Geology Reference
In-Depth Information
settles down with the emission of a 1.46-MeV photon. The half-life of
40
K
is 1.47
10
9
years for K-capture.
The other important primeval radioisotopes decay into nuclei that are
themselves unstable. As with
40
K, there may be more than one possible
decay mode, and the decay chains are quite complicated. All, however, end
in stable isotopes of lead. The decay chains for
238
Uand
232
Th are shown in
Table 4.1.
235
U makes up only about 0.7% of naturally occurring uranium but
is less stable than
238
U and responsible for almost 5% of uranium radiation.
Even so, it may be ignored for most geophysical purposes.
Not all decay events produce significant radiation. The first stage in
the decay of
232
Th involves only weak gamma activity, and the strongest
radiation in the chain (a 2.615-MeV photon, the most energetic radiation to
come from any terrestrial source) comes from the decay of
208
Tl, near the end.
In the
238
U chain,
214
Bi is notable for the numbers and energies of the
gamma photons produced. Those at 1.76 MeV are taken as diagnostic of the
presence of uranium but the gaseous radon isotope,
222
Rn, which precedes
214
Bi in the chain, has a half-life of nearly 4 days and so can disperse quite
widely from a primary uranium source. Gaseous dispersion has much less
effect in thorium decay because the half-life of
220
Rn is less than a minute.
×
10
9
×
years for beta decay and 11.7
4.1.6 Radioactive equilibria
If a large amount of a primeval isotope is present, and if all the daughter
products remain where they are formed, an equilibrium will eventually be
established in which, for each element, the same number of atoms are created
in a given time as decay. Only the concentrations of the two end members
of the series change.
In equilibrium decay, each member of the chain loses mass at a rate
equal to the mass of the element present, multiplied by the appropriate
decay constant. Equilibrium masses are therefore inversely proportional to
decay constants. If more (less) of an element is present than is required for
equilibrium, decay will be faster (slower) than the equilibrium rate until
equilibrium is re-established.
Equilibrium can be disrupted if gaseous or soluble intermediate products
have half-lives long enough to allow them to disperse before they decay.
Dispersion of radon from uranium ores notably disrupts equilibrium, and
the primary source of a 'uranium' (actually
214
Bi) anomaly may be hard to
find.
Roll-front
uranium ores are notorious for the separation between the
uranium concentrations and the zones of peak radioactivity.
4.1.7 Natural gamma-ray spectra
Natural gamma-ray energies range from above 3 MeV for
cosmic
(mainly
solar) radiation down to the 0.12 MeV of the most energetic X-rays. A typical