Construction And Application Of Neutron Activation Analysis Platform Based On D-T Neutron Generator

The D-T neutron generator has the advantages of good neutron monochromaticity,controllable yield and high security.It has broad application prospects in fast neutron radiation damage,neutron photography,neutron activation analysis,oil logging,explosive detection,etc.The neutron activation analysis method has the advantages of good penetration,high measurement accuracy,short measurement time,multi-element measurement,and strong anti-interference ability.Food safety is one of the important issues related to the national economy and people's health.It is of great practical significance to carry out an on-line detection for food production.This paper aims to establish a set of neutron activation analysis system f or on-line detection of talc content in flour.Although the neutron activation analysis method based on the D-T neutron generator is clear in principle,some technical problems are still difficult to solve.It involves neutron measurment,neutron protection,neutron reflection and moderation,detector improvement,spectral decomposition algorithm,etc.A neutron activation experimental platform based on D-T neutron generator was constructed,and the neutron activation method was applied to the field of food safety.By optimizing the experimental conditions and improving the spectral decomposition algorithm,the measurement accuracy was improved.At present,the commercial applications of on-line neutron activation analysis are mainly concentrated on the heavy industries such as coal mining and cement production.The application in food safety expands the application scope of neutron activation analysis.The main research contents of this thesis are as follows:?1?Based on theoretical calculation and MCNP simulation,a neutron shielding structure with 50 cm thick concrete and 50 cm thick water tank was designed and built,and an L-shaped labyrinthwall was set to prevent neutron leakage caused by neuron scattering.The simulated values of neutron and?dose are in agreement with the measured values,which are all lower than the occupational exposure dose limit.Compared with other similar work,it achieves a better protection effect in a smaller space.?2?Three common materials:iron,copper and aluminum were irradiated in the neutron field.By measuring characteristic?rays generated by neutron activation reactions,the neutron yield of D-T neuton generator was calculated.An empirical formula of neutron yield,high voltage and beam intensity were measured and fitted.The neutron angular distribution of the D-T neutron generator was measured and discussed.By optimizing the position of the irradiated sample in the neutron field,the irradiation efficiency can be improved.?3?The delayed gamma-ray neutron activation analysis method was used to measure the talc content in flour.According to the radioactive growth and cooling curve of the irradiated sample,sufficient characteristic ray counts were obtained by optimizing irradiation and measurement time.The measurement results of 3-inch Na I?Tl?detector and 2-inch La Br₃?Ce?detector were compared.When the 3-inch Na I?Tl?detector measures 1779 ke V peak,the linear correlation coefficient between the characteristic peak count and the talc content was more than 99%.The relative error of 20measurements on the same sample was 2.07%,and the lower detection limit of talc was0.07%.The delayed gamma-ray neutron activation analysis method has the advantages of small sample quality,low detection limit,good measurement accuracy,but it takes a long time,so it is suitable for off-line sampling measurement in laboratory.?4?Prompt gamma-ray neutron activation analysis method?PGNAA?was used to measure the talc content in flour.Accurate content results can be obtained by library least-squares method.In order to reduce the systematic error caused by the variation of neutron energy spectrum and?self-absorption in different samples,the library least-squares method was improved.As a result,the average relative deviation between the calculated value and the theoretical value was reduced from 6.11%to 0.33%.The relative error of 30 measurements on the same sample was reduced from 4.81%to 2.98%.Although the detection limit of the PGNAA method is worse than that of the delayed gamma-ray neutron activation analysis method,its detection time can be reduced to 1min,and the measurable sample mass is larger,so it is suitable for on-line measurement of flour.The innovations of this thesis are as follows:?1?The neutron angular distribution of the D-T neutron generator was measured and discussed to improve the neutron irradiation efficiency.Beside the intrinsic angle distribution of the d-t reaction,the shape of the tritium target and the neutron scattering in the tube are also important factors affecting the angular distribution.By optimizing the position of the irradiated sample in the neutron field,the irradiation efficiency can be improved;?2?An empirical curve for thickness,characteristic peak count and talc content was established,so that influence of the thickness on the measurement result can be corrected;?3?The spectral decomposition algorithm for PGNAA was modified to reduce the system error caused by the variation of neutron energy spectrum and?self-absorption in different samples,and the accuracy of measurement results is improved.