Detection of delayed neutrons from neutron activation of fissionable substance samples. Monte Carlo modelling of response of the DET-12 device

Abstract

Activation of fissionable elements by neutrons has been considered as a possible diagnostics of D-D and D-T fusion plasma. Fission caused by fusion neutrons leads up to emission of secondary neutrons: prompt and delayed. Proper interpretation of the time decay of delayed neutrons enables an assessment of the parameters of the primary neutron flux inducing fission. Monte Carlo calculations have been carried out by means of the MCNP code in order to elaborate the method considered. Three nuclides: pure 235U, 238U and 232Th, and additionally sintered UO2 have been selected as possible materials for the sample to be irradiated. Four energies of neutrons irradiating samples have been chosen: thermal, fast and two high ones. Computations have been accomplished for two variants: pure physical effect and real experimental conditions of neutron registration in the DET-12 device designed and built in IFJ PAN. Decay curves have been obtained for each case. Detection efficiency of DET-12 has been also estimated.