Simulation of neutron spectrum and time distribution for long counters

The long counters are widely applied among various types of neutron sources.

In this work, neutron spectra in the long counters are specifically studied, in order to obtain a better understanding of the influences on the detection efficiency due to the size of moderators.

According to the basic structure of long counters, a simple model is built to systematically simulate the spectrum and time distribution of neutrons entering the proportional counter tube from a pulsed fast neutron source.

The calculated results show that the evolution of the neutron spectrum is rapid at first, and becomes slower later. After 31 μs, the neutron spectrum almost no longer changes. The time distribution is different for neutrons of different energy. The lower the energy, the wider the distribution. For the energy of thermal neutrons, the time lasts more than 1 ms. Utilizing the time distribution of different energy, the change of counts of the long counter over time is calculated.

Basically, the flux and spectra of neutrons which enter the long counters do not change with the variation of the moderator radius when it exceeds 20 cm. This result can provide a reference for the optimal design of the long counter.