I. INTRODUCTION
Tritium, an only radioactive isotopes of hydrogen and important fuel in fusion research, will be handled in large amount in fuel recycling system of ITER (International Thermonuclear Experimental Reactor) [1, 2]. Its β–rays are averaged at 5.7 keV with the maximum energy of 18.6 keV. Due to the radioactivity, tritium operation is commonly limited in confinement system, such as stainless steel tube, glovebox, and concrete walls [3]. Tritium concentration in each level of a confinement system shall be monitored to provide an early warning to tritium leakage [4].
Flow-through ionization chamber is widely used in tritium real-time measurements for its fast response and simple structure [5-7]. However, a sampling system must be used to pump gas into the sensitive region of ionization chambers [8]. This increases the risk of tritium leakage and the amount of radioactive waste. In addition, gas sample should be de-ionized by ion traps before it enters ionization chambers. Memory effect is another critical problem in the usage of flow-through ionization chambers, which may undermine the accuracy of measurements [9, 10].
In this work, a tritium monitor system with an open-walled ionization chamber was developed to meet the requirements of tritium measurements in gaseous form for both glovebox and workplace. In the measurements, tritium in gaseous form can diffuse into the sensitive region of the chamber without pumping. The structure of two mesh walls can serve to de-ionize ions ionized by tritium β-rays before it enters the sensitive region of the chamber. Memory effect is also reduced by the use of mesh wall. Ions are collected by an electrometer, and control software was developed to receive and store data from the electrometer. The design and performance of the monitor system will be specified in this article.
II. BASIC PRINCIPLES
Ionization chamber for tritium measurements is operated in saturation mode, in which all the ions ionized by β-rays emitted by tritium will be collected. In saturation mode, signal output (Is) can be described by Eq. (1).
where, E is the average energy (eV) of β-rays emitted by tritium. e is the electron charge (C),
Signal output of a chamber is contributed by tritium in the sensitive region, tritium absorbed onto the inner walls and tritium outside the sensitive region. The second term of Eq. (1) is called the memory effect. Therefore, it is necessary to minimize the influence of the second and third terms of Eq. (1) to improve performance of the monitor. So, a novel tritium monitor system based on open-walled ionization chamber was developed.
III. DESIGN OF THE MONITOR SYSTEM
A. General design
As shown schematically in Fig. 1, the monitor system consists of three parts, a detector, an electrometer, and an industrial personal computer (IPC). Signals generated in the chamber are delivered to the electrometer. A special software installed on the IPC is used for data acquisition and data-handling (conversion, display and storage).
B. Specification of the ionization chamber
Instead of sealed walls of a common flow-through chambers, the open-walled ionization chamber has two mesh walls,. In measurements, the open-walled ionization chamber is placed into the glove box or workplace. Tritium gas diffuses into the sensitive region of the chamber without pumping (Fig. 2(a). With the two mesh walls, the electric field of the chamber is shown in Fig. 2(b) when a negative high voltage is applied to the cathode. The electric field between the anode and cathode serves to collect electrons in the sensitive region of the chamber—the first term of Eq. (1), while the electric field between the cathode and shield wall will prevent ions generated outside entering the sensitive region of the chamber. Therefore, the value of the third term in Eq. (1) will be zero in theory. In addition, the use of mesh wall as cathode instead of sealed wall will lead to half decrease of the surface area, which significantly lowers the influence of memory effect, as denoted by the second term of Eq. (1). As a result, with the structure of open-walled ionization chamber, the influence of both terms 2 and 3 will be diminished greatly, and the signal output of the monitor will be mainly devoted by β rays emitted by tritium within the sensitive region of the chamber.
C. The software for data processing
As described in Fig. 1, a special program was developed to receive and process data from electrometer. Its functions are: (1) receive data from electrometer, (2) convert current into tritium concentration and display it on the screen, (3) store data and make some calculations, such as summing concentrations within a period, doing average, marking maximum and minimum values etc., and (4) provide sound alarm when tritium concentration exceeds threshold.
IV. RESULTS AND DISCUSSIONS
A. The monitor system
For tritium measurements of gaseous form in workplace, the detector is placed over the equipment cabinet and is supported by a bracket. The TMS(Tritium Monitoring System)-I tritium-in-air monitor system is shown in Fig. 3. The chamber, which is 1.6 m in height (at about the nose of an adult), is installed with four wheels, being convenient to move it around. The open-walled ionization chamber can be placed into the glovebox during operation. Signals are delivered to electrometer through the cables, as shown in Fig. 1.
B. Background of the monitor
Background of the monitor was tested in laboratory. From the measurement results of about 6 h (Fig. 4), the monitor works stably. The open-walled ionization chamber is 1.0 L in volume. According to Eq. (1), neglecting the influence of terms 2 and 3, the background of this monitor system is about 3.7×105 Bq/m3.
C. Saturation character of the monitor
Saturation character of the monitor system is tested at tritium concentrations of 7.0×109 Bq/m3 and 6.9×1011 Bq/m3. The negative cathode bias voltages vary from 0 V to 1000 V. As shown in Fig. 5, the signal output increased at first with the absolute voltage and saturated at a certain voltage. Higher voltage is necessary to ensure saturation for higher tritium concentration. This is mainly caused by ion loss due to recombination and diffusion in the chamber. For tritium concentrations of 7.0×109 Bq/m3 and 6.9×1011 Bq/m3, the chamber worked in saturation mode at about 100 V and 150 V, respectively. Therefore, 300 V is recommended as operation voltage to ensure the chamber working in its saturation mode.
D. Saturation character of the monitor
The linear response was examined by exposing the open-walled ionization chamber to a point γ-ray source placed at different distances. The dose rates varied from 0.045 mGy/h to 2.57 mGy/h, which is equivalent to 1.8×107–1.03×109 Bq/m3. The results are shown in Fig. 6, and one sees a very good linear response of the tritium monitor.
E. Memory effect
Memory effect on the monitor system was checked at tritium concentration of 6.9×1011 Bq/m3 in a sealed container. After putting the open-walled ionization chamber in the container for about 4 h, the tritium was pumped away and dry air was used to purge the container. Background of the monitor recovered to 3.7×105 Bq/m3 in about 10 min. In comparison, background of a sealed wall chamber was higher than 1.0×107 Bq/m3 after exposing it to tritium gas of the same concentration, because of the tritium adsorption on the walls.
V. CONCLUSION
A novel tritium monitor system has been developed for tritium measurements in gaseous form, especially for glovebox and workplace. The introduction of open-walled ionization chamber significantly simplifies the structure of commonly used tritium measurement system and lowers the memory effect of it. All the operations can be done on the IPC with the special designed software. With a 1.0 L ionization chamber, the background of the monitor is 3.7×105 Bq/m3. It is suitable for tritium measurements in glovebox and workplace.
Review of deuterium-tritium results from the Tokamak Fusion Test Reactor
. Phys Plasmas, 1995, 2: 2176-2188. DOI: 10.1063/1.871303In-vessel tritium measurements using beta decay in the Tokamak Fusion test reactor
. Rev Sci Instrum, 1999, 1: 1119-1122. DOI: 10.1063/1.1149294Tritium instrumentation for environmental and occupational monitoring
. Health Phys, 1974, 26: 165-178. DOI: 10.1097/00004032-197402000-00002An open-walled ionization chamber appropriate to tritium monitoring for Glovebox
. Rev Sci Instrum, 2010, 81: 073302-1-073302-4. DOI: 10.1063/1.3458012In-line process tritium monitors
. Fusion Technol, 1992, 21: 340-345.Portable intelligent tritium in air monitor
. Fusion Sci Technol, 2005, 48: 390-392. OSTI: 20854172.Tritium measurement and monitoring in experimental and process system with ionization chambers
. Fusion Technol, 1992, 21: 629-635.Ionization chamber system to eliminate the memory effect of tritium
. Nucl Instru Methods Phys Res A, 1989, 278: 525-531. DOI: 10.1016/0168-9002(89)90875-9Improvement and characterization of small cross-piece ionization chambers at the Tritium Laboratory Karlsruhe
. Fusion Sci Technol, 2011, 60: 968-971.