External Exposure
Control
External Exposure
Control
What are external radiation exposures?
External radiation exposures refer to those from a radiation source that
is outside of the body.
Can all types of radiation cause external radiation exposures?
Typically, photon (x-ray and gamma ray) and neutron radiations represent
external radiation hazards. Depending on its energy, beta radiation can be
an external hazard as well. Alpha radiation is not considered an external
hazard for people because it cannot penetrate the outer skin layer.
What are some protection methods for external exposures?
There are several ways to protect oneself from external sources of radiation.
However, the three most important ones are the concepts of "time", "distance",
and "shielding".
How does the concept of "time" influence the amount of radiation exposure
received?
The time spent in a radiation field is the first major method for limiting
your exposure to an external radiation source. To apply this method, it is
important to realize that the amount of radiation one receives is directly
proportional to the time spent in the radiation field. This means that if
you spend twice as much time in the same field of radiation as another person,
you will receive twice the exposure. Therefore, to minimize the amount of
radiation received, the time spent in the radiation field should be kept
as short as possible.
If I know the exposure rate in a particular area, how can I determine
what my overall exposure will be based on time?
This is a straight-forward calculation that is also a good example of how
"time" influences the magnitude of radiation exposure. Let's say, for example,
that a radiation detection instrument, held in a particular room, gives a
reading of 50 microroentgens per hour. If I stand in that location for one
hour, I will receive a total exposure of 50 microroentgens. However, if I
stand in that location for two hours, my exposure will increase to 100
microroentgens. On the other hand, if I only stand there for a half an hour,
my exposure will decrease to 25 microroentgens.
How does the concept of "distance" influence the amount of radiation exposure
received?
Distance is another major exposure-reducing method. A very common and extremely
effective way of reducing exposure is to increase the distance between yourself
and the radiation source. In essence, the exposure drops by the "inverse
square" of the distance from the radiation source.
Can I have an example of the "distance" concept work?
Certainly. If the exposure is 50 microroentgens at one (1) foot from a radiation
source, the exposure at two (2) feet from that same source drops by a factor
of one over two squared, or one fourth, or 12.5 microroentgens. As you can
see, the distance between a radiation source and a person has an even greater
influence over the total exposure than the amount of time one stands in the
radiation field. While the exposure-time relationship follows a direct dependence
(i.e., reducing the time spent in a radiation field by one-half reduces the
exposure to the worker by one-half), doubling the distance from a source
reduces the exposure by a factor of four! (It is important to note here that
there are some situations where the "distance" concept does not work exactly
as described here. However, generally speaking, the farther one moves away
from a radiation source, the lower one's exposure will be.)
How does the concept of "shielding" influence the amount of radiation
exposure received?
Different types of radiations can be stopped or "blocked" by using shields,
just like the heat from a hot pot can be blocked using a pot holder.
How much shielding is required to eliminate radiation exposure?
Well, that depends upon the type and energy of the radiation in question.
How about for alpha particles?
Alpha particles typically have energies on the order of four (4) to eight
(8) million electron volts, or "MeV". However, they quickly lose this energy
through the ionization process. Hence, the range of an alpha particle in
a particular type of material is quite short. In fact, a single sheet of
paper can be used to completely eliminate the exposure from most alpha
radiations.
Is the same thing true for beta particles?
Not quite. Beta particles lose their energy just like alpha particles do
- through the ionization process - but they do not ionize to the same degree
as alpha radiation. Therefore, they have a greater range than an alpha particle
in various types of matter. Nonetheless, it is still fairly easy to shield
or "block out" beta radiation. For average beta energies of less than one
(1) MeV, a few inches of wood or a few tenths of an inch of aluminum will
effectively eliminate the exposure.
What about gamma rays?
Because they are uncharged, gamma radiation is more difficult to shield than
particles like alphas and betas. The basic approach to gamma ray shielding
is to optimize the thickness and density of the shield such that the intensity
of the beam is reduced to an acceptable level. In other words, a very thin
layer of lead or steel may be just as effective of a shield as a much thicker
layer of concrete or wood.
Neutrons are uncharged. Are they shielded like gamma rays?
No. Neutron shielding is based on "moderating" or slowing down neutrons until
their energy is low enough to permit them to be "captured" by a near-by atom.
A wide range of shielding materials are used for these purposes, with some
of the most common being water or cement.
What is the relationship between the energy of a radiation and its range
in a shield?
Charged particle radiations like alphas and betas travel fixed distances
in different materials, with the actual distance dependent upon their energies
(in MeV). The greater the energy, the greater the distance the radiation
will travel, and the thicker the shield must be to stop it. However, shielding
for gamma rays and neutrons is more complex. In reality, the range for these
radiations is indefinite. It is possible for these radiations to travel for
miles before being stopped.
We've spoken of the concepts of "time", "distance" and "shielding" in
regard to reducing external radiation exposures. Are there other factors
that should be considered?
Yes. The quantity (amount) of radioactive material in the source also influences
the mount of exposure received. If one source has only half the amount of
radioactivity in it as another equivalent source, only half of the radiations
will be emitted. Therefore, the exposure rate at a given location away from
the small source will be only half that of the larger source.
In general, radiation sources are designed to achieve a given purpose. Therefore, reducing the radioactivity in them may not be practical. However, if radiation sources are no longer necessary or needed, its a good idea to reduce the inventory. Likewise, when residual radioactivity in the form of "contamination" is present, it is clearly prudent to reduce its amount to the greatest extent practical so that the exposure rate from it will likewise be reduced. Therefore, good housekeeping is just as important in the business of radiation and radioactivity as it is anywhere else.
Copyright © 1998 Integrated Environmental Management, Inc.