Background: Prompt fission neutron uranium logging is a method for uranium exploration.Pulsed neutron source and epithermal neutron detector are used to detect the prompt epithermal neutron from the fission of thermal neutron and 235U.Purpose: The efficiency of epithermal neutron detector of the logging instrument need to be improved.Methods: The energy distribution of fission neutron detected by the epithermal neutron detector

as well as the detection efficiency of epithermal neutron detector with different sizes of moderator are studied by Monte Carlo simulation.Results: Under the studied conditions

the optimal sizes of neutron moderating material and neutron detector are obtained

which is the combination of 0.5-mm thick cadmium

1.1-cm thick polyethylene and 2.6-cm diameter3He tube.Conclusions: In the space with outer diameter of 4.8 cm

the maximum of the epithermal neutron detection efficiency was achieved by combination of a 1.1-cm thick tube and a 2.6-cm diameter3He moderator when using polyethylene or organic glass as the moderator material.