(250-600)kV Low Air Kerma Rate X-ray Absolute Measurement Method And Research

X-ray was discovered by the German physicist Roentgen when he was engaged in cathode ray research in 1895.After more than 100 years of development,X-ray has been widely used in many fields such as industrial,medical,radiation protection,environmental monitoring and safety inspection,which makes the traceability of X-ray measuring instruments more important.The low air kerma rate series is suitable for measuring the energy response of the transfer ionization chamber due to its low resolution and good monochromaticity.However,internationally,the X-ray air kerma value reproducible range of the low air kerma rate series only reaches(10-240)kV,but it is not involved for the higher energy stage,which will cause the transfer ionization chamber's energy response curve is incomplete in this energy segment,which makes it difficult to pass the ionization chamber measurement integrity and accuracy,so it is necessary to study and absolute measurement(250-600)kV low air kerma rate X-ray air kerma.The existing X-ray air kerma benchmarks in the world use free air ionization chambers for absolute measurement and magnitude reproducibility,but for high X-ray energy(usually greater than 400 keV),the free air ionization chamber is not suitable due to the loss of electrons and the increase of the primary electronic track.This study will apply the graphite cavity ionization chamber with good energy response to the higher energy segment as the reference ionization chamber for absolute measurement.Firstly,according to the linear relationship between the tube voltage and average energy of the low air kerma rate series in ISO 4037 specification,the average energy with the tube voltage range of(250-600)kV is derived.Using BEAMnrc simulation,the deviation between average energy of energy spectrum and extrapolated average energy of different materials with different thickness was within ±5%.Then,according to the simulation results,the(250-600)kV X-ray low air kerma rate series standard radiation field was established.The half value layer was measured by curve fitting method,and homogeneity coefficient and effective energy were calculated simultaneously.The actual energy spectrum of the radiated material in the standard radiation field is simulated by BEAMnrc.In the study of X-ray air kerma with the graphite cavity ionization chamber,the ionization current,the composite loss correction factor,the stem scatter correction factor,the beam axial direction and the radial non-uniformity correction factor were measured by experiments.The EGSnrc program was used to simulate the wall correction factor,graphite and air stopping power ratio,air and graphite mass energy absorption coefficient ratio and bremsstrahlence fraction constant.The whole experimental process and method were verified under a narrow spectrum of 300 kV radiation mass.On the one hand,compared with the free air ionization chamber results,the relative deviation of air kerma rate is 0.85%.On the other hand,1L sphere was calibrated,and the deviation between the experimental scale factor and the theoretical scale factor was 0.25%.Finally,the uncertainty of each component in the reappearance process is evaluated.When the uncertainty of air ionization power is 0.15% and 0.35% respectively,the uncertainty is0.32% and 0.45%,which meets the requirement of the reappearance of the air ratio released kinetic energy in the graphite cavity ionization chamber,providing reference for subsequent studies.