Performance of the electrostatic collection type radon and toron detector

The major background source in the underground experiments, such as a search for the double beta decay, is recognized as the radio-active materials, specifically the daughter nucleus of toron, ²⁰⁸Tl, in the environment as well as the detector itself. To measure such contamination, a large volume electrostatic collection-type detector having radon sensitivity, or the detection limit, of the order of 10mBq/m³ is commonly used. In case, for example, of an experimental configuration utilizing a drift chamber, the measurement of radio-active contamination in the filling gas is crucial. The present detector seems to be useful in the estimate of minute amount of the radon as well as the toron in the gas. To understand such type of the detector, we have studied a general behavior of the 1 l volume detector, inside of which is a PIN photodiode held at a negative potential. The radio-active ions, such as poloniums and bismuth, are collected and their a decays are detected as clearly separated several peaks. Some results are presented in this report for the lifetime measurements of natural radon ²²²Rn and toron ²¹²Pb. Other results also will be presented, such as a relation between the collection efficiency of the daughter nucleus, ²¹⁸Po, and the humidity.