Research On The Radiation Field Of Air-kerma Standards In X-rays

With the progress of technology and development of society, X-ray has been applied widely for its importance in medicine, production, science and daily life. With the increasing device quantity of X-ray performance testing and quality assurance, it is urgent to solve the problem in which many indexes have not been regulated so far. As one of the important physical quantity, Air-kerma is the important research foundation in measure of X-ray.The thesis is based on the project of national institute of metrology P.R.china: the research of medium-energy X-ray and measure method of low-energy X-ray air-kerma. With the background of ionizing radiation field quantity and, through experimental studies and theoretical studies, from study of irradiation quantity to Air kerma, we establish Air kerma standard of medium-energy and low-energy X-ray which is supposed to handle the quantity source problem of medical treatment level standard dosimeter, diagnosis level standard dosimeter, protection X-ray radiation dosimeter. As one of the key comparison quantity, after the research of the project, we can fill the blank of the absolute measurement of low-energy X ray air-kerma.In this thesis, our research is mainly about X-ray air-kerma radiation field, which is of great importance in the research of X-ray air-kerma. The thesis is discussed in details of establish of base system, measurement of HVL, uniformity of Radiation field and measurement of energy spectrum.Low-energy X-ray air-kerma and medium-energy X-ray air-kerma are in consisted of base systems. Base system include X-ray Occurrence and collimating device, X-ray filter device, Mechanical-locate–Screen devices, Free air ionization chamber and the weak current measurement system. Here are the mainly hardware of the system: X-ray machine,filter, Mechanical positioning guide device, High Voltage Power, Electrometer, Digital voltmeter, High Resistance Meter, High capacitance, Temperature Barometer and so on. With LABVIEW software we can complete the entire base performance testing. In the initial establishment of system, we study the radiation field by physical experiments.In the establishment of standards system, low-energy and medium-energy X-ray machine are industrial applications. Considering the stability, YXLON MG324 and YXLON MG165 which are suitable for values repetition of X-ray and whose performance meet the requirements of standards established are used to the medium-energy X-ray machine and the low-energy machine respectively. In order to ensure that the X-ray machine is stable and accurate installation, the optical platform of X-ray machines is designed to as its carrier. The expected radiation field is ensured by the designed alignment device. The X-ray filtering device is indispensable in the research of X-ray radiation field, which may realize the automation and mainly consists of the filter, the filter disks, rotating platform and controller. The purity of 99.99% aluminium and 99.95% copper are selected for the filter. The mechanical positioning device consists of laser tracking instrument and mechanical rail, which ensure the geometric conditions are strictly identical when standards values is repetition, and which make the positioning accuracy of the dot of standards values repetition reach±0.1mm. Monitoring system, including video monitoring, safety monitoring and ionization chamber monitoring, is designed to ensure the accuracy and safety of the operation of standards system.The free air ionization chamber, used by the absolute measure of the air-kerma for standards system.According to the design principles, it is consists of high-voltage polar, collector, protective polar, protective ring, aperture and shielded enclosure. The collector of the free air ionization chamber is embedded in the grooves of protective polar and fixed by the amber screws. The protective system is made up by 18 circles hard aluminum rectangle ring. The limited aperture is composed of tungsten alloy, whose ingredients are 89% of tungsten and 7% of nickel and 4% of the copper.The weak current automatic measurement system of datum system which is based on the principle of Thomson balance method, is composed by remote source table (6430), high insulation resistance meter (6517A) and high-accuracy capacitor etc. The software component of monitoring and control unit, which take LabVIEW which is based on virtual instrument technology as a development tool, has developed the monitoring and control software. The design method of the operating software of monitoring and control system is modular design. The whole monitoring and control software can be divided into subscriber management, equipment parameter setting, compensation setting, data processing, database management and other modules.When the establishment of the X-ray air-kerma datum system was finished, we started our research on X-ray air-kerma datum radiation field. As an important parameter of the description of X-ray beam quality, HVL, which is the thickness of standard that required to make air-kerma rate (or exposure rate) of X-ray beam reduce by half at a certain point, is the measurement of the effective average energy of ray. In this paper, we firstly took a comparative study on the method of the HVL measurement and made a conclusion that we should choose interpolation measurement when the interpolation space selecting is appropriate. Then we detailedly designed an experiment of HVL, from what we got 5 energy specifications in the low-and-medium-energy X-ray ranges.Then compared with the BIPM, it achieved the desired results.In this paper, we also studied on the stability of tube-voltage and tube-current of X-ray generator, measured the influence on the dose rate of radiation field that caused by the value changing, and analysed the influence of performance index of the X-ray generator that currently used which works on the uncertainty of realization of the reference quantity. According to the experimental results, the international key comparison of the radiation beam using standard conditions, the tube voltage fluctuation of 0.01%, the dose rate changes from 0.017 to 0.04%; tube current volatility of 0.01%, 0.01% change in dose rate.The results show that the subject of X-ray tube voltage and tube current target is in line with benchmarks to establish repeatability and value transfer request.Uniformity is an important index of radiation field. We studied on the uniformity in low and medium energy air-kerma datum radiation field respectively, and got the beam ranges of radiation field in different uniformities. The result contented the use of realization and transfer of the quantity of X-ray air-kerma datum radiation field.The spectrum of medium-energy X-ray is measured researchfully and it's spectrum in different specification is obtained. This paper calculated peak positions, FWHM and other related information and analyzed. The shield alignment system,NaI (TI) 2×2 in detector and digital spectrometer, are adopt for spectrum measurement devices. Finally, discussion result of this X-ray spectrum measurement is given and the related spectrum measurement work in the future is prospected in this paper.