Observation of High-Lying Levels inBe8fromα−αElastic Scattering

Abstract

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...

New broadBe8nuclear resonances

“…(1, 2) − 1(24), the reason is that these resonances were observed in reactions other than those analyzed here [1]. Of the reactions studied here, the 4 + 0(20) resonance is only non-negligibly observed in 4 He(α, α 0 ) [1], and data from the experimental reference [9] are not included here.…”

Cluster formation and the virial equation of state of low-density nuclear matter

“…In Fig. 4, we show The phase shifts for elastic αα scattering δL(E) versus laboratory energy E. As discussed in the text, the phase shifts are taken from Afzal et al [40] and from Bacher et al [41].…”

“…3. The low-energy phase shifts are taken from Afzal et al [40] and the phase shifts for energies between 30 MeV and 70 MeV are from Bacher et al [41]. The phase shifts display a pronounced 0 + resonance at very low energies that corresponds to the 8 Be ground state.…”

Discrete ambiguities in phase-shift analysis

“…For convenience we shall therefore consider zeros of the amplitudes in the z 2 plane. First we discuss in detail the analysis of Bacher et al 12). From 17.6 MeV to 27.0 MeV four partial waves have been used in the fit, while at higher energies this number was five.…”

“…In fact we obtained The second process we want to discuss in elastic ,¢-~ scattering. Here we have used two sets of input: the analysis of Darriulat it), and the more recent one of Baeher et al 12). The energy region of interest for our work is from the first inelastic threshold (p-7Li) at a c.m.s, energy of 17.4 MeV to about 60 MeV, the highest energy where we have a phase-shift analysis available.…”