Uranium pit attribute detection is an important subject in the research of arms control and verification technology.235U and238U which are the main isotopes of uranium having weak spontaneously γ-radioactivity

and the 235U characteristic γ-rays are of lower energy

hence the difficulty of detecting and verifying a uranium pit attribute using passive detection technique.In this paper

we explore the method to derive the multiplication factor by analyzing the intensity relationship between delayed and prompt neutron which are released from uranium pit excited by high-energy photons and outer neutron.Some reactions such as(γ

n) and(γ

2n) occur when the uranium material is excited by high-energy photons and the neutron emitted uranium nuclides will further induce uranium fission

while the outer neutron will cause scattering and induced fission.Approximation formulas about the radio of delayed-prompt neutron and the multiplication factor in different measuring conditions are obtained by analyzing the transport process

and accuracy of the formulas based on the numerical simulations is verified.