Sper Scientific 840007 Radiation Detection Meter - Manual (Page 10)

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given off from natural radioactive minerals in the earth's
crust is a major constituent of background radiation. For the
most part, it is quite low, due to the long time required for
the remaining radioisotopes to decay. In atomic reactions
(either natural or forced by man) the decay process is sped
up by the effect of neutrons given off in the fission process
interacting with more unstable isotopes to cause immediate
decay. While this allows the energy of the isotope to be
harvested in a conveniently short time, the unstable decay
products produced generally have short half-lives, on the
order of seconds to centuries, and are very radioactive. As
a result of this process, considerable larger quantities of
short half-life (high decay rate) isotopes become a part of
the world we live in. This is the basis for the controversy
and concerns on the subject of nuclear power generation,
waste disposal, and nuclear weapons.
INTERACTION OF RADIATION WITH MATTER
The particles and photons that result from nuclear decay
carry most of the energy released from the original unsta-
ble nucleus. The value of this energy is expressed in elec-
tron Volts, or eV. The energy of beta and alpha rays is in-
vested in the particles' speed. A typical beta particle from
Cesium-137 has an energy of about 500,000 eV, and a
speed that approaches that of light. Beta energies can
cover a wide range, and many radioisotopes are known to
emit betas at energies in excess of 10 million eV. The
penetration range of typical beta particles is only a few mil-
limeters in human skin.
Alpha particles have even shorter penetration ranges than
beta particles. Typical alpha energies are on the order of 5
million eV, with ranges so short that they are extremely dif-
ficult to measure. Alphas are stopped by a ~nin sheet of
paper, and in air only travel a few inches at most before
coming to a stop. Therefore, alpha particles cannot be de-
tected without being in close contact with the source, and
even then only the alphas coming from the surface of the
source can be detected. Alphas generated within the
source are absorbed before reaching the surface. Due to
short range, alpha particles are not a serious health hazard
unless they are emitted from within the body when their
high energy, in close contact with sensitive living tissue, is
an extreme hazard. Fortunately, almost all alpha-emitting
substances also emit gamma rays, allowing for their detec-
tion.