Contamination of Ionization Chambers Exposed to Tritium Gas and Tritiated Water - Cu, Ni-plated, and Au-plated Chambers -

Ionization chambers are widely used for measuring tritium in the gas phase. Recently, however, it has been perceived that the contamination of the chamber wall due to the adsorption of tritium significantly impairs the reliability of the ionization chambers when they are used for measuring tritium at high concentrations. As a first step to overcome this difficulty, we studied the contamination of ionization chambers with tritium gas (HT) and/or tritiated water vapor (HTO) by using different component materials (Cu, Ni-plated, and Au-plated chambers). In case of the Cu chamber, the concentration of tritium gas in the chamber decreased exponentially with time due to purge by fresh air to a certain level. Then, the decrease rate slowed down drastically (the memory effect). This is caused by adsorption of tritiated water on the surface of the material; the tritiated water being formed in the chamber due to exchange reaction with water vapor. In case of the Ni-plated chamber, the memory effect was also observed; however, this is not due to the adsorbed tritiated water. Instead, it is thought to due to adsorbed tritium such as T(a) or HT(a). With respect to the Au-plated chamber, no memory effect took place when it was exposed to tritium gas (HT). On the other hand, when the chambers were exposed to tritiated water vapor, all showed a large memory effect. It was clarified that the memory effect principally arose from the physically adsorbed tritiated water. In conclusion, the Au-plated chamber is most preferable for measuring tritium gas. However, the memory effect due to the adsorption of tritiated water vapor is as large as that for the other two. Therefore, it is necessary to develop suitable decontamination methods for the contamination will have to be avoided by the selection of more suitable materials