H. G. Pugh

The (α, α), (α, α′) and (α, 3He) reactions on 12C at 139 MeV

Abstract The nucleus 12C was bombarded with 139 MeV α-particles to study the characteristics of the elastic, inelastic, and (α, 3He) reactions. An optical model analysis of the elastic data yielded a unique family of Woods-Saxon potential parameters with central real well depth V ≈ 108 MeV, and volume integral J 4A ≈ 353 MeV · fm 3 . By comparing the present results with those of other studies above 100 MeV, we find that the real part of the α-nucleus interaction decreases with increasing energy; the fractional decrease with energy is roughly one-half that observed for proton potentials. Using the optical potential parameters derived from the elastic scattering, first-order DWBA calculations with complex first-derivative form factors reproduced the inelastic scattering data to the 4.44 MeV (2+) and 9.64 MeV (3−) states of 12C. For the 0+ state at 7.65 MeV it was necessary to employ a real, second-derivative form factor to fit the data. The deformation lengths βl Rm and deformations βl obtained in this and other experiments are summarized and compared. DWBA calculations using microscopic model form factors were also performed for the 2+ and 3− states using the wave functions of Gillet and Vinh Mau. These reproduced the shapes and relative magnitudes of the differential cross sections. We also fit the shape of the 0+ differential cross section using a microscopic form factor which contains a node, which is similar to that occurring in the collective model second-derivative form factor. In the ( α, 3 He ) reaction the differential cross sections to the ground state ( 1 2 − ) and the 3.85 MeV ( 5 2 + ) state in 13C could not be reproduced by zero-range local DWBA stripping calculations; it was necessary to employ finite-range and non-local corrections in the local-energy approximation. This DWBA analysis is notable in that unambiguous optical potentials were available for both entrance and exit channels. The ground state spectroscopic factor is in agreement with the prediction of Cohen and Kurath, while the relative spectroscopic factors agree fairly well with the rather few existing measurements of this kind.

The (p, pα) and (α, 2α) reactions on 6Li and 7Li at 60 MeV

Abstract An investigation of the alpha cluster knockout reaction has been made by studying the 6,7Li(p, pα)2,3H and the 6,7Li(α, 2α)2,3H reactions using approximately 60 MeV incident particles. Both energy sharing and angular correlation experiments have been performed near a quasi-free angle pair, and alpha cluster momentum distributions ¦φ(q)¦2 extracted by means of the plane wave impulse approximation. The momentum distributions extracted from the (p, pα) reactions for the two experimental techniques are in good agreement and have the shape expected for the knockout of an L = 0 α-cluster from 6Li and an L = 1 α-cluster from 7Li. The shapes of the ¦φ(q)¦2 extracted from the (α, 2α) energy sharing experiments are in reasonably good agreement with the (p, pα) results; the agreement in magnitude being within the experimental uncertainties. However, the angular correlation results for 7Li(α, 2α)3H show large discrepancies, casting doubt on the analysis of the (α, 2α) reaction with the normal impulse approximation. A discussion of the effects of various methods of making the quasi-free approximation is given.

Off-mass-shell effects and nuclear structure in the 6Li(α, 2α)d reaction

Abstract The 6 Li(α, 2α)d reaction was studied at 50.4, 59.0, 60.5, 70.3 and 79.6 MeV bombarding energy. For each bombarding energy, several coincident energy spectra of the two emitted α-particles were measured. Special emphasis was placed on measuring spectra at pairs of angles where zero laboratory momentum was possible for the residual deuteron. The spectra were analyzed with the plane wave impulse approximation. From those points in the spectra which corresponded to zero deuteron momentum, off-mass-shell α-α cross sections were extracted. These were found to be in excellent agreement with free α-α cross sections, if free cross sections for the final state α-α c.m. energy were chosen for the comparison. Off-mass-shell α-α cross sections were also extracted for data where the residual deuteron had a momentum of 30 MeV/ c . These cross sections were also found to agree with free α-α scattering, but it was necessary to introduce an ad hoc shift in the α-α scattering angle to produce this agreement. Predictions of off-mass-shell α-α cross sections were made using a potential model. These indicate that the off-mass-shell cross section should indeed be very similar to the on-mass-shell cross section at the final state energy. Using the plane wave impulse approximation a momentum distribution for α-particles in 6 Li was extracted from the experimental data. A cluster model for 6 Li was devised to fit the binding energy and rms charge radius of 6 Li, as well as the 3 S 1 α-d scattering phase shifts. For comparison with the experimental data, the momentum wave function of the α-particle in 6 Li was calculated by taking the Fourier transform of the α-d relative motion. The theoretical and experimental momentum distributions were found to be in serious disagreement, both in magnitude and width at half maximum. By introducing a cut-off radius into the theoretical wave function, the discrepancies between theory and experiment were accounted for. It was also found, that if the cut-off radius is used as an adjustable parameter, then this 6 Li wave function and reaction model explains the magnitudes and widths of the α-d relative momentum distri butions determined from a wide variety of other reactions.

The (p, 2p) reaction on 2H, 3He and 4He

Abstract The (p, 2p) cross sections on 2 H, 3 He and 4 He were measured at 65, 85 and 100 MeV. The experimental data show rather strong energy and target mass dependence, particularly for 4 He.

A comparison of the 3He(p, 2p)d and 3He(p, pd)p reactions☆

Abstract The 3 He(p, 2p)d and 3 He(p, pd)p reactions have been compared at three bombarding energies from 65 to 100 MeV. A comparison of plane wave impulse approximation calculations to the experimental data indicates that multiple scattering effects are large and energy dependent but that they primarily produce a uniform reduction in cross section. Although multiple scattering effects are large the ratio of the cross sections for the two reactions is in agreement with that predicted by the impulse approximation.

Two searches for an excited state of 3He

Abstract The 3He(α, α′)3He∗ reaction was studied at 115 MeV and the 4He(p, d)3He∗ reaction was studied at 65 MeV. The 3He continuum spectra show no evidence for production of an excited state of 3He at 20.4 MeV.