NEUTRON DETECTION TIME DISTRIBUTIONS OF MULTISPHERE LII DETECTORS ANDAB REM METER AT A 20 MEV ELECTRON LINAC

Neutron detection time distribution is an important factor for the dea d-time correction for moderator type neutron detectors used in pulsed radiation fields. Measurements of the neutron detection time distribut ions of multisphere LiI detectors (2'', 3'', 5'', 8'', 10'' and 12'' i n diameter) and an AB rem meter were made inside an ANL 20 MeV electro n linac room. Calculations of the neutron detection time distributions were also made using Monte Carlo codes. The first step was to calcula te the neutron energy spectra at the target and detector positions, us ing a coupled EGS4-MORSE code with a giant-resonant photoneutron gener ation scheme. The calculated detector spectrum was found in agreement with the multisphere measurements. Then, neutrons hitting the detector surface were scored as a function of energy and the travel time in th e room using MCNP. Finally, the above neutron fluence as a function of energy and travel time was used as the source term, and the neutrons detected by Li-6 or B-10 in the sensor were scored as a function of de tection time for each detector using MCNP. The calculations of the det ection time distributions agree with the measurements. The results als o show that the detection time distributions of detectors with large m oderators depend mainly on the moderator thickness and neutron spectru m. However, for small detectors, the neutron travel time in the field is also crucial. Therefore, all four factors (neutron spectrum, neutro n travel time in the field, detector moderator thickness and detector response function) may play inter-related roles in the detection time distributions of moderator type detectors.