Synergist - March 2012

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:
•;Have;we;chosen;the;appropriate;instrument;for;the;type;of
radiation and safety decision we want to make?

•;How;can;we;know;if;the;chosen;instrument;is;working;properly?

•;After;verifying;operation,;are;we;prepared;to;use;the;instru-ment properly—that is, the way it was calibrated?

•;Once;measurement;data;has;been;gathered,;what;do;these measurements mean and are they defensible?

•;Do;these;measurements;support;expensive;decisions;for 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 lower energy.