Haruo Ui

Exchange effect in the scattering of an α-particle by light nuclei

Abstract Exchange effects are discussed in the elastic and inelastic scattering of an α-particle by light nuclei. It is pointed out that, in addition to the well-known exchange scattering, there is another sort of exchange effect which corresponds to the heavy-particle-stripping in a nuclear rearrangement collision. This process is shown to play an important role in the angular distribution in the backward direction when the structures of both the target and residual nuclei are amenable to the α-cluster model. In the cut-off Born approximation the cross section for the process is presented in such a form as to contain no energy-dependent adjustable parameters, by making use of the technique developed in our previous papers. The elastic scattering of α-particles by C12 is analysed in the energy region between Eα = 15 MeV and 42 MeV; it is found that the very sharp backward peaking observed by Wong and Bleuler and by Corelli, Bleuler and Tendam in the angular distribution at about Eα = 18 MeV can be satisfactorily reproduced by the calculation. The reduced width of the α-particle is determined for the ground state of C12. The cluster structure of C12 is discussed. It is suggested that the ground state of C12 should be a mixture of the fourth order oscillation 3s, 2d and 1g states in the Be8-α-cluster representation, the mixing ratio of the 3s state being at most 1 2 . Several experiments are discussed which could confirm the reaction mechanism leading to backward peaking.

Nuclear direct interaction in the C12(t, α)B11 reaction

Abstract By taking into account in detail the cluster structure of C12 and B11, the angular distributions of α particles leading to the ground and first three excited states of B11 from the C12(t, α)B11 reaction at Et=10.06 MeV are treated by the direct-interaction, i.e. the heavy-particle-stripping, pick-up and knock-on reactions. Expressions for the differential cross sections which have been derived previously by the cut-off Born approximation are adopted. The spectroscopic factors contained in all the relevant reduced widths are calculated by employing the harmonic oscillator shell model in the LS coupling scheme, viz. the α particle and proton reduced widths for the ground state of C12 and the triton reduced widths for the first four low-lying levels of B11. It is pointed out that three states in Be8, viz. p4[4]11S, 11D and 11G are equally important in the wave function of the ground state of C12 when decomposed into the Be8-α cluster representation and that these states contribute coherently to the heavy-particle-stripping reaction as the core. It is shown that the angular distributions at large angles can be well reproduced by the theory of the heavy-particle-stripping reaction, provided the nuclear cluster structure is correctly taken into account. Further, the angular distributions at small angles are found to be well explained by the pick-up and knock-on reactions.

The heavy-particle-stripping reaction in (α, p) reactions: (II) The Li(α, p) Be and the F(α, p) Ne reactions

Abstract In the same way as that presented in the preceding paper, the following reactions are analysed in detail in terms of the heavy-particle-stripping reaction: the Li 6 (α, p 0 ) Be 9 (ground state) reaction, the Li 7 (α, p 0 ) Be 10 (ground state) reaction and the Li 7 (α, p 1 ) Be 10 (1st excited state) reaction at E α = 30 MeV and the F 19 (α, p 0 )Ne 22 (ground state) reaction, the F 19 (α, p 1 ) Ne 22 (1st excited state) reaction and the F 19 (α, p 2 )Ne 22 (2nd excited state) reaction at about E α = 20 MeV. It is shown that the angular distributions of these three (α, p) reactions for Li nuclei can be satisfactorily explained by the heavy-particle-stripping reaction except for their behaviour at small angles. It is further shown that the differential cross sections at larger angles for the F 19 (α, p) Ne 22 reactions quoted above are also explained reasonably well by the heavy-particle-stripping reaction. In particular, good agreement is obtained between theory and the experiment with respect to the very sharp peak at the backward direction appearing in the differential cross section for the F 19 (α, p 0 )Ne 22 (ground state) reaction. In this connection, some remarks are added concerning the shell model wave functions of O 18 and F 19 calculated by Elliott and Flowers and by Redlich. The reduced width { O α Θ α Θ p } 2 extracted from the experimental data of the F 19 (α, p 0 )N 22 reaction is found to be very small in comparison to that of the C 12 (α, p 0 )N 15 reaction obtained in the preceding paper.