R.G. Allas

Radiative capture of protons by B11 and the giant dipole resonance in C12

Abstract The radiative capture of protons by B11 has been studied in the energy range 4 MeV ≦ Ep ≦ 14 MeV. The ground-state gamma-ray yield follows the giant resonance observed in photonuclear studies with some structure apparent. The gamma ray to the first excited state also shows a giant resonance whose centre is displaced upward by about 4 MeV from the ground-state giant resonance. Considerable structure is present in the giant resonance built upon the first excited state. For both gamma rays, the angular distributions vary but little with energy-they are peaked at 90° for the ground-state transition and nearly isotropic for the excited-state transition. For both gamma rays, the distributions exhibit a positive cos θ term which increases with increasing energy. The angular distribution data, analysed in terms of Legendre polynomials, are compared with theoretical predictions and shown to be in agreement with the results of particle-hole configuration calculations.

Giant resonances and fine structure in Si28 from the Al27(p,γ)Si28 reaction

Abstract The reaction Al 27 (p,γ)Si 28 has been studied over the energy range 4 MeV E p 0 and γ 1 that appear in th capture spectrum correspond to transitions to the ground state and to the first excited state of Si 28 at 1.78 MeV. A 90° yield curve taken at 15 keV intervals shows that for each gamma ray the cross section fluctuates widely and rapidly about an average which traces out a giant resonance. The γ 1 giant resonance is displaced upward from the γ 0 giant resonance by an energy about equal to the energy of the first excited state of Si 28 . Transitions to the second and/or third excited states of Si 28 are also observed but not resolved from each other. There is evidence that this group (γ 2 +γ 3 ) also displays a giant resonance at a still higher excitation energy. The fluctuations in the cross sections are analysed and shown to resemble Ericson fluctuations. There is also evidence for structure of an intermediate width. The angular distributions, measured at 15 keV intervals over representative energy regions, are relatively constant as compared with the fluctuations in the cross sections. The distributions are approximately W ( θ ) = 1+0.07 P 1 for γ 0 and W ( θ ) = 1+0.10 P 1 –0.50 P 2 –0.10 P 3 for γ 1 . Invariant angular distributions suggest fixed configurations for the giant resonances; thus it is not possible to identify the observed structure with individual particle-hole levels. Possible assignments for the giant resonances are discussed.

Energy levels of Ar36 and Ar38

The energy-level schemes of Ar36 and Ar38 were measured up to excitation energies of 8.3 and 6.9 MeV, respectively. We used the reactions K39(p, α)Ar36 and K41(p, α)Ar38 and studied the α-particke groups leading to the different states in Ar36 and Ar38. We found many more states in both nuclei than were known previously; in particular, closely spaced levels lying less than 100 keV apart are rather numerous in Ar36. Each of these nuclei shows a complicated energy-level scheme which cannot be explained solely by the excitation of the nucleons in the 2s12 and the 1d32 shells.

Radiative capture by 19F: The giant dipole resonances in 20Ne

Abstract The radiative capture of protons by 19 F has been studied over the range of proton bombarding energies from 2.88 MeV to 12.80 MeV. The most intense capture gamma rays are γ 0 to the ground state and γ 1 to the first excited state in 20 Ne. The yield curves for both γ 0 and γ 1 are dominated by the E1 giant resonance with the γ 1 giant resonance displaced upward in excitation energy from the γ 0 giant resonance. Strong, well correlated structure is present in the yield curves; the structure has a characteristic width of about 175 keV. Extensive angular-distribution measurements showed that the angular distributions do not vary greatly with energy. The angular distributions are incompatible with γ 0 and γ 1 emanating from the same levels of the compound nucleus. The integrated yields, structure and angular distributions are compared with the results of other (p, γ) studies of the E1 giant resonance and some of the features of this and other radiative-capture experiments are discussed in terms of the shell model.

Study of the 39K(p, α)36Ar reaction

Abstract The reaction 39 K(p,α) 36 Ar was investigated in the incident proton energy range 10.0–13.0 MeV in steps of 20 keV and 100 keV at eight angles from 25° to 160°. The excitation functions of the α-groups leading to different final states in 36 Ar exhibit strong fluctuations in forward and backward angles. These fluctuations usually are not correlated. The analysis was made in terms of the Ericson statistical model. The coherence with Λ is around 10–15 keV. There are some indications for non-fluctuating contributions (direct interaction). Tentative spin assignments of the first two excited states in 36 Ar are discussed on the basis of the 2 I +1 rule.