Calculations and measurements of neutron moderation time distributions for multisphere LiI detectors and AB remmeter at 20-MeV electron linac

Neutron moderation time distribution is an important factor for the dead-time correction for moderator type neutron detectors used at pulsed neutron fields. Measurements of the neutron moderation time distributions for multisphere (bare, 2 inch, 3 inch, 5 inch, 8 inch, 10 inch and 12 inch diameter) LiI detectors and AB remmeter have been successfully made inside a ANL 20-MeV electron Linac room. Equivalent calculations were also made using MCNP4a. First, the neutron energy spectrum at the detector position was calculated using a coupled EGS4-MORSE code with a giant-resonant photoneutron generation algorithm. The calculated spectrum agrees with the multisphere detector measurements from the unfolding. Then, neutrons arriving at the detector surface were scored as a function of energy and time using MCNP4a. Finally, the above neutron fluence as a function of energy and time was used as the source term and the neutrons detected by 6Li or 10B in the sensor were scored as a function of the moderation time for each detector. The calculations are in good agreement with the measurements. The results also show that the moderation time distributions for large detectors, e.g., the 12 inch detector and AB remmeter, depend mainly on the neutron spectrum and moderator thickness. However, for small detectors, the neutron travel time from the source target to the detector surface in the radiation field is also crucial. Therefore, all four factors (neutron spectrum, neutron travel time in the field, detector moderator thickness and detector response function) play interrelated roles in the time distribution of a moderator type detector