Research On The Micro-current Measurement System In Ionizing Radition Metrology

Ionizing radiation measurement is a discipline that concentrates on the research of ionizing radiation measurement unit. It fully reflects the interaction between materials and ionizing radiation. It involves nuclear physics, radiation protection, biological medicine, metrology, environmental engineering, clinical medicine and other fields. As one of the most important manifestation modes of the interaction between materials and ionizing radiation, weak current is the important mean to research basic physical quantity of rays in the area of ionizing radiation measurement.The thesis is based on the project of national institute of metrology P.R.china: the research of X-ray radiation field measurement and measure method of low-energy X-ray air-kerma. With the background of participated Asia low-energy X-ray air-kerma key comparison,through engineering design and experimental analysis,we established tow measurement system based on the Townsend compensation method and high precision celectrometer.The establishment of the system will become an important component in the low-energy X-ray of air-kerma standard and provide essential support in this international comparison. The system can be applied toγ-ray measurement of ionizing radiation dose activity measurement, radionuclide field ionization current measurements, and has a very important promotional value for the transfer and test and calibration instrumentation.This paper focuses on the micro electric current measurement system of ionizing radiation metrology. Micro electric current is not only the result of interaction between materials and ionizing radiation, but also the main physical quantity during the process of using the ionization chamber to detect rays. From the foundation knowledge of radiation measuring technology, this article introduced the basic principle and method for detecting X-ray ionization current successively. Radiation field device that used to produce ionization current, the scheme of micro electric current measurement and the actual engineering design was evaluated through the test contrast in physical experiment finally.The process of ionization currents and detection methods are introduced in the basic principles and methods of detecting X-ray ionization currents, and detailed the micro-current's principles and characteristics of using high resistance, drift method and the measurement of Townsend compensation.Low-energy X-ray standard radiation device is an important part in a national primary standards and national standards of measurement instruments, also is indispensable conditions in the micro-current measurements.The meaurement and research of radiation field has great meaning toward national benchmarks for the air kerma value reproduction, international comparison and value transfer. Standard X-ray radiation device including X-ray machine system, automatic filter, positioning and monitoring system, control software. Low-energy X-ray radiation field uniformity of ionization current measurement accuracy is also an important factor. Poor uniformity of radiation field is bound to increase the measurement uncertainty, in order to measure the value of micro-current accurately, thus to achieve low-energy X-ray value reproduction of national benchmarks and achieve instruments value traceability,it provides the reference conditions for the chamber calibrations are as follows: distance from the focal spot to the reference plane (the front surfaxe of the chamber) is 1000mm and radiation field size at the reference plane is 10cm in diameter. We used film dosimeters and two-dimensional matrix flat-panel detectors and ionization chambers measured the low-energy radiation field, and achieved good results.There are many factors to impact measurement accuracy of ionization current, one of the main factors is the system's leakage current. This paper analyzes the sources of leakage current and the methods to reduce the leakage. Towards the principles and characteristics of low-energy X-ray ionization currents, we established three measurement systems based on the Townsend compensation (automatic, semi-automatic, manual), and high-precision Keithley 6517 electrometer, PTW UNIDOS dosimeter micro-current measurement system. Townsend compensation measurement system includes the following components: current compensation circuits, voltage feedback, feedback compensation, measurement and control unit and so on. Hardware of the system includes the voltage source table, electrometer, high precision digital multimeter, precision components such as capacitors and precision timers. System software can be based on LabVIEW graphical programming, developed a set of instrument control, data acquisition and processing, stored in a multi-purpose control software. Keithley 6517 electrometer system is designed using LabVIEW, it can real-time display and storage the value of the ionization current and charge. After the system is established, we conducted a preliminary performance test, and results show that the system has good repeatability and short-term stability.In this paper transfer ionization chamber AE1340C which is used for key comparison of low energy x-rays air kerma in the Asian-Pacific region was selected to test the performance of the system. Based on the five radiation qualities that recommended by CCRI, we measured the size of ionization current on the reference plane about 1000mm from the lightpipes focal spot. This paper emphatically introduced the townsend compensation system, 6517 electrometer system current and charge gear measurement result, and analyzed the possible existence of deviation. At last it made a brief evaluation for the adapt occasions of the several systems.