Astrophysical Reaction Rates for 10^{10}B(p,α\alpha)7^{7}Be and 11^{11}B(p,α\alpha)8^{8}Be From a Direct Model

The reactions $^{10}$B(p,$\alpha$)$^{7}$Be and $^{11}$B(p,$\alpha$)$^{8}$Be are studied at thermonuclear energies using DWBA calculations. For both reactions, transitions to the ground states and first excited states are investigated. In the case of $^{10}$B(p,$\alpha$)$^{7}$Be, a resonance at $E_{Res}=10$ keV can be consistently described in the potential model, thereby allowing the extension of the astrophysical $S$-factor data to very low energies. Strong interference with a resonance at about $E_{Res}=550$ keV require a Breit-Wigner description of that resonance and the introduction of an interference term for the reaction $^{10}$B(p,$\alpha_1$)$^{7}$Be$^*$. Two isospin $T=1$ resonances (at $E_{Res1}=149$ keV and $E_{Res2}=619$ keV) observed in the $^{11}$B+p reactions necessitate Breit-Wigner resonance and interference terms to fit the data of the $^{11}$B(p,$\alpha$)$^{8}$Be reaction. $S$-factors and thermonuclear reaction rates are given for each reaction. The present calculation is the first consistent parametrization for the transition to the ground states and first excited states at low energies.Comment: 27 pages, 5 Postscript figures, uses RevTex and aps.sty; preprint also available at http://quasar.physik.unibas.ch/ Phys. Rev. C, in pres

Dependence of calculated binding energies and widths of η\eta-mesic nuclei on treatment of subthreshold η\eta-nucleon interaction

We demonstrate that the binding energies and widths of eta-mesic nuclei depend strongly on subthreshold eta-N interaction. This strong dependence is made evident from comparing three different eta-nucleus optical potentials: (1) a microscopic optical potential taking into account the full effects of off-shell eta-nucleon interactions; (2) a factorization approximation to the microscopic optical potential where a downward energy shift parameter is introduced to approximate the subthreshold eta-nucleon interaction; and (3) an optical potential using on-shell eta-nucleon scattering length as the interaction input. Our analysis indicates that the in-medium $\eta$N interaction for bound-state formation is about 30 MeV below the free-space $\eta$N threshold, which causes a substantial reduction of the attractive force between the $\eta$ and nucleon with respect to that implied by the scattering length. Consequently, the scattering-length approach overpredicts the binding energies and caution must be exercised when these latter predictions are used as guide in searching for $\eta$-nucleus bound states. We also show that final-state-interaction analysis cannot provide an unequivocal determination of the existence of $\eta$-nucleus bound state. More direct measurements are, therefore, necessary.Comment: 28 pages, 1 figur

Spin excitations in pion inelastic scattering

The data on spin excitations observed in pion inelastic scattering are reviewed. A predominant feature of this process is the selectivity with which high-spin unnatural-parity states are excited. Constant-q excitation functions have proven valuable in identifying unnatural-parity states because of the unique signature of ..delta..S = 1 transitions. It has recently been shown that angular distributions measured for transitions to natural-parity states are quite different for ..delta..S = 0 and ..delta..S = 1 transitions. Pion scattering should continue to prove useful in studying the spin structure of nuclear transitions because of the sensitivity of both excitation functions and angular distributions to the spin transferred to the nucleus. In particular, pion scattering measurements may be helpful in searches for spin-mode giant resonances. In addition to the ability to distinguish transitions dominated by ..delta..S = 1, comparisons of ..pi../sup +/ and ..pi../sup -/ scattering can be used to determine the relative contributions of neutrons and protons to inelastic transitions. In each N not equal to Z nucleus studied there have been large ..pi../sup +//..pi../sup -/, asymmetries observed for some transitions to stretched states. This results in information that is not obtainable from 180/sup 0/ electron scattering

Floating-wire measurements of the magnet used for large-angle pion-nucleus scattering at EPICS

Floating-wire measurements carried out on the circular magnet used for a modification of the EPICS system for large-angle pion-nucleus scattering are described. The region of the incoming pion beam and that of the scattered pion were both covered. The effective radius of the magnet was determined to an uncertainty estimated to be +-1.5 mm. The resulting uncertainty in the scattering angle of the pion in experiments using this system is estimated to be +-0.22/sup 0/, with an upper limit of +-0.50/sup 0/