Reducing noise in uranium exploration

A method and apparatus are described for reducing or removing the background noise caused by thoron gas (220Rn) in uranium exploration conducted by the detection of radon gas (222Rn) emanating from the ground. This is accomplished by the use of a number of alpha particle detectors, each of which is disposed in a protective enclosure. A permselective membrane, which permits, but selectively retards, the passage therethrough of gases is disposed in the path to be traversed before such gases can reach the alpha particle detector. The retarding influence of the membrane should be sufficient to make the concentration of thoron inside the enclosure small relative to the concentration of thoron outside the enclosure. The influence of the membrane of radon should be negligible, i.e., the radon concentration inside and outside the enclosure should be substantially equal