Capture reactions with protons, neutrons, and alpha particles

A review of recent capture experiments which discusses polarized proton, polarized neutron, and alpha particle capture reactions is presented. The giant dipole resonance region is examined chiefly in terms of the information which can be obtained from angular distribution measurements. Experimental results regarding the relative transition matrix elements and their phases are shown to be quite well accounted for by the direct-semidirect reaction theory. Some of the new information obtained from such experiments in the case of the three and four-body problems is described. The utility of polarized capture for the study of M1 and E2 strength is exhibited by examples. Several cases are shown in which the E2 strength observed in light nuclei via p capture is accounted for by a purely direct E2 mechanism. The virtues of polarized neutron capture studies are explored, and the results of the first experiments are described. Alpha particle capture is compared with $p$ and $n$ capture. Fundamental differences in the reaction processes are described which can account for the relatively small $a_1$ coefficients observed in $n$ versus $p$ capture and lack of E1-E2 coherence observed in several $\alpha$-capture experiments.