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need to be shielded, although the alpha-emitting materials should be prevented from
entering the airways (e.g., with filtering masks) to protect delicate lung tissue. On the
other hand, high-energy gamma rays need to be shielded with very thick lead or concrete
because they interact so little and penetrate so deeply. As noted earlier, lightweight
materials such as Plexiglas ® or aluminum are preferred as shielding for beta particles to
reduce bremstrahlung.
Radiation Units
The measurement of radiation required the creation of new units to describe various
aspects that were new to science. The major units used to describe radiation and its
effects are discussed below. This discussion will include both the older American
nomenclature and the newer nomenclature under the System International (SI), because
both are still widely used.
Radioactivity; curie/becquerel. The units used to describe radioactivity are the curie
and the becquerel (SI). Activity represents the number of radioactive decays that take
place in 1 second. One curie (Ci) is the number of decays that occur in 1 gram of pure
radium-226 in 1 second (i.e., 3.7 × 10 10 disintegrations/second). The becquerel (Bq) is
one disintegration per second, so that there are 3.7 × 10 10 Bq per Ci.
The specific activity (activity per unit of mass) varies from radionuclide to radionuclide,
so that the same weight of different materials does not necessary have the same level of
radioactivity. For example, 1 curie of 226Ra weighs about 1 gram, as noted above, while
1 Ci of 232Th weighs just under 10 million grams (ten metric tons).
Exposure; roentgen. Formally, the roentgen (R) is a unit of electrical charge liberated by
photons per kilogram of air. This unit is specific to x- and gamma rays and cannot be
applied to any other form of radiation or to any other medium. It does not have an SI
equivalent.
Absorbed dose; Rad/Gray. “Absorbed dose” is the amount of energy absorbed per unit
of mass, which applies to any type of radiation in any type of matter. The Rad (radiation
absorbed dose) is defined as 100 erg per gram. The SI equivalent is the Gray (Gy),
defined as 1 joule per kilogram. One Gy is equivalent to approximately 100 Rad. Often
the unit of centiGray (cGy) is used because it is equal to 1 Rad.
Effective dose. Absorbed dose is a very useful concept, but it does not account for the
different biological effects of different types of radiation on different types of tissues.
Two artificial weighting factors are used to represent the impact of these differences in
“effective dose.”
Relative biological effectiveness. This weighting factor accounts for how strongly a given
radiation interacts with the particular mix of elements that makes up the human body.
This factor does not account for shielding and is partially dependent on energy. Note that
this weighting factor seems to match up reasonably well with the discussion of LET. For
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