Radiometric method - Geophysics for the Mineral Exploration Geoscientist - page 199

a)

b)

K-Capture

22

Potassium

40 Ar

40 K

20

β -Emission

40 Ca

α -Emission

18

Number of protons ( Z ) 14 16 18 20 22 24 26

Number of protons ( Z )

c) 238 U

146

Uranium

144

234 Th
234 Pa

γ -Emitter

Non- γ -emitters 234 U

142

d)

230 Th 232 Th

140

142

Thorium

226 Ra

228 Ra

138

140

228 Ac

222 Rn

228 Th

136

138

224 Ra

218 Po

134

136

218 At

214 Pb

220 Rn

132 134

214 Bi

216 Po

214 Po

130

132

210 Tl

212 Pb

128 130

210 Pb

212 Bi

210 Bi
210 Po

212 Po

126

128

208 Tl

206 Tl

124 126

208 Pb 206 Pb

80 82 84 86 88 90 92 80 82 84 86 88 90

Number of protons ( Z ) Number of protons ( Z )

Figure 4.5 Changes in atomic number (Z) and neutron number (N) associated with radioactive decay. (a) The scheme for showing the
changes associated with

-emission); (b) 40 K series, (c) α - and β -emissions and K-capture (there are no changes in Z and N with γ

238 U series, and (d) 232 Th series.

4.3 Measurement of radioactivity in the field

measured over a series of integration periods of the same
duration will generally show a statistical scatter, or devi-
ation, about a mean value. This is known as statistical
noise, and the standard deviation (SD) is given by

The intensity of the radiation measured, from either a
stationary or moving survey platform, depends on a number
of parameters. These include the actual intensity of the
radiation emitted by the radioactive sources, the spatial size
of the radioactive sources with respect to the spatial
sampling interval, the type and nature of the detector, and
the time available to make individual measurements. These
parameters also affect the accuracy of the measurement.

p N

SD ¼ ð 4 : 6 Þ

where N is the number of emissions counted during the
integration period. This can be expressed as a percentage
statistical error of N and given by

p

ð Þ¼ = ð : Þ

Statistical error % 100 4 7

This shows that the error in a measurement is inversely
proportional to the square root of the total number of
emissions recorded ( Fig. 4.7 ) . A small count has an inher-
ently higher error, so it is preferable to measure a large
number of emissions in order to obtain an accurate

4.3.1 Statistical noise

The random nature of the radioactive decay process
requires measurements to be made over comparatively
long integration periods. The number of emission products