INTEGRATED RADON MAPPING IN THE EARTH - ASSESSMENT OF THE RN-220 SIGNAL AND ITS EXCLUSION

In many searches for uranium in the earth by mapping alpha decay activity from **2**2**2Rn, there is a background of **2**2**0Rn activity due to thorium. Both sources of activity are measured, unless special measures are taken to exclude the alpha particles from **2**2**0Rn decay. Laboratory experiments and field observations are used to demonstrate that the signal including **2**2**0Rn activity is commonly about twice that from **2**2**2Rn activity alone, but that the **2**2**0Rn/**2**2**2Rn activity ratio can vary widely. Use of anti minus **2**2**0Rn membranes is, therefore, important in making meaningful surveys of **2**2**2Rn activity. In some cases, anomalously high field readings arise from unusually high local radon emanation. These cases can be recognized by high **2**2**2Rn activity measured in the laboratory using separate soil samples from each site used for a reading in the field.