Studies on the in-situ method of the radon measurement in the underground water

We have developed an underwater radon detector of the use of a PIN photodiode (PD) on 1996. This detector was put into the monitoring well at the depth of a meter, and has been monitoring the continuous radon concentration in the well. A microporous flat sheet membrane is attached to the window of the bottom of the detector. Gas dissolved in the underground water pass through the membrane of polypropylene with the hydrophobic properties. The principle of radon detection which is used in the detector is the electrostatic collection of the daughter nuclei of 222Rn, and the energy measurement of the alpha decay with PD. More than 90% of 218Po atoms, which was one of the daughter nuclei of 222Rn, tended to become positively charged. A negative high voltage is supplied to the p-layer of PD. The detector vessel is grounded. Then, an electric field is produced in the vessel. The daughter nuclei which ionized positively are collected on the surface of PD, the energy of alpha decay are measured. The small-sized underwater radon detector (PMT underwater radon detector) made of a Photomultiplier Tube and a ZnS scintillator. They say that the Photomultiplier Tube isn't influence by neutralization. We compare PMT underwater radon detector with PD underwater radon detector. (J.P.N.)