L. Meyer-Schützmeister

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.

The 23Na(p, γ)24Mg reaction and the giant dipole resonance in 24Mg☆

Abstract The 23 Na(p, γ) 24 Mg reaction has been studied over the region of excitation corresponding to the giant dipole resonance. Gamma rays to the ground state and to the first and to the second plus third excited states have been observed. The (p, γ 0 ) cross section is on the average smaller than it is in other doubly even nuclei in this region. The (p, γ 1 ) cross section is considerably larger than the (p, γ 0 ) and clearly follows a giant-resonance shape. A part of the giant resonance built upon the second and/or third excited states is also observed. A fluctuation analysis finds the coherence width to be about 75 keV. The angular distributions of both γ 0 and γ 1 show the usual small energy variation that has come to be associated with the giant resonance.

The giant dipole resonance excited by α-capture

Abstract We have studied the reactions 24Mg(α,γ)28Si from E α = 5.3 MeV to 14.5 MeV , 26 Mg(α,γ) 30Si from 4.0 MeV to 13.5 MeV and 28Si(α,γ)32S from 7.0 MeV to 12.0 MeV. These α-energies lead into the region of the giant dipole resonance in the compound nucleus. The yield of γ-rays leading to the ground state of the final nucleus, γ0, was studied for each of the three targets; and for 24Mg(α,γ) the radiation γ1 going to the first excited state was also studied with resonable accuracy. The angular distributions of γ0, which were measured in 100 keV steps over a wide range of energies for the 24Mg and 26Mg targets and at two energies for the 28Si target, showed a dominant electric-dipole transition. This indicates that the (α,γ0) reactions lead predominantly through the giant dipole resonance in 28Si and 30Si and most likely also in 32S. The gamma-ray yield, which was measured in steps of 30 or 100 keV over the energy ranges studied, exhibited strong fluctuations in each case. For α-capture both in 24Mg and 26Mg, statistical analysis of the fluctuations showed that the reactions proceed nearly 100% through compound-nucleus formation and that the average width of the compound-nucleus resonances is about 60 keV. The integrated cross sections of these three alpha-capture reactions led to the conclusion that formation of the giant resonance by alpha capture is strongly inhibited - even in cases in which the alpha capture is isospin allowed. This is expected since the giant dipole resonance is supposed to consist predominantly of particle-hole states. Earlier experiments of proton capture by 27Al had shown that only a small part of the giant dipole resonance in 28Si leaks into the more complicated nucleon configurations of the compound nucleus. The observation that this part of the giant dipole resonance in 28Si decays with comparable strength by α-particle and proton emission indicates that it has a strong admixture of isospin T = 0.

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.

Absolute cross sections for production of prompt nuclear

Cross sections have been measured for the production of prompt nuclear

gamma

gamma

rays by fast pions

rays in the pion bombardment of Al, Ca, V, and

Study of 47Ti with the 45Sc(3He, p)47Ti and 45Sc(3He, pγ)47Ti reactions

sup 60