Reaction channel contributions to proton scattering at 65 MeV

Abstract

Background: Well-established coupled channel (CC) and coupled reaction channel (CRC) processes make contributions to elastic scattering that are absent from local density folding models.Purpose: To establish and characterize the contribution to the proton optical model potential (OMP) made by the coupling to neutron pickup channels, in particular the proton OMP for 65 MeV protons on $^{48}\mathrm{Ca}$ and $^{40}\mathrm{Ca}$. Also to relate this contribution to results for ${}^{40}\mathrm{Ca}$ at lower energies; to investigate the dynamical nonlocality of this contribution; to characterize the effect on the OMP of breakup of the deuteron.Methods: CRC calculations of neutron pickup and CC calculations of collective states, provide the elastic channel $S$ matrix ${S}_{lj}$. Inversion of ${S}_{lj}$ produces a local potential that yields, in a single channel calculation, the elastic scattering observables from the CC/CRC calculation. Subtracting the bare potential yields a local and $l$-independent representation of the dynamical polarization potential, DPP. From the DPPs due to a selection of channel couplings the influence of dynamically generated nonlocality can be identified. The effect of coupling to the deuteron breakup continuum is also identified.Results: For $^{40}\mathrm{Ca}$, coupling to pickup channels has an effect on observables somewhat weaker than that at 30 MeV, and much less than for pickup coupling for $^{48}\mathrm{Ca}$. The DPPs have similar general properties in each case, but are much larger in magnitude for $^{48}\mathrm{Ca}$. Subsequent breakup of the deuteron makes a large contribution to the DPP, and hence to the OMP. The formal DPPs due to pickup coupling exhibit dynamical nonlocality.Conclusions: The DPPs challenge local density folding models for elastic scattering. The breakup of the deuteron must henceforth be included in calculations of the DPP due to neutron pickup in proton scattering. Pickup coupling effects are still substantial at 65 MeV. No smoothly varying global OMP could fit proton elastic scattering from both ${}^{40}\mathrm{Ca}$ and ${}^{48}\mathrm{Ca}$.