Materials identification by X-ray and photoneutron transmission

In this paper, we present the method research of materials identification based on X-ray and photoneutron transmission. X-ray is produced by 7MeV LINAC and photoneutron is emitted as the byproduct of X-ray when beryllium convertor is used to convert X-ray to neutron. The different attenuation coefficients of X-ray and neutron in materials are used to form a factor, named V, to discriminate materials. The main problems of MIXPT (materials identification by X-ray and photoneutron transmission) come from (1) the interference of X-ray pulses to the detection of photoneutrons and (2) varied V value of materials originated by continuous spectra of X-rays and photoneutrons when penetrating materials; To the first problem, a thermal neutron detector is designed to detect the fast penetrating photoneutron. With the aid of neutron moderation substances surrounding BF₃ detector, fast photoneutron is thermalized and timely delayed thus could be detected free of X-ray pulses interference and with high detection efficiency. A time window of 950¿s width after each X-ray pulse is carefully determined according to both simulation and experimental research to measure the penetrating photoneutron. To the second problem, the result of simulation with MCNP5 shows that each material has a definite curve of V versus mass per unit area and could be discriminated from other materials effectively. The experimental results of polyethylene, aluminium, iron, copper, lead and explosive simulant also confirm these curves.