Dose Measurement System Research Based On Cavity Ionization Theory

Ionizing radiation is widely used in agricultural breeding,radiation processing,medical diagnosis and treatment,industrial inspection,nuclear powerand other fields.In the activities of exposure to ionizing radiation,the human body by the irradiation dose exceeds a certain limit,harmful effectswill happen to the body to produceradiation injury.Under the effect of ionizing radiation,the body to suffer the damage degree depends on the type and dose of ionizing radiation,irradiation conditions and the sensitivity of the body,so the radiation dose of accurate measurement or not relates to the radiotherapy patients and staff's health and life safety.Cavity ionization chamber whichis the most widely usedin ionization radiation dosimetry isthe one of the most important sensors for radiation measurement,and it's usually used for irradiation dose and absorbed dose measurement of X-raysand gamma rays.Cavity ionization chamber adopts the cavity ionization theory,namely the radiation doseabsorbed in the media can be measured by the ionization in gas cavity.The absorption dose is measured by the cavity ionization chamber,and ionization quantity of cavity is converted to the absorption dose of the medium.However in practical application,how to design the ionization chamber for various measurement conditions by using the theory of cavity ionization and a dose meter designed for weak current signal measurement with high accuracy and stability are two key technologies of dose measurement system.In this paper,the energy response of the ionization chamber under different structures and materials is exploredfor the technical difficulties in the measurement of CT dose index and dose area product by using Monte Carlo simulation method which based on the theory of cavity ionization.Under the guidance of the theory and the simulation,the CT ionization chamber and the dose area product ionization chamber are designed.The energy response problem in the measurement of the ionization chamber is successfully solved,and a multi channel CT dose measurement system and dose area product measurement system were constructed.So the work in this paper has important research significance and practical value.The main research content of this paper are:(1)the Bragg-Gray theory,Spencer-Attix theory,and the theory of Burlin model and equivalent electronic source cavity ionization theory and scope of various kinds of theory are introducedrespectively,and the structure and working principle of the ionization chamber are studied in this paper.(2)Simulate X-ray transport process in the ionization chamber using Monte-carlo method,and study on the relationship between structure or materials of ionization chamber and the energy response.(3)Develop a long pencil-type CT ionization chamber and a transparent parallel plate ionization chamber.Research the structure and materials of chamber,and do some experimentsfor comparing with the results of simulation.(4)Study the amplification of weak current bycombination of long pencil-type CT ionization chamber and transparent parallel plate ionization chamber.Discussthe principles of preamplifierand the process of assembly measures in weak current measurement,and analyze the noise model of weak current measurement,then improvethe weak current measurement accuracy greatly.(5)Develop a weak current signal source used in weak current detection and a small precision high voltage power supply for ionization chamber,and it provides the technical support for the development of the dosimeter.(6)Research on the weak current measurement from output of CT chamber,and build a precise measurement system for multi-channel weak current.(7)Research on the weak current measurement from output of dose area product chamber,and develop a dose area meter which can be integrated into diagnostic X-ray machine.There are four research results and innovation pointsin this paper:(1)The simulation calculation of energy response of chamber under different wall material has been carried on using a monte-carlo simulation programand it providedthe design data reference for the development of the new type of ionization chamber.(2)Anew-typelong pencil-type CT ionization chamber used for CT dose index measurement and a new-typetransparent parallel plate ionization chamber used for dose area productmeasurement have been developed,andthe technical performance of ionization chambers reached the same level of foreign countries' products.(3)A multi-channel CT dosimeter has been successfully developed for CT measurement,and its patent for invention and utility model has been obtained.The multi-channel CT dosimeter developedsimplifies the measuring process,and provides the powerful technical equipment for the quality control of CT machine in our country.(4)A dose area product meter for monitoring radiation dose area product level of patients have been successfully developed,and realized the integrationwith the diagnostic X-ray machine which communicates other devices using CANopen protocol.It conforms to theinternational standard ofdose area product meter IEC 60580-2003,and can effectively replace the similar foreign products.