Physics for Geologists, Second edition

66 Electromagnetic radiation
radiation travels, whereas wavelength is not (as we saw when considering
the refraction of light).
Cosmic radiation
Cosmic radiation, which was discovered early in the twentieth century, con-
sists of very high frequency, high energy X-rays together with very high
energy particles. The general level was found to be roughly constant over
24 hours, so clearly the radiation does not come from the Sun. The inten-
sity was found, by apparatus flown in balloons, to increase up to a height
of about 12 km (a height now flown by many aircraft!), then to decrease;
so some cosmic radiation, or its effect, is clearly generated within the
atmosphere. Cosmic radiation is much more intense in the polar regions
than the equatorial, so clearly some of the particles are charged positively,
others negatively. The equatorial regions are protected by the magnetic field's
being approximately tangential to the Earth's surface near the Equator.
Gamma rays
y-rays are high-energy radiation from radioactivity with frequencies greater
than 100 x 1018 Hz (wavelengths shorter than about 10-12 m, or 1 pm), and
their significant property is that they are not affected by magnetic fields so
they are not charged particles. a and 0-particles are affected by magnetic
fields.
y-rays are emitted spontaneously from the nucleus of some unstable isotopes.
They are gradually absorbed when passing through solids, but can
penetrate some millimetres of steel, some centimetres of concrete, and rather
more centimetres of sedimentary rock. The main isotopes yielding y-rays are
of the uranium and thorium families, and 4 0 ~
(potassium-40). 4 0 ~
occurs in
the micas and K-feldspar, and clay minerals by cation-exchange. Mudrocks
or mudstones have a level of y-radiation that is significantly higher than
that of sandstones and most carbonates. The natural y-radiation is, there-
fore, a useful borehole-logging parameter. When y-rays are absorbed by
a nucleus, neutrons and a-particles are emitted.
Catalytic activity is enhanced by y-rays. Most clay minerals act as catalysts
in petroleum-chemical reactions, and most emit y-rays spontaneously, and
this may be significant in the generation of petroleum. The catalytic cracking
unit in petroleum refineries used to use natural Fuller's Earth (which contains
montmorillonite and other clay minerals) as the catalyst to improve the yield
of high-octane-number gasoline and reduce the gas yield.
X-rays
X-rays were discovered in 1895 by Wilhelm Rijntgen when he found fluo-
rescence in a cathode tube when electrons were beamed from the cathode
Copyright 2002 by Richard E. Chapman