Advances in Realistic Neutron Spectra: Progress in Fluence Monitoring of DD Reaction

Abstract

As part of a programme on simulation of realistic neutron spectra at workplaces, a new facility based on the 2 H(d,n) 3 He reaction (abbrev.: DD) which yields a 3.3 MeV neutron field, is being developed at the IPSN/SDOS Laboratory in Cadarache. Additional shields at the exit of an accelerator target can provide, in the laboratory, a replication of some spectral conditions encountered in practice. Spectra resulting from this reaction will complete those obtained from the DT process in the same laboratory. Reasons for using the DD reaction and the associated particle counting technique as a monitor of neutron flux are presented. Instead of detecting the alpha particle from the DT reaction, protons are counted from the competitive reaction 2 H(d,p) 3 H with respect to the DD reaction. The fact that the two last reactions are involved in the monitoring and are markedly anisotropic in the CM system, means that a more complex situation has to be dealt with to calculate the neutron yield per proton measured in a solid angle. A conversion factor between neutrons and protons must be calculated and compared with reference measurements performed with several instruments. Based on the new assembly, a 'realistic' spectrum calculated with the MCNP code is proposed. Results from the MCNP code are multiplied by a (n,p) factor calculated by another program, MONITOR, which takes into account the kinematics, the cross sections and the energies involved in the reactions. The theoretical fluence predicted at a reference point in this spectrum is validated directly with the area monitor previously calibrated through a specific methodology. This work provides a new tool of investigation to obtain realistic neutron spectra to be used in the calibration of radiation protection instruments that should fulfil the ICRP recommendations, as given in ICRP Publication 60.