The Measurement Of Gaseous Tritium Based On Proportional Counter And The Research Of Calibration Methods

Tritium (3H) is a radioactive isotope of hydrogen, which can decay β-raysspontaneously. Inhaled a large number of tritium would arising the internal radiationhazards on human body, so around the nuclear power plants, nuclear reactors andrelated nuclear facilities, we need to measure the concentration of tritium in theenvironment. Due to the rapid development of nuclear technology industry,especially after the launch of the hydrogen bomb and other human testing of nuclearweapons, the amount of tritium released had reached200times compared to thenatural amount, resulting in the rapid increase in the concentration of tritium in theatmosphere. After the1970s, the concentration of tritium in the environment hasdeclined, and the current natural tritium concentration is about1.4×10-3Bq/m3.Because of lack of standards gaseous tritium source,the calibration of themeasurement results is particularly important. The use of the gas proportional counterto measure tritium activity concentration is an absolute precision β activitymeasurement method which is internationally recognize, it can accurately measureand obtain results fast and efficiently. But at the same time it faced with calibrationand optimization problems of proportional counter detection.In this paper, based on an independent research of tritium concentrationmeasuring apparatus, a large number of experimental data is used to analyze thefactors that affect the tritium concentration measurement and calibration methods. TheMonte Carlo simulation results and experimental measurements are combined tofurther optimize the detection system. An ionization chamber and the end effect, thewall effect are used to correct the results of research and analysis, to improve theaccuracy of measurement results. And it provides some reference value for themeasurement of tritium concentration.The main contents are as follows:(1) For in-depth studying the ionization process and the factors in theproportional counter, use the Monte Carlo method to simulate the changes of countingrate in proportional counter under different counting gases，different working gaspressure, and under adding the shielding. Simulation results show that the gas samplecomponents (P20, P15, P10, P5, pure argon) has small changes on the counting rate; when working gas pressure dropped from100kPa to50kPa, the relative uncertainty is1.12%.When the pressure over200kPa, the wall effect is less than1%. Detectionsystems after adding anti-coincidence shielding-the plastic scintillator, made the totalnumber reduced of83.54%, which significantly reduced system background. Throughsimulation results which can further optimize the measurement system.(2) Proportional counter measure tritium concentrations were studied. After theevacuation of the self-designed and developed gaseous tritium concentrationmeasuring instrument, filling with counting gas and working gas to measure. Studythe background values of different proportional counters, system stability, andhigh-pressure plateau curve, and different concentrations of tritium measurementcurve, determine the optimum working conditions for each branch proportionalcounter.(3) The experimental results were analyzed using an ionization chamber detectorcalibration, to study the calibration methods of different concentration. While use theinflated volume subtraction to achieve the absolutely measurement of tritiumconcentration, the uncertainty of high concentration and low concentration of tritiumsample were1.19%、0.65%，which introduced by the length compensation. Thoughthe relationship of the pressure and the counting rate value to research the loss of thewall effect, the relative uncertainty is13.1%, which is far more than the simulationresults, the wall effect losses at this time can not be ignored. Achieve the calibrationmethod to proportional counter tritium measurements though the analysis of the endeffect and the wall effect.The main innovations of this paper are:(1) The Monte Carlo simulation and experimental results are in contrast to jointlyanalyze the impact of the wall effect in tritium concentration measurements. Theproportional counter coated with anti-coincidence unit, optimize the measuring devicevia simulation results.(2) In the evaluation of the stability of the system,increase the impact oflong-term measurements to the position of β peak of tritium. Use changes in therelative positions of the peaks combined with the traditional triple variance methodtocommon evaluatethe stability of tritium measurement system.(3) Measure the unknown concentration of tritium samples, taking the detectionlimit of proportional counter into account, first using the ionization chamber to do thepreliminary measurements.For high concentration of tritium samples, after diluted,use theproportional counter for accurate measurement. (4) In the study of calibration methods, the priority isusing ionization chambermeasurements for calibration, to establish a standard curve of measurement andexperimental values. And to eliminate end effects though the length compensationmethod, to obtain the wall effect though the relationship between the working gaspressure and the counting rate,co-calibrate theresults of measurement.