Effect of neutron–photon converter materials (Cd, Gd, and Sm) on the positron production in a reactor-based slow positron beamline

Abstract

The change in the amount of positron production when Cd cap, installed as a neutron–photon converter, is replaced with Gd or Sm caps was simulated by Monte Carlo calculations in the slow positron beamline at the Kyoto University Research Reactor, especially focusing on the change in the amount of positron production induced by neutrons emitted from a reactor core. Based on the simulation results, the amount of positron production induced by neutrons obtained using Gd and Sm caps was estimated to decrease to (69±4)% and (54±3)%, respectively, compared with the amount obtained using Cd cap. Meanwhile, Cd and Gd caps with a thickness of 1 mm were evaluated to have almost the same burn-up lifetime. Therefore, a Cd cap is the best choice, while a Gd cap, which has a higher melting point, could be an alternative if the structure of the positron source makes effective cooling difficult.