Measurement of prompt X-rays in 238 U(n,f) from threshold to 400 MeV

Prompt K X-ray emission yields in the fission induced by neutrons on 238 U have been measured for the first time for incident energies ranging from below 1MeV up to 400MeV. Results are used to investigate the evolution with incident neutron energy of the fragment elemental distribution and the X-ray emission probability per element. The progressive increase of the symmetric fission probability with neutron energy is observed in qualitative agreement with Wahl systematics for the primary fission fragment charge yields.