| As an important non-destructive testing technology,neutron radiography（NR）can provide quantitative information on the spatial distribution of different elements and isotopes in research samples,which has a wide range of applications in military industry,nuclear industry,aerospace industry,medicine,archaeology and other fields.Since the neutron beam is electrically neutral,it can easily penetrate the electron layer and react with the nucleus.Therefore,the attenuation coefficient of the neutron is related to the incident neutron energy and the nucleus cross section of the substance,which has no obvious regularity with atomic number.Because of the unique characteristics of the interaction between neutrons and matter,it can be used as X-ray radiography and gamma-ray radiography complementary methods for non-destructive testing.From the perspective of neutron energy,NR can be divided into fast neutron radiography（FNR）,thermal neutron radiography（TNR）,cold neutron radiography（CNR）,and so on,among which TNR is more popular in practical applications due to its advantages of high resolution.At present,the neutron sources used for TNR in the world are usually nuclear reactors and spallation sources,which can provide relatively high neutron yield and flux for imaging,so as to shorten the imaging time and improve the spatial resolution.However,such sources are very huge,expensive and professional.Their TNR system are unable to realize mobile detection and lack of flexibility,which limits the application and popularization of NR to a certain extent.Therefore,it is an inevitable trend technology for NR to explore and develop a compact TNR system which is convenient to move and easy to operate.Compared with other neutron sources,sealed neutron tube has the characteristics of miniaturization,low price and easy to popularize,which can be used as a potential neutron source in compact TNR system.Therefore,the development of the compact TNR system based on portable neutron tube neutron source is of far-reaching significance for the long-term development of NR technology.In this thesis,the compact thermal neutron transmission imaging system and the compact thermal neutron computational ghost imaging（TNCGI）system were designed based on the DSNH type miniaturized long-life neutron tube independently developed by Northeast Normal University.The key components in the proposed system were designed and simulated by Super MC software,which is developed by the FDS team of the Institute of Nuclear Energy Safety Technology of the Chinese Academy of Sciences.The specific research work and innovation are as follows:1.Design of the moderating collimation system for the compact thermal neutron transmission imaging.（1）Design of the moderator.The D-T neutron source model was established according to the structure analysis and performance test of DSNH miniaturized long-life D-T neutron tube.On this basis,the moderator structure combined with heavy metals,hydrogen containing materials and graphite was proposed to effectively improve the thermal neutron flux at the sample position by analyzing the moderating properties of different materials and optimizing the structure size of the system.At the same time,the position of thermal neutron beam outlet was determined.Besides,the interference of gamma rays to the system was effectively reduced by designing shielding materials.The simulation results show that when the yield of neutron tube is 1.14×109 n/s,the thermal neutron flux is up to 3.58×104 n·cm-2s-1 at the exit plane of the moderating collimation system.The farthest position that can be detected is about67 cm away from the neutron source with the collimation ratio of 25.And the thermal neutron flux here is 1.05×103n·cm-2s-1with n/γ of 1.42×1010n·cm-2Sv-1,which can meet the requirements of NR.（2）Design of the collimator.The safety and practicability of the system were improved by arranging polyimide shell to wrap the 45# transformer oil around neutron source.Based on the structural design of the moderator,the moderating collimation system was further optimized from the perspective of material selection and structural layout.In order to improve the collimation of neutron beam and reduce the influence of stray neutrons on imaging quality,a vertical-divergent collimator equipped with air gap structure and divergence angle of 3° was obtained through the research and analysis of important parameters such as collimator channel shape,inlet diameter,lining material and divergence angle.The results show that the non-uniformity of thermal neutron distribution at the exit plane of the moderated collimation system is as low as 4.6%,and the thermal neutron flux within the field of view at this place is significantly higher than that around it.2.Design of the neutron source system for the compact TNCGI device.Ghost imaging technology based on the principle of correlation operation can obtain the object image under the condition of low flux source,which can reduce the high requirements for source intensity in traditional imaging systems and make up for the low flux of compact sources.Based on the development of ghost imaging,the theory and method of computational ghost imaging are applied to the process of NR.A compact thermal neutron computational ghost imaging system with D-T neutron tube as the source was proposed to promote the further miniaturization of TNR equipment.Through the research and analysis of primary moderating layer,secondary moderating layer,reflecting layer and aggregation layer,a neutron source system with high thermal neutron ratio for TNCGI was designed to provide thermal neutrons with sufficient beam intensity.Under the design of sapphire as fast neutron filter material and metal magnesium thermal neutron as aggregation material,the high-quality thermal neutron beam can be obtained at the exit plane of the system with the neutron flux of 3.77×104 n·cm-2s-1,content of 85.20%,and n/γ of 4.59×1011n·cm-2Sv-1.3.Design of the thermal neutron flux spatial modulator for the compact TNCGI system.In order to obtain the thermal neutron speckle field with predictable fluctuations used for CGI,a thermal neutron flux spatial modulator with Gaussian distribution was proposed.The material selection and process preparation were analyzed in advance.On this basis,the influence of speckle statistical properties on the thermal neutron flux distribution was studied in depth,and the cadmium particles with Gaussian distribution with mean value of 500 μm and variance of 0.05 are used as the core of the modulator.The optimized thermal neutron flux spatial modulator was placed in the TNCGI system.When the sampling times are 500 groups,the neutron image of the measured object is successfully obtained through correlation calculation.With the increase of sampling times,the image quality is gradually improved.The results show that it is feasible to develop computational ghost imaging technology with neutron as source.At the same time,the study of TNCGI system will promote the further miniaturization of TNR equipment.In summary,the key components of the compact TNR system were studied through theoretical analysis and simulation methods.The moderator and collimator of the compact thermal neutron transmission imaging system were designed and optimized respectively to improve the thermal neutron flux and parallelism for TNR,and reduce the influence of stray neutrons and gamma-rays on imaging quality.The neutron source components of the compact TNCGI system were optimized to obtain a neutron beam with a better thermalization ratio.Combined with the correlation algorithm,an optimized thermal neutron flux spatial modulator was designed to obtain the neutron image of the object,which verifies the feasibility of applying the ghost imaging theory to the neutron field.The research results of this paper provide technical guidance for the miniaturization of TNR devices,and are of great significance to promote the development of miniaturized TNR. |
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