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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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ical calibration curves cannot be replaced by predictions based on DWBA theory. Calculations were made for the reactions on 52 Cr and 54 Fe. They are in good qualitative agreement with vector-analyzing-power measurements. However, the measured and calculated curves show significant quantitative differences, especially where the vector analyzing power is changing rapidly with angle. In addition, the calculations did not reproduce the shape of the observed cross-section angular distributions at back angles. These deviations cause a considerably poorer fit to the 53 Cr data than the fit shown in Fig. 1. The use of empirical calibration curves has the advantage that it allows a determination of a(i~) and a(f ~) which is independent of any theory.The authors are grateful to P. J. Bjorkholm for preparation of the targets used in this experiment.Accurate measurements of the large intrinsic quadrupole deformation of the first excited 2 + states in 20 Ne, 24 Mg, and 28 Si have recently been performed. 1 Such data are a critical test of the detailed microscopic calculations of nuclear properties which are now being carried out by methods such as deformed Hartree-Fock. 2 Some of these calculations suggest that nuclei of the 2s-ld shell should also have a ground-state hexadecapole deformation which changes both in size and sign through the shell. 3 " 5 The size and sign of Y 4 moments of rare-earth nuclei have previously been determined by a coupled-channels analysis of the cross sections for excitation of the ground state rotational band by 50-MeV alpha particles. 6 The advantage of this method was that all multiple-excitation paths between these states were treated consistently. A similar analysis of scattering data in the 2s-Id shell has been performed only for 24 Mg, but no F 4 deformation was observed in the ground-state band. 7 On the other hand, previous inelastic-scattering results analyzed with the distorted-wave Born approximation, Austern-Blair, and other less so-*Work supported in part by the U. S. Atomic Energy Commission. *T.