R.D. Bent

Microscopic description of the A(p, π)A + 1 reaction

We have developed a flexible and thoroughly tested computer program for microscopic calculations of proton-induced pion production in nuclei at bombarding energies up to 500 MeV. The model used includes explicitly both the one-nucleon (pionic stripping) mechanism and the resonant p-wave rescattering part of the two-nucleon mechanism. Initial and final state interactions are included through proton-nucleus and pion-nucleus optical model distortions, with careful consideration of the multiple scattering series. The intermediate delta in the two-nucleon mechanism is treated as static with possible inclusion of a local density approximation for the delta-nucleus interaction. We explain the philosophy of the model, detail the parametrization of the microscopic dynamics, and give the formalism for the calculation of experimental observables. We also describe the testing of the computer program and present preliminary results for the 3He(p, π+)4He reaction.

Two-proton pickup in the zirconium region using the (6Li, 8B) reaction

Abstract Spectra at θlab = 8° have been measured for the 92, 94, 96, 98, 100Mo(6Li, 8B)90, 92, 94, 96, 98Zr and 90, 92, 94Zr(6Li, 8B)88, 90, 92Sr two-proton pickup reactions at 90 MeV. The strongest transitions to levels in the Zr isotopes are the 0+ → 0+ transitions to the g.s. and first excited 0+ states. The salient feature of the five Zr spectra is the large increase in 0+2 transition strength relative to that of the g.s. which is observed in the two heaviest isotopes, 96, 98Zr. In 90Zr, the 0+2 transition is weak whereas it is the dominant transition in 98Zr with twice the g.s. strength. These large variations in relative cross section, which are reproduced by DWBA calculations, are attributed to changing g.s. proton configurations in Zr. In the Sr spectra, which are distinctly different from the Zrspectra, the strongest transitions to excited states are to 2+ rather than 0+ states. The (6Li, 8B) reaction seems to be adequately described as a one-step cluster transfer of a T = 1, S = 0 proton pair. The suitability of this reaction for measurements of two-proton pickup is discussed.

The dominance of high-spin two-particle one-hole transitions in (p,π−) reactions

Abstract Measurements of broad-range momentum spectra for (p, π − ) reactions near threshold on a number of target nuclei with 13⩽A⩽92 have revealed a striking and systematic selectivity for one or a few low-lying states In the residual nuclei. We propose that the dominant transitions lead to high-spin stretched (or nearly stretched) two-particle one-hole states (with respect to the target-nucleus ground-state configurations), as favored by a two-nucleon pion production mechanism. Experiments which might confirm this explanation and possible future applications of the (p, π − ) selectivity are discussed.

Correlation between (6Li, 8B) two-proton pickup and (d, 6Li) ϵ-particle pickup in the zirconium region

Abstract Spectra have been measured for the 94,98 Mo( 6 Li, 8 B) 92,96 Zr reactions at 90 MeV. Differences in transition strengths for the first excited 0 + states, first noted in some recent (d, 6 Li) work, are quantitatively accounted for by the changing proton configurations with neutron number.

Energy dependence of the 3He(p, π+)4He and 3H(n, π−)4 He reactions

Abstract We examine in detail the energy dependence of the various components of a microscopic calculation of the 3 He(p, π + ) 4 He reaction from the near-threshold region ( T lab p = 178 MeV, T c.m. π = 10.5 MeV) to an energy at which the Δ 1232 resonance should clearly dominate the reaction mechanism ( T lab p = 325 MeV, T c.m. π = 110 MeV) and demonstrate the sensitivity of the calculations to pion and proton distortions. The discrepancy in size between the calculated and measured cross sections at the lower energies ( T lab p = 178 and 200 MeV), persisting in two earlier publications, is found to disappear when Coulomb effects are properly taken into account. Our present calculations give a reasonably good description of the measured cross section and analyzing-power data at these lower energies and predict a smooth energy dependence of the cross section at the higher energies reflecting the increasing importance of the Δ-resonance. Calculations for the charge-symmetric 3 H(n, π − ) 4 He reaction are compared with existing cross-section data for the time-reversed 4 He(π − , n) 3 H reaction at the higher energies using detailed balance, while differences between the 3 He(p, π + ) 4 He and 3 H(n, π − ) 4 He reactions due to Coulomb effects are investigated.

A QDDM magnetic spectrograph system in use for nuclear pion production experiments at IUCF

Abstract The quadrupole-dipole-dipole-multipole (QDDM) magnetic spectrograph and associated experimental methods employed at the Indiana University Cyclotron Facility for studies of the A (p, π ± ) A + 1 reaction are described. The techniques have been used successfully for extensive measurements of the energy-mass- and state-dependence of (p, π + ) differential cross sections in the near threshold region ( T π = 9 to 60 MeV), and for measurement of the polarization asymmetry in the ( p , π − ) reaction. Backg round levels of about 0.1 nb/sr and an energy resolution of 63 keV fwhm have been achieved.

The (p, π±) reactions on 12,13C at 200 MeV

Abstract Angular distributions from the (p, π ± ) reactions have been measured at 200 MeV on 12 C and 13 C leading to discrete final states in the residual nuclei. A comparison of the cross sections is made between transitions to isobaric analog and non analog final states. Differences between the (p, π + ) and (p, π − ) reactions are found in the shape and magnitude of the differential cross section as well as in the energy variation.

Role of the Δ-isobar in the A(p, π)A+1 reaction

Abstract We have applied a microscopic model of the A(p, π)A+1 reaction that includes both the one-nucleon mechanism and the resonance p-wave rescattering part of the two-nucleon mechanism to the 16 O ( p , π + ) 17 O and 12,13,14 C ( p , π − ) 13,14,15 O reactions leading to two-particle one-hole final states to test separately the two-nucleon mechanism part of the model. The calculations reproduce fairly well the differential cross section and analyzing power data for 16 O ( p , π + ) transitions leading to high-spin, two-particle one-hole states in 17O at 7.76 MeV ( 11 2 − ) and 15.8 MeV ( 13 2 − ) at bombarding energies of 200, 250 and 354 MeV, but underestimate appreciably the differential cross sections for 12,13,14 C ( p , π − ) transitions leading to the ground states 13,14,15O and the 7.28 MeV ( 7 2 + ) excited state of 15O and fail to reproduce the analyzing-power angular distributions for these (p, π−) transitions. These results verify that NN → NNΔ → NNπ is the underlying two-nucleon process in the 16O(p, π+) reactions, but that non-Δ channels dominate the 12,13,14C(p, π−) reactions. The latter conclusions is consistent with results of recent studies of the pn ↔ π−pp reaction.