28 Chapter 1. Properties and Sources of Radiation
Fortunately the earth’s atmosphere acts as a shield to the worst of these radiations,
such as ultraviolet rays from the Sun are blocked by the ozone layer. However not all
of the harmful radiation is blocked and some reach the surface of earth causing skin
burns and cancer in people who remain exposed to sun light for extended periods of
time. The situation is even worse in places where the ozone layer has depleted due
to some reason.
On top of these localized sources of radiation there is also a background radiation
of low energy photons. This radiation is thought to be the remnant of the so called
big bang that created this universe. It is known as cosmic microwave background
radiation since the photon spectrum peaks in the microwave region of the electro-
magnetic spectrum. Although these photons reach the earth’s surface but due to
their low energies, they are not deemed harmful.
Apart from photons, there are other particles as well that are constantly being
produced in the outer space. Most of them, however, never reach the earth either
due to magnetic deflection or the earth’s upper protective atmosphere. Some of the
particles, like muons, electrons, and neutrinos, are produced when other cosmic par-
ticles interact with atoms in the upper atmosphere. Shower of these particles reach
earth’s surface time but due to their low energies and low interaction probabilities,
they do not pose any significant health hazard.
Muons and neutrinos directly produced by luminous objects in space also man-
age to reach earth due to their low interaction capabilities but are not considered
hazardous to health due to their extremely low interaction cross sections.
A.2 TerrestrialRadiationSources
This type of radiation is present in small quantities all around us and is more or less
inescapable. Our surroundings, the water we drink, the air we breathe in, and the
food we consume, all are contaminated with minute quantities of radiation emitting
isotopes. Although these isotopes, in general, are extremely hazardous, they are not
supposed to cause any appreciable harm to our bodies except when the are present
in higher than normal concentrations.
The main source of terrestrial radiation is the element uranium and its decay
products such as thorium, radium, and radon. Although the overall natural concen-
tration of these radioactive materials is within the tolerable range of humans, some
parts of the world have been identified where higher levels of uranium and thorium in
surface soil have increased the radiation to dangerous levels. Unfortunately man has
also contributed to this dilemma by carrying out nuclear explosions and by dumping
nuclear waste.
The two isotopes of radon,^222 Rnand^220 Rn, and their daughter products are
the most commonly found hazardous radioactive elements in our surroundings. The
main cause of concern with respect to theseα-emitting isotopes is their inhalation
or digestion, in which case the short rangeα-particles continue to cause damage to
internal organs that can lead to cancer.
A.3 InternalRadiationSources
Our bodies contain some traces of radioactive elements that expose our tissues
to continuous low level radiation. This internal radiation primarily comes from