Measurement And Research For Air Kerma Of X-ray Narrowspectrum Series

With the continuous development of science and technology and the economic globalization,the level of national measurement technology has become an important means and technical guarantee for realizing high-tech industrialization and improving the overall strength of the country.As the national measurement institution,the National Institute of Metrology of China represents the highest level of national measurement technology.Based on the measurement science research,it establishes and improves the national measurement standards,participates in the comparisons at home and abroad actively,and completes the system construction of quantity transferability and traceability.In the field of ionizing radiation,the National Institute of Metrology of China has established(10-60)kV X-ray air kerma primary standard and(60-250)kV X-ray air kerma primary standard,but has not achieved comparison of national air kerma standards for narrow spectrum series.In order to solve the problem of quantity traceability of the radiation protection instrument under X-ray radiation quality,under the support of the subject of the central public welfare institutes “Measurement methods research on X ray air kerma in environmental level”(project number :25-AKY1707)"Measurement and Research for Air Kerma of X-ray Narrow-spectrum Series" was specially developed to complete the quantity reproducibility for Air Kerma of X-ray Narrow spectrum Series.According to the ISO 4037-1 international standard published by the International Organization for Standardization,we designed independently and processed a measuring device for half-value layer.By selecting different tube voltages,tube currents,and additional filtration including lead,tin,copper,and aluminum,we established the corresponding reference radiation.The narrow spectrum series in the range from 60 kV to 300 kV of X-ray reference radiation were established on the medium energy X-ray reference experimental device.compared with the standard values given in ISO 4037-1,the thickness deviation of all additional filtration and the deviation between the first half-value layer and the second half-value layer are all within ±5%,which meets the requirements of the standard specification.Under the narrow spectrum series of X-ray reference radiation,the absolute measurement of the air kerma rate was carried out by using free-air ionization chamber standard and microcurrent measurement system.The correction factors required for the absolute measurement were obtained by experimental measurement and Monte Carlo simulation,thus the air kerma rate at a distance of 1 m from the focus of the spot can be determined.The A5 ionization chamber used in protection level was calibrated at a distance of 1 m from the focus of the spot,in order to obtain the calibration coefficient under the narrow-spectrum series.When positioning the A5 ionization chamber at a distance of 3m from the focus of the spot,we can determine the air kerma rate at that point.By using the equivalent substitution method,the PTW32002 ionization chamber was calibrated at a distance of 3m from the focus of the spot,therefore we can get the calibration coefficient under narrow-spectrum series of the PTW32002 ionization chamber.The calibration coefficient of the PTW32002 ionization chamber under narrow-spectrum series was obtained by relative measurement method.By comparison,the deviations of the calibration coefficient were all within 0.7%.The air kerma rate at a distance of 4m from the focus of the spot was determined by the PTW32002 ionization chamber.Two high-pressure ionization chambers were calibrated at these radiation qualities,including X-ray radiation qualities of narrow-spectrum series,Am-241 and Cs-137 gamma radiation qualities.Then we can get energy response curve of two chambers,where we can find the energy response of two chambers are all good enough to meet the test requirements.At the end of this paper,the uncertainty of air kerma rate under the narrowspectrum series at a distance of 1m from the focus of the spot was evaluated.The type A uncertainty was 0.46%,the type B uncertainty was 0.37%,and the combined standard uncertainty was 0.59%.This research work has perfected China's X-ray radiation protection system,laying a solid foundation for the subsequent comparison of national air kerma standards for narrow spectrum series.