| In order to ensure the accuracy and reliability of the measurement data of neutron radiation monitoring instruments,the International Organization for Standardization(ISO)and relevant countries have developed standards for verifying the measurement value of the instruments and calibrating the instruments.The relevant professional institutes for ionizing radiation have generally built reference neutron radiation field for the verification and calibration of neutron radiation monitoring instruments.Generally,reference neutron radiation field is a fixed reference radiation field with large size and reinforced concrete shielding structure outside the field.At the same time,there are some problems between the users of neutron radiation monitoring instruments and the professional ionizing radiation metrology institution,such as the long distance and period of verification,and the process of verification affecting the normal operation of other equipment.Therefore,the development of neutron radiation calibration and metrology technology in site is conducive to the realization of neutron radiation calibration from the appointed verification at fixed period to the verification in site at fixed period,and thus improve the verification efficiency.To solve the problems caused by appointed verification,we have proposed minitype reference neutron radiation(MRNR),which is an important part of the neutron radiation calibration and metrology technology in site.The device of neutron radiation calibration and metrology in site obtained by materializing MRNR has the characteristics of small size,light weight and mobility,which can be sent to the neutron radiation operation site for verification and calibration of the instrument on-site.The resulting problem is that the scattering neutron components in MRNR are complex,which will affect the calibration results of the instruments seriously.At present,there is no systematic research on MRNR at home and abroad,and no relevant literatures have been found.Therefore,relevant studies on the existing reference neutron radiation field can provide the necessary theoretical basis for the in-depth study of MRNR.The research work of the thesis mainly includes:(1)On the basis of extensive investigation and reading of domestic and foreign literatures on reference neutron radiation,the basic principle of neutron and material interaction is clarified.Based on above,this thesis firstly proposed and researched the technical route for the study of the dose field characteristics of MRNR and two key technical problems——the scattering neutron contribution correction and measurement uncertainty during calibration.(2)Monte Carlo particle transport simulation software MCNP5 was used to study the field metrological characteristics of the free-field reference neutron radiation(FRNR),the minimum size of standard reference neutron radiation field(SRNR)specified in GB/T14055 series standards and an example reference neutron radiation field(ERNR).It is found that air has little effect on the dose rate and energy spectrum distribution in FRNR,and the contribution of air scattering dose rate is less than 2%,which can be considered as the ideal reference neutron radiation field.In SRNR of different shapes,sizes and shielding materials,the dose rate of each point of test has increased,and there was a certain difference between the energy spectrum in the field and the neutron source energy spectrum.In SRNR shielded by polyethylene containing 5% boron or concrete,the contribution proportion of scattering neutron dose rate obtained by shadow-cone method is close to the theoretical value,while the effect of scattering neutron on the field dose rate,contribution proportion of dose rate and energy spectrum distribution of SRNR can be reduced in SRNR shielded by polyethylene containing 5% boron.Compared with SRNR,the field dose rate and the contribution proportion of scattering neutron dose rate in ERNR are smaller,and has a higher similarity with the energy spectrum of the neutron source.The contribution proportion of scattering neutron dose rate obtained by shadowcone method is closer to the theoretical value,and the calibration range of instruments to be calibrated is also larger if the contribution proportion of scattering neutron dose rate is less than 40%.(3)The influence of reference radiation field size and shadow-cone method to modify scattering neutrons on the metrological characteristics of MRNR is discussed,and compared with typical reference neutron radiation fields.It was found that the size of MRNR is smaller,the more the scattering neutrons contributed to the dose rate of each point of test in the fields,and the number of neutrons in the energy range of 0.25-0.5Me V increased significantly.Considering the size of MRNR,the volume of subsequent internal equipment and the size of calibration range,it is planned to choose the MRNR of2m×1.2m×1.2m.At the same time,it is found that the shadow-cone method is no longer applicable to modify the effect of scattering neutrons in MRNR.In the range of energy higher than 7Me V,the energy spectrum curve of MRNR is basically coincident with the energy spectrum curves of three typical neutron reference radiation fields,which indicates that the neutron energy spectrum distribution in this energy segment is basically independent of the shape and volume of the reference neutron radiation fields.Furthermore,the influence of the detector on the metrological characteristics of MRNR and typical reference neutron radiation fields is analyzed.It can be found that the detector is placed after,the dose rate and energy spectrum distribution of the four kinds of reference neutron radiation fields in the front,inside and back of the detector all change significantly. |
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