Preequilibrium spin effects in Feshbach-Kerman-Koonin and exciton models and application to high-spin isomer production

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Abstract
We describe how the Feshbach-Kerman-Koonin (FKK) theory can be used to obtain residual nucleus spin distributions following preequilibrium decay, by removing the assumption of zero intrinsic spins in multistep direct reactions. By making use of parallels between the exciton model and the FKK multistep direct theory we also obtain a straightforward method for determining spin distributions in the exciton model. We compare these two approaches and apply them to high-spin isomer production cross sections in 14 MeV neutron reactions on hafnium. We obtain reasonable agreement with measurements, though there is evidence that the FKK theory underpredicts high spin transfer reactions. Comparisions with the exciton model suggest that multistep processes in FKK are underestimated, and that an FKK formulation incorporating non-normal DWBA transitions may yield results in closer agreement with the exciton model, and with experiment.