Tiezhu Li

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 950μs 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;

Explosives detection using dual energy X-ray imaging and photoneutron analysis

To find the explosives concealed in cargos, X-ray penetration information and neutron induced γ information are both helpful. The combination of them would improve the capability of explosives detection in complex situation like metal shielded or other organic materials disturbed. In this paper, we present the research of combining dual energy X-ray imaging and photoneutron induced γ-ray analysis. The former technique provides the materials identification capability based on effective atomic number that indicates organic, light metal, inorganic and heavy metal areas. The area containing organic material will be suspected further that will undergo neutron analysis. High energy X-ray induced photoneutron analysis provides the 2-dimensional elemental distribution of hydrogen, nitrogen and iron. Area wealthy of nitrogen and hydrogen element may be suspected as to be explosive. Several different shielding situations are researched in the experiments to evaluate the explosive detection capability, including explosive standing alone, shielded by 2cm iron plate, surrounded by sugar and so on. 9MeV and 6MeV X-ray pulses are produced interlaced with 5μs pulsed width and 3000rad/min@1m to form dual energy X-ray image. Neutrons are produced by X-ray with 20kg heavy water converter. The experiments results show that areas of organic materials could be first notified by dual energy imaging effectively, then the 2-dimensional elements information of nitrogen and hydrogen are used to confirm the existence of explosive.

Positron analysis based on high energy X-ray source

Positron analysis can reveal the information of atomic level defect of materials like metals or polymers through the study of Doppler broaden 511keV γ spectra of positron with electrons that have different momentum distributions relating with the inner defect of materials. The usually used positrons come from isotope or accelerator positron source that irradiates the materials with positrons from outside. Because of the short range of positron in matter, only the surface of millimeter depth inside metals or polymers could be analyzed. To fulfill the demand of positron analysis inside the materials, high energy X-ray is used to produce positron. Two kinds of reaction is feasible, (1) is positron decay after the (γ, n) reaction of high energy photon with low photoneutron threshold nuclide, like N or Fe; (2) is the pair production. We use the first reaction in this paper. The algorithm to analyze the broaden 511keV γ spectra is S factor or FWHM method. Maximum likelihood expectation method is also researched to deconvolute the spectra to remove the detector response for more precise electron momentum information extraction. We setup the facility with 15MeV LINAC and HPGe detector, and then test several specimens including low carbon steel, cast iron, aluminium and copper. The experimental results of five low carbon steel specimens show appreciable correspondence between positron analysis result and tensile elongation. This indicates that the atomic level defect of low carbon steel is within the range of positron analysis. On the other hand, cast iron, aluminium and copper do not show such relation. This is perhaps due to the void in these materials is so much that positron analysis method is saturated.