The Circuit Design Of Current To Frequency Converter For Dose Monitoring In Heavy Ion Therapy

As the physical properties of energy loss focused on the end of range and the high relative biological effectiveness(RBE),heavy-ion tumor therapy is considered as the best radiotherapy for cancer.Based on the Heavy Ion Research Facility in Lanzhou,Institute of Modern Physics(IMP),Chinese Academy of Sciences,has designed a heavy ion cancer therapy system and has completed shallow-tumor therapy and deep-seated tumor therapy.China has become the fourth country in the world to carry out heavy ion clinical experiments.In order to make sure that the tumor is irradiated with a uniform and adequate dose and the surrounding tissues are damaged as little as possible,assure the safety and reliability of medical treatment in heavy ion clinical experiments,we need to monitor and measure the irradiation dose in real time.The dose-monitoring probe of the beam intensity monitoring system in the deep-seated tumor therapy is an integral ionization chamber: its output current signal is proportional to the beam intensity.The intensity of the beam in the deep-seated tumor therapy is 0~109pps(particles per second),and the output signal of the ionization chamber is less than 1?A,which belongs to the small signal.In order to measure the current signal,a new current to frequency converter was designed in this paper.During the design process of the circuit,we analyze kinds of the method of weak current measurement.Finally,we choose the method of integrating input current comparing with a variable threshold voltage and discharging when the threshold reaches its maximum value.This paper introduces the hardware and logic part of the circuit in detail,and shows the debugging and test results of two versions of circuits designed.We design two kinds of circuits with different precision: 10 p C/pulse and 0.5p C/pulse.The circuit with precision of 10 p C/pulse can measure current from 0.01 n A to 60?A,it can measure the bipolar current,and doesn't need complex operation when current's polarity is changed.The linearity is better than 0.46434% when measures negative current from 0.1n A to 70?A.It is better than 0.34288% when measures positive current from 1n A to 60?A.The circuit with precision of 0.5p C/pulse can measure current from 0.01 n A to 500 n A,and the linearity is better than 0.04168% in full range.The current to frequency converter can satisfy the requirements in practical applications.Finally,the paper summarizes the structure and function of the current to frequency converter,gives the plans for the next phase of work and prospect for circuit improvement in future.