Study Of Elemental Distribution In A Bulk Sample Based On PGNAA Technique

In the process of aerospace spacecraft manufacture and mission implementation,neutron detection and its correlation technology are important research contents.The elemental imaging of sample based on neutron technology is of great significance for the preparation,assembly and detection of large components of aerospace equipment.It’s also important for developing new materials,evaluating their properties and guiding the production process,and is a hotspot of man-machine and environmental field.Prompt Gamma Activation Imaging（PGAI）is a technique that can be used for element composition and position-sensitive analysis of the bulk samples.It can supply information such as the state of interest area of sample,element migration and damage analysis,which plays a strategic role for aerospace industry,energy technology and weapons determination.However,the current PGAI technology can only measure sample with small volume.When mearing bulk sample by PGAI,the analytical accuracy is not good and the determination length is limited.Meanwhile,the traditional PGAI facilities are mainly performed on large platforms that can provide thermal/cold neutrons,which may limit its application.Thus,in this work,we focus on studying the physical mechanism that affects the measurement results when measuring bulk samples by PGAI technology,and establishing the correction model.Meanwhile,we also study the feasibility of building a PGAI platform based on a portable neutron source.The aim of this work is for improving the analytical accuracy of PGAI,and getting rid of the constraints of large platforms to expand the application of this technology.Firstly,we study the main physical mechanism and propose a correction model based on the reactions between neutrons and nuclides.Secondly,a PGAI facility composed of a ²⁵²Cf neutron source and a high-purity germanium detector is developed and is used for measuring a bulk nonuniform sample.Then the results are corrected by the proposed model.Last,we study the feasibility of fast neutron induced PGAI based on D-T neutron generator.The main results and conclusions are summarized as following:（1）Based on the neutron transport method,we studied the physical mechanism of inhomogeneous neutron field distribution inside sample,which was caused by the neutron attenuation and neutron scattering effects.The theoretical analysis was verified by bulk aqueous sample determination.The neutron field distribution was also calculated by the Monte Carlo simulation,and a preliminary correction model was established.（2）A PGAI facility composed of a ²⁵²Cf neutron source and a high-purity germanium detector was designed and optimized.Based on the optimization results,the facility was built.A series of benchmark experiments were performed including the detector,neutron source,and gamma-ray spectra acquisition.The experimental measurements were compared with Monte Carlo simulations.The results showed the proposed Monte Carlo model was reliable after considering the different sources of uncertainty.The following experiments could be accurately simulated with the proposed Monte Carlo model.（3）The bulk inhomogeneous sample was measured with the designed PGAI facility.A new correction model was proposed to correct the results.First of all,a series of samples with different cross-sections were proposed.The activation foils combined with neural network were used to calculate the macroscopic cross-sections of different areas of an unknown sample.Then,the neutron field distribution and the detection efficiency were calculated by Monte Carlo simulations.Finally,the intensities of prompt gamma-rays obtained by scanning measurements were used to calculate the elemental distribution.Both the simulated and experimental results show that the model improves the analytical accuracy.（4）The feasibility of fast neutron induced PGAI based on D-T neutron generator was studied.A fast neutron induced PGAI facility was designed and optimized.Based on the optimization results,the fast neutron induced PGAI facility was built.A bulk metal sample was measured,the sample was composed of iron,copper and titanium block.The results showed that the PGAI method based on fast neutron was feasible,and the element distribution could be determined.After neutron field distribution correction,the results were more accurate.