A Study of the (³He, n) Reaction at 25 MeV

Energy spectra of neutrons from the ¹²C (³He, n)140; ²⁷A1(³He, n)²⁹P; ²⁸Si(³He, n)³⁰S; and ^58,60,62Ni(³He, n)^60,62,64Zn reactions have been measured at 25 MeV incident ³He energy using time-of-flight techniques. An overall energy resolution of about 1 percent for detection of 10-35 MeV neutrons provided accurate determinations of level excitations. Differential cross sections were extracted for many two particle states with up to 10 MeV excitation in the residual nuclei. By varying the beam incident angle and leaving the flight path somewhat fixed, angular distributions from 0 to 35° lab were obtained. Making use of heavy shielding, neutron-gamma discrimination techniques and a cosmic rat anti-coincidence detector, background was reduced to the extent that cross sections on the order of 50 microbarn/sr could be measured.

The shapes and relative magnitudes of the experimental angular distributions were compared with the theoretical predictions of distorted wave calculations, thus yielding orbital angular momentum (ΔL) transfers. Certain simplifying but well verified assumptions about the reaction mechanism enabled assignments of definite J^π values from the orbital angular momentum transfer values which best fit the shapes of the angular distributions. In some cases sensitivity of the calculations, mostly in magnitude, to the choice of bound state wave functions permitted determinations of the dominant two particle shell configurations of the final states.