Development Of Ambient Dose Equivalent Secondary Standard Ionization Chamber

In 1985,the thirty-ninth report published by the International Commission on Radiation Units and Measurements?ICRU?recommended three operational quantities of radiation protection,namely,ambient dose equivalent,directional dose equivalent and personal dose equivalent.They can not only provide effective estimation for the protection quantities,but can also be used as the calibrating quantity of dose instruments in radiation monitoring.Among them,the ambient dose equivalent H^*?10?is used to describe the practical quantity of radiation monitoring in the environment or workplace under strong penetrating radiation?incident radiation energy>15keV?,and it's the only basis for area monitoring to estimate the effective dose equivalent H_E.In the monitoring of X and gamma external irradiation,the physical quantity obtained by the monitoring instrument is usually the air kerma K? or the exposure X,but the ambient dose equivalent secondary standard ionization chamber can be applied to directly determined the ambient dose equivalent of the reference point in the radiation filed.The traditional ambient dose equivalent chamber usually uses the compensatory materials?Al,Pb,Si,etc.?to change the energy response curve on the ionization chamber wall?usually high pressure pole?.The ionization chamber is mainly composed of high voltage pole,collector pole,support bar,insulation terminal and working gas.In previous studies,the working gas usually used by researchers is free air,and the internal cavity of ionization chamber is interconnected with the outside atmosphere,which may cause the working gas be affected by the factors such as temperature,humidity,air pressure and dust in field environment and it's not conductive to the on-site measurement of the ionization chamber.In addition,because of the charge pressure of the working gas is proportional to the sensitivity of the ionization chamber,therefore,the sensitivity of chamber with ordinary pressure is lower than the situation with higher pressure,in the actual field measurement,the scattering effect caused by walls and air will produce o lot of low energy photons,which would affect the accuracy of the measurement of low energy radiation.It is obvious that the traditional ionization chamber is not conductive to the on-site consideration of ambient dose equivalent.It is necessary to develop the environmental radiation monitoring,nuclear facility routine and emergency monitoring,so it is important to realize the development of the secondary standard ionization chamber suitable for field measurement.Accordingly,this paper relies on the field calibration technology of GM tube gamma radiation dosimeter,which is based on the metrology and testing department of China Institute of Atomic Energy,an ambient dose equivalent secondary standard ionization chamber is developed.Firstly,the determination of the key parameters of the ionization chamber are completed with the Monte Carlo simulation.Secondly,the design,processing and assemble of the ionization chamber are completed with the cooperation of the factory.Finally,the study of radiation dosimetry characteristics of ionization chamber is completed,including energy response and linear testing,and the accuracy of ambient dose equivalent measurement is verified by experiments.This work has finally realized the development of the ambient dose equivalent secondary standard ionization chamber,which has the advantages of free from ambient temperature,humidity and air pressure,wide detection range and high sensitivity,it can be used to directly measure the ambient dose equivalent.The main research results are as follows:?1?Using Monte Carlo simulation to determine the energy compensation scheme of the ambient dose equivalent secondary standard ionization chamber,and the design of the ionization chamber was completed.By contrast,the experiment energy response curve of the ionization chamber is in good agreement with the simulated one,it greatly reduced the time and effort when compared with the experimental method.The feasibility of Monte Carlo method in the development of the ambient dose equivalent secondary standard ionization chamber is verified.?2?The design scheme of the ionization chamber of the stainless steel wall material and the 8atm argon gas are selected for the first time.The closed cavity structure makes the ionization chamber not affected by the temperature,humidity and air pressure of the external environment,which satisfies the requirement for the on-site determination of ambient dose equivalent value.?3?The ambient dose equivalent secondary standard ionization chamber is placed in gamma ray reference radiation field produced by ¹³⁷Cs and ⁶⁰Co for linear test.The results show that in the range of 0.684?Gy/h¹.01mGy/h of kerma rate and the range of 0.8208?Sv/h¹.212 mSv/h of ambient dose equivalent rate has a linear relationship with the ionizing current,and the fitting relation curves are obtained respectively.?4?The feasibility and accuracy of the on-site determination of the ambient dose equivalent secondary standard ionization chamber produced by the self-shielded gamma ray irradiation device were studied and compared with the PTW ionization chamber.The experimental results show that the relative deviation between the two measurements is less than 5%,which verifies the accuracy of ambient dose equivalent secondary standard ionization chamber.The uncertainty of ambient dose equivalent rate measurement at typical reference point is U=7.4%?k=2?.