"Fission-Electron Collection" Neutron Detection Technology Research

For some nuclear species,the cross section of neutron-induced fission show flat characteristic over a wide energy range,and fission-based neutron detector has a potential to achieve a flat energy response consequently."Fission electron-collection" neutron detector collects escaped electrons induced by fission fragments in fission material in vacuum environment to produce signal.Due to direct conversion from fission energy to electric signal and the avoidance of secondary reaction between fission products and intermediate material,"fission electron-collection" neutron measuring technique could reflect the flat characteristic of fission cross section.Meanwhile,this technique could achieve rapid time response since electrons are collected in vacuum.In this thesis,the "fission electron-collection" neutron measuring technique was studied systematically both with theory and experiments.The "fission electron-collection" neutron detector consists of two metal electrodes,labeled as the coated and collection electrodes.With physical analysis and Monte Carlo simulation,detector physical design and optimization,including choices of fission material and electrode material,and coating thickness determination,were accomplished.On this basis,we have determined the structural design of vacuum chamber,the design of electrode mounting,the scheme for vacuum obtaining and maintenance,and finally obtained the mechanical design scheme of the detector prototype.Also finished were works of development and verification,including the detector prototype fabrication and verification,large-area coated electrode manufacturing technique researches,neutron verification experiments on reactor and neutron calibration test on accelerator.Through the above-mentioned studies,we obtained some major conclusions:1)the energy response flatness of "fission electron-collection" detector exhibits a similar trend to the flatness of fission cross section;2)the escaped fragments,the choice of electrode materials and the electrode gap have no essential impact on detector output;3)in a certain range,the neutron sensitivity of detector increases with increasing layer thickness,and tends to be a constant beyond that range;4)the electrons move fast in vacuum,and the collection time of electric charge is on nanosecond scale;5)positive voltage of hundreds of volts would give several folds of promotion for sensitivity.Higher voltage would not have a better result,hence factors of linear current as well as vacuum maintenance should be considered in the choice of working voltage;6)the disturbance is difficult to be avoided in the sensitivity calibration tests for DT neutron,and the interference factors include circuit noise and scattering background;7)with combined measures of enhancing neutron fluence at measurement position and reducing noise current,the impact of circuit noise could be effectively reduced;8)the symmetrical structure of electrode has a good suppression effect for ? rays induced by neutron inelasticity scattering,and the impact of scattering neutrons could be eliminated with mathematical method by utilizing the difference of distribution of source neutrons and scattering neutrons.? rays are the main noise source for neutron measurements in mixed radiation field.On the basis of analyzing transport characteristics of ? rays and its secondary electrons,the limitation of y sensitivity suppression methods reported by paper was demonstrated.Meanwhile,we proposed a new scheme for y sensitivity suppression via Monte Carlo methods.The proposal balances the gain and loss of electrons for coated electrode by adjusting the electrode thickness and distance,and was well validated experimentally.According to the theoretical and experimental studies,the fundamental knowledge of the"fission electron-collection" neutron measuring techniques were obtained,and several key problems were solved which include the suppression of y sensitivity,the calibration in weak signal and high background environment,and the manufacture of large-area coated electrode.For the detector prototype,the sensitivity of DT neutron is 8.65×10-20C·cm2,the sensitivity of 60Co y ray is 1.79×10-21C·cm2,and the ratio of the two sensitivities is greater than 40:1.The FWHM of the response function of prototype is less than 3 ns.The "fission electron-collection" neutron detector provides several advantages of rapid and flat-energy response and high discrimination of n/? as well.The shortcoming of the detector is its low-usage of fission energy,so the sensitivity is low but do not have the function of signal amplification.The results from this work could provide high precision diagnostic measures for the study of high-energy-density physics,advanced fusion energy source,and neutron physics.