Analyzing power and cross section distributions of the C12(p,pα)Be8 cluster knockout reaction at an incident energy of 100 MeV

The (p,p alpha) reaction on C-12 was investigated experimentally using polarized incident protons of 100 MeV. The scattered proton and alpha particle, from the knockout reaction, were detected in coincidence. Coincident data, which were obtained at ten quasifree angle pairs for proton angles ranging from 25(degrees) to 110(degrees), were analyzed in terms of the distorted-wave impulse approximation (DWIA). Calculated energy-sharing cross section and analyzing power distributions reproduce the data reasonably well, indicating that a quasifree knockout mechanism dominates the reaction. Since measurements of analyzing powers were made, spin-orbit distortions were included in the DWIA calculations. The effects of this were found to be very small near zero recoil momentum and did not destroy the validity of the factorization approximation where the two-body p-alpha cross section enters as a multiplicative factor in the three-body (p,p alpha) cross section expression. Spectroscopic factors derived from the data are consistent with theoretical predictions. Analyzing power data also follow the trend of free p-He-4 scattering data, and comparisons with DWIA predictions are in reasonable agreement. Because the two-body interaction response between the projectile and the alpha cluster was found to resemble the scattering of protons from a free alpha particle to a remarkable degree, the present results would strongly imply the existence of preformed alpha clusters in C-12.