| Since the discovery of X-rays by German physicists in 1895,the study of X-rays has never stopped.X-rays are widely used in various aspects of society due to their high penetrability,such as industrial flaw detectors and medical CT.However,X-rays are generated by a group of high-speed electrons striking a substance,It has high penetrability but at the same time inhibits and damages human tissue cells.if the human body is exposed to X-rays for a long time,it will make people Leukopenia in the body causes infection or decreased resistance.Even cause mutations in the body's internal cells to cause cancer.However,a small amount of X-rays does not pose a danger,so the measurement of X-rays becomes extremely important.X-ray measuring instruments makes the traceability of measuring instruments become extremely important.Due to the high energy resolution of the narrow spectrum series,the narrow spectrum of radiation quality is often used to calibrate and determine the relationship of the dosimeter response as a function of photon number.At present,the world's most reproducible narrow-spectrum series is Germany's PTB reaching 400 kv.For the measurement instruments that require higher energy reproduction requirements,it is very inconvenient.To perfect the reproducibility of the narrow-spectrum series,it is of great significance to study the measurement and research of(250-600)kv X-rays.The primary task of this project is to establish the radiation quality of the re-filtered narrow-spectrum series.The energy of the tube voltage(250-400)kv reproduced by the German PTB is based on the linear relationship between the narrow-band series tube voltage and the average energy in the ISO 4037 specification.The average energy of this energy segment is introduced.Then Monte Carlo simulation is used to simulate the energy spectrum of different energies.It is necessary to add metal sheets of different thicknesses and different materials to adjust the average energy of the energy spectrum,so that the simulated average energy is basically the same as the average energy of the external push.The deviation is within ±5%.This completes the establishment of the radiation quality.Next we will calculate the homogeneity coefficient and the effective energy by measuring the half-value layer and use the actual energy spectrum simulated by BEAMnr to establish the standard radiation field.After the establishment of the standard radiation field,the recurrence of the selectedmagnitude of the ionization chamber and the evaluation of the final uncertainty are considered.Because of the electron loss in the free energy ionization chamber of the high energy section of 400 kv and the sharp increase in the amount of primary electrons,we chose the graphite cavity ionization chamber.In the process of recurring the magnitude,we also need to get the values of 8 correction factors,some of which need to be measured experimentally,and the rest need to be simulated by the EGSnrc program.After obtaining the air kerma rate,we need to verify the results.On the one hand,we need to use the free air ionization chamber and the graphite cavity ionization chamber to simultaneously measure the air kerma rate of the N250 radiation to verify the performance of the graphite cavity ionization chamber.As a result,the relative deviation of the air kerma rate was found to be 0.4%;on the other hand,the A5 ionization chamber was scaled to observe the trend of the scale curve and the theoretical scale curve.The theoretical scale factor under N250 was obtained by interpolation and compared with the scale factor calculated by us.The results show that the deviation between the experimental scale factor and the theoretical scale factor is 0.34%.Finally,the uncertainty is evaluated,in which the class A uncertainty is 0.15%,the class B uncertainty is 0.46%,and the synthetic standard uncertainty is 0.44%. |
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