Because radiation does not give us any physical sensa- tion, we are forced to rely on radiation instruments to provide us with information to use as a basis for mak-
ing decisions about safety. The challenge is determining if the
instruments are telling the truth. There are many factors that
can affect the quality of radiation measurements, and several
important questions to consider include:
radiation and safety decision we want to make?
•;After;verifying;operation,;are;we;prepared;to;use;the;instru-ment properly—that is, the way it was calibrated?
measurements mean and are they defensible?
safety, such as evacuation, cleanup or other costly actions?
To answer these questions, the Ionizing Radiation Committee
has presented a PDC on radiation instruments at AIHce for the
past two years. The course will also be presented at this year’s
conference in Indianapolis. The instructors—the authors of this
article—have more than 60 years of radiation instrument experience, and the course has ranked among the top 10 PDCs on
multiple occasions. The PDC focuses not on theory or math but,
rather, on practical information about radiation instruments to
allow industrial hygienists to do their jobs well. (See the sidebar
on page 24 for more information about the PDC.)
The following are some of the specific topics related to radiation instruments.
Choice of Instruments for Exposures
Before choosing an instrument, know the purpose for making
a radiation measurement. For example, are you determining
someone’s external exposure to gamma radiation or X-rays?
If so, the first choice should be a standard or pressurized ion
chamber (see Figures 1 and 2). These are the only instruments
that give a true reading of exposure—transfer of energy through
the air—in milliroentgens per hour or microsieverts per hour. All
other instruments with roentgen scales are surrogate or secondary measurements. This does not mean they are unacceptable;
you just have to be aware of their limitations.
Standard ion chambers are slow, erratic and not very sensitive, meaning they are not able to measure normal background
radiation. In addition, they may be affected by air pressure
(altitude) or temperature. Pressurized ion chambers are more
sensitive, but only if the signal has a high enough energy to
penetrate the heavy walled pressure chamber. For example,
part of any X-ray signal will always have energies at 50 keV
or below, and most of this signal will not be measured by a
pressurized ion chamber. The rule of thumb is that the bulk of
an X-ray signal will have energies at about one-third of the
applied voltage in the direct beam. Scattered X-rays, which we
measure for safety decisions to protect people nearby, will have
Figure 1. Standard ion chamber for exposure measurements in
milliroentgens per hour (mR/hr).
Figure 2. Pressurized ion chamber for X-ray and gamma ray