RAD7 H2O is an accessory to RAD7 that enables measurement of radon in water over a concentration range from ,10 pCi/l to.4105 pCi/l. The lower limit of detection is ,10 pCi/l. The equip- ment is portable and battery operated, and the
measurement is fast. Figure 2 shows the schematic diagram of RAD H2O. RAD H2O gives results after a 30 min analysis with a sensitivity that matches or exceeds that of liquid scintillation methods. The RAD H2O method employs a closed loop aeration scheme whereby the air volume and water volume are constant and independent of the flow rate. The air recirculates through the water and continuously extracts radon until a state of equilibrium develops. The RAD H2O system reaches this state of equilib- rium within about 5 min, after which no more radon can be extracted from the water. The extraction effi- ciency or the percentage of radon removed from the water to the air loop is very high 94 % for a 250- ml sample. The exact value of the extraction effi- ciency depends somewhat on ambient temperature, but it is almost always well above 90 %. The RAD7 detector converts alpha radiation directly into an electric signal. RAD7 has the ability to distinguish between old and new radon and radon from thoron(6). As per EPA recommendations that all con- tinous radon monitors be calibrated at least every 6 months in a radon calibration chamber, the instru- ment was calibrated recently in September 2011 by International Environmental Consulting, New Delhi, India. The spectrum obtained from RAD7 at the end of a run was carefully observed and there were clearly defined peaks and no noise across the