| Radiation dose rate changes rapidly from nature background to several Gray per hour(Gy/h)in a nuclear explosion accident or some serious nuclear and radiation events,and the change of dose rate range generally exceeds 8 orders.Due to the interplay between detection efficiency and intrinsic dead time,the traditional gas radiation detector such as Geiger-Müller(G-M)counter,is quite limited in measuring the whole range with only one counter except for cooperating with another type of counter,and with several counters operating,it would be prone to saturation and false alarm in a higher dose rate or more rapidly changeable radiation field.Thick Gas Electron Multipliers(GEMs),studied in the dissertation have large sensitive volume and fast response time,can effectively solve the contradiction between sensitive volume and intrinsic dead time,so a single thick GEM can measure wide range dose rate.Since thick GEMs were invented,the detectors are mainly used in the field of high energy radiation detection,and seldom applied in radiation monitoring,especially in portable monitoring instruments,due to the flow-air operating mode and complex readout system.For the first time,this dissertation systematically studied the X/? dose rate response characteristics of the thick GEMs,and explored sealing and integration technologies for them,which could provide some new technicics for the development of radiation monitoring instruments.The main contents and conclusions are summarized as follows:(1)The physical response mechanisms of thick GEMs to X/? rays were analyzed with the method of multi-physics field coupling.Using the finite element method(ANSYS),the transfer fields of the thick GEMs were analyzed,and the mechanisms of electron multiplication were studied by combining the fluid dynamics model(COMSOL)and gas discharge simulation method(Garfield++),thus it is explained that thick GEM has fast response time.The intrinsic dead time of the thick GEM in X/? rays detection was simulated and calculated by the Monte Carlo method and gas discharge simulation in the dissertation.According to the relationship between the gain and the signal discrimination efficiency,this study proposed the theoretical analysis method in detection efficiency measurement of the thick GEMs,as well as the theoretical model of the whole physical process from radiation to signal,which provided a theoretical basis for the detector design.(2)In this dissertation,a gas-flow type thick GEM detector was designed for X/?dose rate response test.In the study,the structure size of thick GEM film was calculated theoretically,and one of the five films with the best comprehensive performance was selected as the key component of the detector,these films had the same structure but different insulation substrates.The starting voltage of the chosen film was as low as510 V,with the operating voltage range over 160 V,the average gain over 8000,and better consistency than the other films.Through the optimization of structure and electric field as well as materials selection,a gas-flow chamber of the thick GEM detector was designed,with the incident window made of a 10?m thick copper-coated polyimide film,the drift electrode made of cellular-like stainless steel sheet with tungsten plated,and the drift region distance of 3 mm,collecting area distance of 2 mm.The chamber wall was made of organic glass,and the holes of gas-flow passed through the lateral wall,in turn,connected with the cylinder pressure reducing valve,flow meter and bubble bottle.(3)The main performance characteristics of the thick GEM detector for dose-rate measurement were comprehensively grasped.In this dissertation,a set of flow-gas thick GEM detector testing system was built using the separated high-voltage and modular readout circuits.The linear response range of the dose rate of the thick GEM detector to the X/? rays was measured from 0.3?Gay/h to 8Gy/h,using counting and current mode switching,so the range reached 8 orders of magnitude.The energy response and angular response characteristics of thick GEM detector were studied for the first time,which provided reference for the design of detector in the further study.This paper also focused on the irradiation stability of the thick GEM detector,analyzed the effect of "Charging up/down" using the multi-physics field coupling method,and explained the reason that the counting rate kept rising in the first half an hour after power on.Comparing with the performance of G-M counter,the detector has obviously advantages in the wide range dose rate measurements.(4)The key technologies of sealing and integration for thick GEM detector were explored.Based on a domestic pocket high voltage module and ASCI integrated chip,the resistance-chain type high voltage circuit and integrated readout system were designed respectively.The sealed thick GEM detector was designed using vacuum materials with low out-gassing rate and high precision air pressure control system.The characteristics of the detector under low pressure were studied,that provided a reference for the gas filling process and operating voltage of the sealed thick GEM detector.This dissertation also studied the long-term stability of the sealed thick GEM detector,where continuous working time was more than 60 days,and the stable working time was more than 30 days(the relative variation error of counting rate was not exceed ±15%).The initial achievements and academic contributions of this dissertation are mainly reflected in: 1)For the first time,the thick GEM detector was applied to X/? rays dose rate measurement,the response range of the dose rate with a single detector was extended to 8 orders of magnitude by the method of counting and current switching,which was wider than the range of traditional gas detector,that provided a new technic support for the application design of radiation detection device.2)Using the method of multi-physics field coupling analysis,the whole process simulation method fromradiation interaction mechanism to detector performance analysis was established,which provided a new idea for the virtual design of nuclear radiation detector.3)The characteristics of energy response and angular response were studied at the first time,"Charging up/down" effect for thick GEM detector was analyzed,and the effect of conduction current on gain stability was put forward.4)Closed and integrated technologies for thick GEM were studied.An integrated readout system was designed using ASIC integrated chip and resistance chain type high voltage module.Through the design of high vacuum sealed chamber and the optimization of materials with low out gassing rate,the detector can work continuously and stably for more than 30 days under the closed condition. |
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