| Tritium on-line measurement is very important for tritium safety and control in tritium operating system, and it is also one of the key issues in DT fuel cycling system for fusion reactor. Ionization chamber is the main tool for tritium on-line measurements in tritium processing system. However, there are some problems still unsolved, such as energy deposition process in the chamber, ions recombination during ion collecting process, and memory effect caused by tritium absorption. Furthermore, the mechanism about enhancement of output signal in ionization chamber operating with He (H2) as carrier gas still remains unclear. Therefore, ionization chamber for tritium on-line measurement needs to be improved to meet the requirements for future tritium system handling with large amount of tritium. The main purpose of this thesis is to solve the basic problems about ionization chamber during its application in tritium processing system, including energy deposition in ionization chamber of P rays from tritium disintegration, counts loss caused by ion recombination, enhancement effect of output signal, and memory effect after exposure to high tritium concentration. In addition, we have designed a prototype of micro ionization chamber for tritium measurements in processing system according to the requirements.According to the distribution of tritium in ionization chamber of cylinder form, we have proposed a theory to calculate energy deposition in ionization chamber quantitatively based on a one-dimension model. Several parameters are included into the developed theory, including gas pressure in the chamber, dimensions of the chamber, materials of the inner wall and gas temperature in the chamber et al. Results indicate that both dimension of ionization chamber and operating conditions will significantly affect energy deposition in ionization chamber, especially for chamber of small size and operating under low gas pressure. For ionization chamber of 1.0 L, energy deposition rate is nearly 100% at 100 kPa, while it is only 81.9% at 5 kPa with air as operating gas and 50.2% in hydrogen at same pressure. Chambers with gold plated wall result in much larger energy deposition rate comparing with chambers using other materials as inner wall, including aluminum, iron, and copper. At pressure 5 kPa, gold plated chamber will lead to 11.3% higher energy deposition rate than aluminum wall. Although the developed theory is useful to calculate energy deposition rate quantitatively, it is applicable only for ionization chamber whose size is larger than the range of ? ray in the chamber. For ionization chamber with size smaller than the range of ? ray, we employ Monte-Carlo simulation to calculate energy deposition in ionization chamber using sampling method instead of analytic method. MCNP 5 is used in the simulation to perform calculations by tracing the transport of both photons and electrons. Results show that for ionization chamber of 1.0 mL, only 0.8% of ? energy deposits into the chamber at 0.1 kPa, which indicates most energy of ? ray will loss into the chamber and make no contribution to output signal. In comparison, energy deposition in ionization chamber of 10 L is one order in magnitude higher than chamber of 1.0 mL at 0.1 kPa.Theoretical method has been proposed to calculate ion recombination based on an assumed two-dimension model which is reasonable for ionization chamber with cylinder form. A formula has been deduced from the proposed theory by some approximations, which can be used to calculate ion recombination quantitatively, and it is also can be extended to three-dimension. With the established formula, ion recombination coefficient for dry air was obtained by fitting experimental data, and it is estimated to be 6.9×10-6 cm3/ion-s, which is accordance with data reported in literature. In addition, results also indicate that ion concentration at low voltage is much higher than ion concentration at high voltage at the same position in the chamber. Therefore, it is helpful to reduce ion recombination by applying higher voltage to the chamber. Furthermore, the proposed theory is also applicable for calculation of ion recombination coefficient for other kinds of carrier gas.Enhancement effect of output signal (Jesse effect) has been theoretically studied by analyzing the interaction process of ? ray and materials, including ionization and excitation. Theoretical formula has been deduced, which can be used to calculate enhancement factor caused by Jesse effect quantitatively. Results indicate that Penning ionization is not the only reason for the enhancement of output signal. Besides Penning ionization, sub-excitation electrons also play an important role in the enhancement process. For He+H2 as carrier gas, the enhancement factor (K) is as high as 1.44 for 1.0% H2 as impurities. As the concentration of hydrogen increases, K saturates gradually. On the other side, helium will be the impurity during the storage of tritium. The value of K increases gradually as the storage time increases due to the increase of helium concentration. After 120 months storage, K is only 1.05 for H2(99.9%)+T2(0.1%). For H2(90%)+T2(10%) as tritium source, K will be 1.05 after 60 month storage, while it will be higher than 1.1 after storage of 148 months. In addition, the theory proposed for He(H2) is also suitable for other kinds of gases, such as He(CxHx) and He(COx).Memory effect of ionization chamber has also been studied in this thesis. After exposure to tritium at the same condition,316 L stainless steel and teflon absorbed less tritium than other materials such as red copper, brass, and aluminum alloy. It also indicates that roughness of sample is important for tritium absorption on sample with the same materials. Smoother the sample, less tritium will be absorbed on it. After exposing to tritium of 2.8×1010 Bq/m3 for 4h, the output signal caused by tritium absorbed on inner wall of an ionization chamber will be negligible comparing with the background level of 1.0 L chamber if the inner chamber wall is made of stainless steel and polished to 0.5 micron. However, if the inner surface is polished to 1.0 micron, signal caused by memory effect will be one order in magnitude higher than background level. To diminish the effect of memory effect, two kinds of ionization chambers have been developed, wire wall chamber and ionization chamber system with two chambers operating at differential mode. In experiments, both kinds of ionization chamber can recover to initial level after measuring 1010 Bq/m3 tritium level for 6 h.Cross-type ionization chamber has been developed for tritium on-line measurement in processing system. The sensitive volume of the developed chamber is 1.9 mL, and its lower detection limit is about 2.0×10 Bq/m. With special designed flange, the leakage rate of the chamber is as low as 0.9x10"7 Pa.L/s. It also shows good performance after tritium exposure in 6.7×1011 Bq/m3 for 6h.It is concluded from the established theories about energy deposition, enhancement effect of output signal, ion recombination and memory effect that key problems of ionization chambers for tritium on-line measurements have been solved, and prototype of micro ionization chamber has also been developed for tritium processing system. All these will provide useful information for the application of ionization chambers for tritium on-line measurements in tritium processing system in the future. |
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