Environmental Engineering FOURTH EDITION

Radioactive Waste 317

Barrel (Pb)

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Source of radioactivity Charged fluorescent screen

Figure 16-2. Controlled measurement of alpha (a), beta (B), and gamma (y) radiation.

inhaled, or otherwise absorbed inside the body, so that organs and tissues more sen-
sitive than skin are exposed to a radiation. Collisions between a particles and the
atoms and molecules of human tissue may cause disorder of the chemical or biological
structure of the tissue.
Beta radiation is a stream of electrons emitted at a velocity approaching the speed
of light, with kinetic energy between 0.2 and 3.2 MeV. Given their lower mass of
approximately 5.5 x g), interactions between /? particles and
the atoms of pass-through materials are much less frequent than a particle interactions:
fewer than 200 ion pairs are typically formed in each centimeter of passage through air.
The slower rate of energy loss enables /3 particles to travel several meters through air
and several centimeters through human tissue. Internal organs are generally protected
from external B radiation, but exposed organs such as eyes are sensitive to damage.
Damage may also be caused by incorporation of /3 emitters into the body and resulting
in exposure of internal organs and tissue.
Gamma radiation is invisible electromagnetic radiation, composed of photons,
much like medical X-rays. y photons are electrically neutral and collide randomly
with the atoms of the material as they pass through. The considerably longer distance
that y rays travel in all media is defined by the relaxation length, the distance that
the y photon travels before its energy is decreased by a factor of l/e. A typical 0.7-MeV
y photon has a relaxation length of 5, 50, and 10,000 cm in lead, water, and air,
respectively - much longer than an a or B particle of the same energy. External doses
of y radiation may have significant human health consequences because the dose is not
greatly impacted by passage of the radiation through air. The properties of the more
common radioactive emissions are summarized in Table 16-2.
When ionizing radiation is emitted from a nucleus, the nature of that nucleus
changes: another element is formed and there is a change in nuclear mass as well.
This process may be written as a nuclear reaction, in which both mass and charge must
balance for reactants and products. For example, the beta decay of C-14 may be written

amu (9.130 x

That is, C-14 decays to ordinary stable nitrogen (N-14) with emission of a beta particle.
The mass balance for this equation is

14 = O+ 14,