D.A. Bromley

Reaction mechanism studies on Si28(d, p)Si29 (II) interference effects

Measurements bearing upon the mechanism for the Si28(d, p)Si29 reaction, including measurements of differential and total excitation functions and of angular distributions for the proton groups leading to the lowest three states of Si29, have been made for incident deuterons of energy 6–10.7 MeV and for proton detector angles 0°–155°. These measurements have demonstrated a dominant direct interaction amplitude, with, however, a small residual compound system amplitude reflected in interference terms in the cross section. The effects of this interference are displayed by prominent resonance structure in the excitation curv≐s, particularly for proton angles off the forward stripping peak. Upper limits of < 15% and < 30% have been established on the intensity of the compound system contributions to the reaction cross sections to the ground state and first two excited states of Si29, respectively. Angular distributions were found to have similar features over the energy range studied and to display diffraction structure which persists even at large angles. The angular positions of the structural features are well fitted by Butler theory, but with the observed envelope decreasing much less rapidly with angle than does the prediction. Empirically it has been demonstrated that adequate fits are obtained by removing the angle dependence contained in the deuteron factor which appears in the Butler expression for the angular distribution. Reduced widths have been obtained using the methods of Amado and of Bowcock. A comparison of these reduced widths with values obtained by the usual peak-fitting technique has demonstrated that the same relative reduced widths are given by the three methods but that the absolute value obtained using the Amado extrapolation is approximately a factor of two less than that obtained with the other two techniques.

Low levels of Al27

Abstract The de-excitation radiation from the 0.84 and 1.013 MeV levels in Al 27 has been studied at a number of resonances in the reaction Al 27 (p,p′γ) with proton energies between 1.7 MeV and 2.8 MeV. The data support assignments 1 2 + and 3 2 + to the 0.84 and 1.013 MeV states. The 1.013 MeV state is observed to be de-excited, 2.4 (± 0.3)% via a 0.170 MeV transition to the 0.84 MeV level and the remainder to the ground state. It is shown that either one or both of these radiations have an E2/M1 amplitude ratio ⪆ 0.2. These results and other properties of the low levels of Al 27 are discussed in terms of the rotational collective model which is found to give a good qualitative description of most of the known data.

Mechanisms for the O16(He3, He4)O15 reaction

BS>Excitation curves for the OO/sup 16/(He/sup 3/, alpha )O/sup 15/ and O/sup 16/(He /sup 3;p) F/sup 18/ reactions show pronounced resonance structure in the He/sup 3/ energy range from 2.0 to 3.0 Mev. It has been previously demonstrated that the energy dependenee of the alpha particle angular distribution coefficients in the region of the strongest resonance structure requires interference between compound states with angular momentum 1/2 and 5/2 and like parity at excitations of 10.44 and 10.46 Mev in Ne/sup 19/. Total and partial widths tor both resonances were extracted from the data; the He/sup 3/ partial width of the 1/2 resonance is 67% of the single particle limit. In fitting the expertmental data, it was necessary to include the energy dependence of the resonance parameters. Alpha particle angular distributions measured between resonances indicate a direct interaction mechanism for the reaction. A small, relatively constant direct interaction reaction amplitude and a markedly energy dependent compound system amplitude which is dominant in the vicinity of compound system resonances are suggested by these measurements. An upper limit of 0.8 mu b was established for the He/sup 3/ capture cross section proceeding through the 1/2 resonance. (auth)

Reaction mechanism studies on Si28(d, p)Si29 (I) distorted wave effects

Abstract A study of the (p, γ) angular correlations involving the 1.28 and 2.03 MeV states in Si29 populated by the Si28(d, p)Si29 reaction has been carried out for deuteron energies in the range from 6 to 9 MeV. In each case measurements were carried out in the (d,p) reaction plane with protons detected on the observed peak of the ln= 2 stripping angular correlation. These data have been analysed within the framework of distorted-wave stripping formalism. The measurements on the 2.03-MeV state alone enable the statistical tensors for the reaction to be determined for each incident deuteron energy. These statistical tensors were then used to compute the angular correlation of the 1.28-MeV radiation, yielding results in good agreement with the experimental measurements and thus supporting the expectation that the matrix elements are essentially independent of the detailed nuclear structure of the final state. The statistical tensors have also been used to predict the general (p, γ) angular correlation function over the sphere as well as the magnitude of the proton polarization for the Si29∗- proton systems involving both the 1.28 MeV and 2.03 MeV excited states.

A search for double dipole de-excitation in C12

Abstract An experimental search for the simultaneous quanta which would result from double E1 electromagnetic de-excitation of the 4.43 MeV, 2+ state in C12, in competition with the normal E2 de-excitation, has yielded negative results. The 4.43 MeV state was populated via the resonant N15 (p, α)C12 reaction; competing proton capture reactions preclude the establishment of an experimental limit on the branching ratio Γ 2E1 Γ E2 of less than 1.7 × 10−4. It is suggested that this limitation applies generally to reaction studies of this branching ratio. Recent theoretical studies of this process suggest that Γ2E1 vanishes in first order, consistent with the experimental observations.