J.V. Maher

The fusion of 12C and 16O with 28,29,30Si

Abstract Complete fusion cross sections have been measured for the systems 12 C and 16 O + 28,29,30 Si in the energy ranges 20 MeV ⩽ E 12 C 49 MeV and 21 MeV ⩽ E 16 O ⩽ 61 MeV . Weak structure is observed in the 12 C + 28 Si excitation function while the remaining five systems exhibit an energy dependence which is consistent with smooth behavior. Maximum fusion cross sections for 12 C + 28 Si and 16 O + 28 Si are measured to be ≈ 1000 mb while those for 29,30 Si peak at ≈ 1200 mb. Data have been analyzed in terms of the Bass model and the model of Glas and Mosel.

Energy dependences of (16O, 12C) transitions☆

Abstract The ( 16 O, 12 C) cross sections have been measured for lab angles, θ ≧ 4° on targets of 24 Mg and 28 Si at lab beam energies ranging from 36 to 53 MeV. Angular distributions have been extracted for transitions to the ground states and first excited states at all beam energies studied. Angular distributions for higher excited states (up to ≈ 8 MeV) have been extracted for 24 Mg( 16 O, 12 C) 28 Si at 48 MeV and for 28 Si( 16 O, 12 C) 32 S at 42 MeV. An analysis has been performed with the full finite range DWBA code LOLA. Although the angular distribution shapes can be reproduced with reasonable DWBA calculations and the resulting relative spectroscopic factor products are energy independent and agree well with ( 6 Li, d) spectro-spectroscopic factors, there are significant difficulties encountered in applying the DWBA treatment. Absolute spectroscopic factors decrease by a factor of ≈ 5 as bombardment energy is increased through the ≈20 MeV range of this investigation and the shape of the DWBA predicted angular distribution is very sensitive to the choice of optical model parameters at bombarding energies near the Coulomb barrier. Interestingly a two parameter Frahn and Venter model calculation reproduces the l = 0 angular distributions for both targets with an energy variation of the absolute normalization factor which is significantly smaller than that resulting from the DWBA analysis.

Structure in 28Si(16O, 12C)32S excitation functions

Abstract Oscillatory structure appears in 28 Si( 16 O, 12 C) excitation functions populating the ground state and first excited state of 32 S over a range of center-of-mass bombarding energy from 22.3 MeV to 32.5 MeV. This structure is weaker in magnitude and less regular than that previously reported for 24 Mg( 16 O, 12 C) excitation functions. The structure in the 28 Si → 32 S ground state excitation function does not correlate with entrance or exit channel elastic scattering excitation functions, but it does correlate with some of the structure observed in excitation functions for 28 Si( 16 O, 16 O) 28 Si ∗ (2 + ). At most bombarding energies the 28 Si → 32 S ground state transition shows angular distributions which are dominated by one grazing partial wave (which can be calculated with the sample semiclassical Frahn-Venter model), but in two narrow energy ranges the angular distributions shift abruptly to oscillatory shapes which are not dominated by the grazing partial wave.

Quasi-free proton-scattering at 164 MeV☆

Abstract Inclusive proton spectra have been measured over the angular range 25° – 150° for 164 MeV protons on 27 Al, 58 Ni, 62 Ni and 208 Pb. At 25° and 30° a peak attributable to quasi-free scattering, accounting for ≈20% of the total number of fast protons at these angles, is present. No quasi-free peak is evident in the spectra at larger angles. These results are in disagreement with previously published work at this energy but in line with results reported at other energies.

Optical potential for 12C + 28Si scattering☆

Abstract 12 C + 28 Si elastic scattering angular distributions are smooth functions, relatively easily fit by optical potential predictions, at laboratory bombarding energies, E 1 ab , within ≈ 10 MeV of the Coulomb barrier (i.e. up to E 1 ab ≈ 27 MeV). Between E 1 ab = 27 and 36 MeV the angular distributions show pronounced oscillatory structure which is not easily fit with an optical potential. No optical potential has been found to give a very good account of all the angular distributions simultaneously; the best simultaneous fit to all the data was achieved with a surface-transparent potential whose real and imaginary well depths are energy dependent.

Elastic scattering of C12 by Si28

/sup 12/C + /sup 28/Si elastic scattering angular distributions have been measured at twenty-three bombarding energies over a range 19 MeV < or = E/sub lab/ < or = 48 MeV. At energies below 30 MeV the angular distributions are smooth and can be reproduced with a wide variety of optical potentials; similarly, above 40 MeV the angular distributions exhibit diffraction oscillations which optical potentials can rather easily reproduce. Irregular structure is observed in the cross sections for angles larger than 50/sup 0/ in the bombarding energy range 30--40 MeV. No optical potential has been found to give a good account of all the data; the best potential is a surface transparent Woods-Saxon potential which has energy dependences for both real and imaginary well depths. Examination of potentials which give reasonably good fits to the 36 MeV data shows that, although these potentials agree on a real well depth at a reasonable strong absorption radius, they can have quite different Argand diagrams: even in the range of the most sensitive partial waves. A Regge analysis finds several equally good families of Regge parameters for the same choice of background potential, but larger angle data might allow this ambiguity to bemorexa0» lifted. A Breit-Wigner analysis gives results which are at least partially consistent with the resonance parameters reported by Ost et al. Coupled-channel calculations with a 20% reduction of real and imaginary optical model well depths give a good account of inelastic scattering to the /sup 28/Si 2/sup +/ state while leaving the elastic scattering essentially unchanged from the predictions of the unmodified one-channel optical model. In the energy range of this study the grazing partial wave is found to be the same as the most important exit channel partial wave of the /sup 24/Mg(/sup 16/O,/sup 12/C)/sup 28/Si ground state transition.«xa0less

Elastic scattering of /sup 12/C by /sup 28/Si

/sup 12/C + /sup 28/Si elastic scattering angular distributions have been measured at twenty-three bombarding energies over a range 19 MeV < or = E/sub lab/ < or = 48 MeV. At energies below 30 MeV the angular distributions are smooth and can be reproduced with a wide variety of optical potentials; similarly, above 40 MeV the angular distributions exhibit diffraction oscillations which optical potentials can rather easily reproduce. Irregular structure is observed in the cross sections for angles larger than 50/sup 0/ in the bombarding energy range 30--40 MeV. No optical potential has been found to give a good account of all the data; the best potential is a surface transparent Woods-Saxon potential which has energy dependences for both real and imaginary well depths. Examination of potentials which give reasonably good fits to the 36 MeV data shows that, although these potentials agree on a real well depth at a reasonable strong absorption radius, they can have quite different Argand diagrams: even in the range of the most sensitive partial waves. A Regge analysis finds several equally good families of Regge parameters for the same choice of background potential, but larger angle data might allow this ambiguity to bemorexa0» lifted. A Breit-Wigner analysis gives results which are at least partially consistent with the resonance parameters reported by Ost et al. Coupled-channel calculations with a 20% reduction of real and imaginary optical model well depths give a good account of inelastic scattering to the /sup 28/Si 2/sup +/ state while leaving the elastic scattering essentially unchanged from the predictions of the unmodified one-channel optical model. In the energy range of this study the grazing partial wave is found to be the same as the most important exit channel partial wave of the /sup 24/Mg(/sup 16/O,/sup 12/C)/sup 28/Si ground state transition.«xa0less