X-rays emitted in neutron-induced fission of 238U(n,f) (Vol. 44 No. 6)

Prompt x-rays emitted in neutron-induced fission help unveil the evolution of fission fragment charge yields as a function of incident neutron energy.

Nuclear fission is accompanied by the prompt emission of neutrons, gamma rays and x-rays. It has been known since the sixties that fission prompt x-rays originate essentially as a consequence of the internal conversions occurring in the prompt gamma deexcitation cascades of fission fragments.

This work presents for the first time a measurement of the prompt fission x-ray yields in ²³⁸U(n,f) for average incident neutron energies ranging from 3 to 200 MeV. Fission fragment charge distributions are derived from the measured x-ray yields using x-ray emission probabilities per fragment obtained in an earlier work on low energy fission. The results are found to be in a remarkable agreement with the Wahl phenomenological systematics for fission product yields, as well as with the more sophisticated GEF fission model. More detailed comparisons demonstrate that x-ray emission evolution with increasing incident neutron energy tends to be dominated by the transition towards lighter fragments which on average are closer to closed-shell nuclei and are thus less subject to internal conversion.