Geology Reference
In-Depth Information
dry soil, recent rain and the presence of puddles of standing water should
always be noted.
The ways in which readings were taken, including time constants or
count periods, must be recorded. Field parameters should not be varied in
the course of a survey and the location of the sensor (e.g. whether hand-held
or in a backpack) should be specified.
4.3.5 Alpha-particle monitors
Because radiation is absorbed by almost trivial thicknesses of overburden,
exploration is sometimes based on soil-gas sampling. Radon gas, which is
a copious source of alpha particles, diffuses readily through rocks and soil
to the surface and also dissolves in groundwater, and can therefore act as a
pathfinder to 'blind' uranium mineralization. The detection methods were
first developed for public health applications, since the gas can be a health
hazard if allowed to accumulate in enclosed spaces.
In uranium exploration a number of special factors have to be taken into
consideration. Dispersal routes can be complex, and locating a primary
source is not necessarily easy. Variations in temperature, atmospheric pres-
sure and humidity can cause the flow of gas to the surface to vary by factors
of three or more in the course of a day and by an order of magnitude over
longer periods, so that, ideally, monitoring should extend over days or even
weeks in the hope that these effects will average out. Exploration practical-
ities usually set limits of one or two weeks, and it is obviously important
that all the detectors are in place for the same lengths of time. Methods that
rely on measuring absorbed or adsorbed radon are unsuitable because the
3.8-day radon half-life implies that, regardless of the length of the sampling
period, the values recorded will relate only to the previous few days. The
traces left by alpha particles when passing through some materials provide
the basis of true cumulative methods. The most common detector is a strip
of plastic film made of CR39 polycarbonate, enclosed in a small chamber to
which only filtered gas has access. The tracks are subsequently made visible
by etching with a caustic solution
In a radon survey, hundreds or even thousands of detectors may have to
be buried across the area of interest, under conditions that are, as nearly
as possible, identical. A hand or, preferably, motorised auger is needed to
prepare the large number of holes required within an acceptable period
of time, and these must be closed in ways that allow the detectors to be
quickly retrieved. Backfilled soil can be contained by plastic sheeting, and
then simply lifted out, or inverted plastic cups can be attached to the bases
of wooden or plastic plugs of the same diameter as the hole (Figure 4.3).
Concentrations of soil gas increase with depth, reaching a plateau at about
one metre below surface in typical loose soils. Ideally, therefore, the holes
