Neutron Detection Time Distributions of Multisphere LiI Detectors And AB Remmeter at A 20-MeV Electron Linac

Neutron detection time distribution is an important factor for the dead-time correction for moderator type neutron detectors used in pulsed radiation fields. Measurements of the neutron detection time distributions of multisphere LiI detectors (2Ó, 3Ó, 5Ó, 8Ó, 10Ó and 12Ó in diameter) and AB remmeter were made inside a ANL 20-MeV electron Linac room. Calculations of the neutron detection time distributions were also made using Monte Carlo codes. The first step was to calculate the neutron energy spectra at the target and detector positions, using a coupled EGS4-MORSE code with a giant-resonant photoneutron generation scheme. The calculated detector spectrum was found in agreement with the multisphere measurements. Then, neutrons hitting the detector surface were scored as function of energy and the travel time in the room using MCNP. Finally, the above neutron fluence as function of energy and travel time was used as the source term, and the neutrons detected by 6 Li or 10 B in the sensor were scored as a function of detection time for each detector using MCNP. The calculations of the detection time distributions agree with the measurements. The results also show that the detection time distributions of detectors with large moderators depend mainly on the moderator thickness and neutron spectrum. However, for small detectors, the neutron travel time in the field is also crucial. Therefore, all four factors (neutron spectrum, neutron travel time in the field, detector moderator thickness and detector response function) may play interrelated roles in the detection time distributions of moderator type detectors.