2.3. 222 Rn activity concentration measurements
A RAD 7 H2O detector (hereafter RAD-W) manufactured by Durridge Company was used to make direct readings of the radon concentrations in the water samples. The RAD-W setup consists of three components, namely:(a) a water vial with an aerator;(b)the desiccant tube and(c)the alpha detector. Fig. 2 shows a diagrammatic illustration of the radon-monitor using a RAD-W for measuring radon concentrations in water samples. For accurate readings, the RAD- W should be dried out thoroughly before making measurements. High humidity levels reduce the efficiency
of collection of the 218 Po atoms, formed when radon decays inside the chamber .As always, the relative humidity inside the instrument will stay below 10% for the entire 30-min measurement period. Then each measurement was carried out for three hours.
The RAD-W method employs a closed-loop aeration scheme whereby the air and water volumes are constant and independent of the flow rate. The air is recirculated through the water and continuously extracts the radon until a state of equilibrium develops. The RAD-W system reaches this state of equilibrium within about 5 min after which no more radon can be extracted from the water. The extraction efficiency (percentage of radon removed from the water in to the air loop) is around 94 % for a 250ml sample. The exact value of the extraction efficiency depends somewhat on the ambient temperature, but it is almost always well above 90%. The most significant background effects in the RAD-W are counts from radon daughters and traces of radon left from previous measurements