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The elastic scattering of a particles from 4 He has been measured in the energy range between 30 and 70 MeV. A phase-shift analysis was used to investigate the even-spin, positive-parity levels in 8 Be in the region between 15 and 35 MeV of excitation. New 0 + and 4 + levels have been located near 20.3 and 25.6 MeV, respectively. The positions of five 2 + levels and an additional 4 + level are compared with previous work and with theoretical predictions based on the intermediate-coupling shell model.Below an excitation energy of 16 MeV, the level structure of 8 Be is well understood 1,2 in terms of states predominantly of a two-a-particle configuration. Above 16 MeV the level structure rapidly becomes more complex as other configurations for these states (e.g., />+ 7 Li, «+ 7 Be, and d + 6 Li) become important. Those states of 8 Be which have both even spin and parity can decay into the 2a channel, and this selectivity is extremely useful in identifying levels of 8 Be that lie above the 7 Li+£ threshold. Although multilevel R-and S-matrix analyses of 7 Li(p, o?) 4 He 3 and 6 Li(tf, a) 4 He 4 data have provided some level assignments between excitation energies of 19 and 26 MeV in 8 Be, these assignments are not unique. However, any state with a significant aparticle width will appear as a resonance in a-a elastic scattering, which permits immediate assignment of spin and parity from a determination of the resonant partial wave. Earlier studies of the elastic scattering provided some information on 8 Be states above 16 MeV 5 ; however, it was clear that more detailed measurements were required in order to remove ambiguities in the phase-shift analysis. We report here new results which are already apparent from a phase-shift analysis of more extensive elastic scattering measurements.The present measurements were performed with a-particle beams from the Berkeley 88-in. cyclotron using a beam-analysis system 6 which provided a high-resolution beam (AE/E-0.02%) of accurately known 7 energy (±0.03%). Measurements were taken at about 100 energies between 30 and 70 MeV (spanning a region of excitation in 8 Be between 15 and 35 MeV). An array of seven detectors was used to obtain cross sections at 21 center-of-mass angles between 16° and 96°. The 4 He gas target with a thin (7500 A) nickel entrance foil was operated at a pressure of 0.1 atm. The relative errors are typically less than ±2%, while the absolute normalization of the cross section is accurate to better than 4%.A standard x 2 minimization search routine was employed to determine phase shifts including I = 8 at the higher energies. A small Z = 6 phase shift («2°) was required at the lowest energies. Continuity of the phase shifts as a function of energy was required for an acceptable solution. This criterion was particularly useful in regions where the level structure was most complex, e.g., near E a = 40MeV, where three resonances overlap. Numerous attempts to locate alternate solutions were unsuccessful. Phase shifts corresponding to the only acceptable soluti...

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Differential cross sections and analyzing powers for polarized (p, p') scattering on "0 have been measured at an incident proton energy of 24.5 MeV, using the Saclay AVF cyclotron. Data have been taken for nine levels, up to 7.11 MeV excitation energy. The analysis has been carried out in the distorted wave Born approximation within the framework of the macroscopic and microscopic models.For the latter the available wave functions of "0 and an approximate description of the nucleon-nucleon interaction, derived from the Hamada-Johnston potential, have been used. The macroscopic model, with just a simple deformation parameter P, accounts reasonably well for some even-parity transitions, and also for the excitation of the 3 level at 5.09 MeV. Furthermore, coupled-channel calculations support evidence for a rotational band based on the ground state and including the 2+ at 1.98 MeV and the 4+ at 7.11 MeV. The analysis also supports a collective character of the 3 level at 5.09 MeV and accounts for the observed strong excitation of this state.Satisfactory fits are obtained for some levels within the microscopic model, even though the absolute values of the cross sections are usually underestimated and large normalization factors needed. The use of an imaginary form factor adds some structure to the calculated angular distributions and improves the agreement between theory and experiment for those transitions for which the real form factor is collectivelike, i.e. , peaked at the nuclear surface.NUCLEAR REACTIONS O(p, p') 0, E = 24.5 MeV; measured 0'(e) and analyzing power for nine levels. Macroscopic and microscopic D%BA calculations,