Forte Energy (Perth) is exploring for uranium in the Sahara Desert of Mauritania with the aid of the Pylon AB-5R. It takes two days of travelling to get to the isolated project area.
The principle techniques are airborne and ground gamma radiometric surveys, with trenching and drilling of suitable anomalies. In some places a thin sand cover is present along the trend of a mineralised zone and as it only takes about 50-70 cm of sand to completely block gamma radiation, we are using radon gas to locate mineralisation below the sand. There are plastic film techniques to detect radon but these require a detector to be left buried in the ground for a month and then sent to a laboratory for processing. This required two visits to the site and a wait until the films are processed, and this is not cost effective.
Instead, Forte Energy is using a Pylon Electronics AB-5R radiation counter with alpha-particle scintillation detectors (Lucas cells) to detect the radon gas. We drill a 50cm hole into the coarse granular soil using a portable gasoline-engined drill and then insert a steel probe with a coupling for the plastic tubes leading to the AB-5R. We measure the cell background for one minute, then use the internal pump to draw up a sample of soil air into the cell where three one-minute counts are then made. Then the cell is flushed with air and the instrument is moved to the next sampling point, usually 10 to 50 meters apart on a grid. Fairly repeatable results can be obtained, the main problem being the 'fit' of the steel probe in the hole. It is usually necessary to hammer it down as the hole tends to fill in with fine sand and this makes an almost ideal gas-tight seal around the sampling tube.
The three counts allow Forte Energy to distinguish the counts due to Radon-222 (a daughter of Uranium) and Radon-220, also known as 'Thoron' (daughter of Thorium) as Thoron has a short half-life of 55 seconds while Radon (222Rn) has a half-life of 3.8 days. Moreover, the count due to 222Rn actually increase by a factor of x3 during the three hours after sampling and this can be discerned in the three mintues after sampling as a slight increase. This is due to the additional alpha-particle emissions from the short-lived daughter products of 222Rn (218Po and 214Pb) which build up rapidly at first and then more slowly until the maximum is reached after about three hours. The 222Rn parent then decays according to the 3.8 day half-life. This can be demonstrated by repeatedly measuring the activity from a cell during the hours and days following sampling.