Proton scattering at 1 GeV

Abstract

The physics of 1-GeV proton scattering on nuclei is discussed in the light of recent calculations, and compared to the Gatchina, Los Alamos, and Saclay data. The impulse approximation (including spin-orbit effects and correlations) is reviewed, and comparison is made with other theories such as the Glauber model and the low-energy optical model. This discussion is addressed to specialists as well as nonspecialists in the field. The neutron distribution is extracted from the data and a detailed comparison is made with other determinations of this distribution and with the Hartree-Fock predictions. The neutron radii are seen to be generally larger than the proton radii. Within a given shell, they increase at a much slower rate ( ∼A 1 8 ) than the A 1 3 rule. Except possibly for 208Pb, they are consistent with the Hartree-Fock predictions, but not with the values obtained from Coulomb energies. The study of inelastic scattering to collective states allows the extraction of neutron transition densities, and in particular the analog B( N, L) of the electromagnetic transition rates B( E, L) one usually considers for the protons. Neutron excitations are seen to be stronger by 20 to 40 % than proton excitation, exceeding the N Z prediction of the collective model. Spin effects lead only to small changes in the cross section, but to a measurable analyzing power. The unnatural parity excitations of the lowest 2 − ( T = 0) state of 16O and the 1 + ( T = 1) state of 12C show that the spin-spin and tensor terms of the nucleon-nucleon amplitude are sizable. Their relative magnitudes are seen to be crucial for explaining the observed cross sections.