0, 9.6, 10.0, 10.5, 15.0, 16.55 and 18.0 MeV, and for the first excited state at 15.0 MeV. The angular distributions measured at 10.5 MeV and below could be fitted very well by a series of the first three Legendre polynomials, and have been interpreted in terms of compound nucleus theory. It is concluded that for these energies the reaction proceeds through two interfering states in Be7 at about 10 MeV excitation with spine of ½+ and 32. An attempt has been made to explain the results obtained at 15.0 MeV and above in terms of a direct reaction theory. The direct reaction theory however gave no semblance of a fit. Calculations have been made in terms of pure phase space considerations in order to explain the nature of the neutron continuum seen in the reaction. It is concluded that the continuum arises chiefly from the four body break-up reaction Li6 + p → He4 + p + p + n with a small contribution from the reaction Li6 + p → Li5 + p + n . An experiment performed to study the neutron decay of excited states in Be9 populated by the beta decay of Li9 is reported in Chapter IV. The neutron energies were measured by the associated particle time of flight technique. It has been found from this experiment that the beta decay of Li9 populates not only the ground state and 2.43 MeV state of Be9 , but also states at 3.35 and 4.6 MeV. These two states, which have not previously been observed, have been given spin assignments of ½ and 32. Assuming that the beta decay branching ratio of Li9 to the 2.43 MeV state in Be9 is 20 per cent, the branching ratios to the 3.35 and 4.6 MeV states is found to be (7 ± 4) per cent and > 1.5 per cent, compared to shell model predictions of (10 ± 2} per cent and (12 ± 3) per cent. Finally, in Chapter V is reported the reaction P31 (Me3, n)Cl33. This experiment was performed to measure the spin of the first T = 32 state in Cl33 . From the angular distribution of the neutrons, which is characteristic of an L = 0 angular momentum transfer, the spin of the analogue state has been found to be ½+. In addition tentative spin assignments of 32+ and ½+ have been given to two other states in Cl33 at 6.30 and 7.45 MeV respectively.
Supervisor: Macefield, B. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Neutron irradiation ; Deuteron reactions ; Nuclear spin