The Application Of Large-volume Graphite Cavity Ionization Chamber In The Absolute Measurement Of ?-ray Air Kernal Energy

Since the discovery of X-rays by Wilhelm Rontgen in 1895,many standardized methods have been designed and built to quantify the ionizing radiation field.It is often the case that using dosimeter measures the dose in medium imparted by the ionizing radiation.At this point,it is necessary to establish a certain relationship between the response of the dosimeter?which can be considered as the reading of the instrument in most cases?and the dose in the medium to achieve the purpose of the measurement.In many types of dosimeters,the ionization chamber is a commonly used gas-filled detector that measures the dose in the cavity primarily by collecting the ionized charge in the cavity gas.In addition,because of the small uncertainty of the kerma in the dissemination of the value of quantity and the close correlation with biological effects,the kerma gradually replaces the application of the radiation in the radiation field.In the ¹³⁷Cs gamma ray reference radiation field,the graphite-walled cavity ionization chamber is often used for absolute measurement of air kerma.The purpose of this study is to investigate the absolute lower limit of ¹³⁷Cs gamma ray air kerma.The significance of this research is that the air kerma below the protection level in the ¹³⁷Cs gamma ray reference radiation field can be obtained by the absolute measurement method,and the measurement uncertainty of the measurement result is reduced compared to the indirect measurement method.The final measurement result can be used to calibrate the dosimeter in the radiation field.From the above purpose of the research,the cavity theory correction factor of the graphite-walled cavity ionization chamber is first calculated.The purpose of introducing the cavity theory correction factor is to reduce the effect of the volume of the ionization chamber on the application of the cavity theory.The fraction of energy deposited by photon F_air and the Spencer-Attix cavity theoretical correction factor k_SA in different volume spherical graphite cavity ionization chambers are calculated by EGSnrc program.According to the above calculated results,the volume of the graphite cavity ionization chamber of this design is determined to be around300cm³.The large-volume graphite cavity ionization chamber is designed with reference to the domestic and international air kerma standard device.In addition,the Monte Carlo code?EGSnrc?is used to calculate the wall effect correction factor,axial non-uniformity correction factor,rod scattering correction factor and cavity theory correction factor of the large-volume graphite-walled ionization chamber,and the recombination correction factor,axial nonuniform correction factor,air density correction factor,and humidity correction factor are determined by experiments.Moreover,the performance of the large-size graphite-walled cavity ionization chamber is also tested,such as saturation curve,leakage current and repeatability of the ionization chamber are obtained.The test results show that the large volume graphite-walled cavity ionization chamber meets the performance requirements mentioned in the JJG 912-2010 Therapeutic Level Ionization Chamber Dosimeter Standard.The lower limit of the effective dose rate of the large volume ionization chamber is determined by comparing the ratio of the leakage current to the ionization current at different dose rate.The absolute measurement method is used to measure the air kerma?rate?in the ¹³⁷Cs gamma ray radiation field at the lower limit of the effective dose rate of the large volume graphite cavity ionization chamber,and the uncertainty of air kerma reproduced by absolute measurement and relative measurement method is compared.The results of this study show that the large volume graphite-walled cavity ionization chamber can meet the performance requirements of the relevant dosimeter standard and can achieve lower limit of the effective dose rate than the small volume graphite-walled cavity ionization chamber.Comparing with the relative measurement of air kerma,the absolute measurement using a large volume graphite cavity ionization chamber can achieve lower uncertainty.This study can provide relevant experience and reference for exploring the absolute lower limit of ¹³⁷Cs gamma ray air kerma and designing,testing and using a large volume graphite-walled cavity ionization chamber.