Geology Reference
In-Depth Information
4
RADIOMETRIC SURVEYS
The radioactivity of rocks can be measured using gamma-ray scintillation
counters (scintillometers) and spectrometers. Although most radiometric
instruments were developed with uranium search in mind, other uses were
soon found. Amongst these were regional geological mapping and correla-
tion, exploration for some industrial minerals and
in situ
determinations of
phosphates. The same instruments can also be used to track the movement of
artificial radioactive
tracers
deliberately introduced into ground water, and
to assess health risks from natural and artificial radiation sources. Radon gas
'alpha' detectors are important in public health applications and have some
exploration uses.
4.1 Natural Radiation
Spontaneous radioactive decay occurs when an unstable atomic nucleus
loses energy by emitting alpha, beta and gamma radiation. Alpha and beta
'rays' are actually particles. Quantum theory tells us that gamma rays,
which are high-energy electromagnetic waves, can also be treated for many
purposes as if composed of particles.
4.1.1 Alpha particles
An alpha particle consists of two protons held together by two neutrons
to form a stable helium nucleus. Emission of alpha particles is the main
process in radioactive decay, resulting in decreases of four in atomic mass
and two in atomic number. The particles have large kinetic energies but are
rapidly slowed by collisions with other atomic nuclei. At
thermal
energies
they soon gain two orbital electrons and become indistinguishable from
other helium atoms. The average distance travelled in solid rock before this
occurs is measured in fractions of a millimetre, and even in air is only a few
centimetres.
4.1.2 Beta particles
Beta particles are electrons ejected from atomic nuclei. They differ from
other electrons only in having higher kinetic energies, and so cease to be
identifiable after being slowed by multiple collisions. Energy is lost most
