of 10 to 100. Conversely, at higher energies, this detector could
respond too low.
Figure 5. MicroR meter—sodium iodide crystal for gamma
radiation measurements.
Calibration and Energy Dependence
Exposure reading instruments are primarily calibrated only for
a cesium signal. A radiation signal from any other source could
give a very different response depending on how the instrument
responds to different energies. When this uncertainty is present, significant errors could occur. The instrument manufacturer
should be able to tell how each instrument will respond at different energies.
In practice, it is helpful to compare your instrument response
with the anticipated energies you wish to measure so you can determine if the instrument is reading too high or too low. To account
for energy differences for activity measurements, the best option is
to calibrate the instrument with the source you wish to measure.
Geometry
Geometry has to do with the orientation of the detector to the
signal. Since all radiation measurements are made by comparison, the best results are achieved by using the instrument in the
same way it was calibrated. For example, a pancake GM probe
is usually calibrated for activity measurements with a source
positioned by a jig, which holds the source at about ¼ inch
Ionizing Radiation PDC at AIHce
The PDC “Practical Applications for Portable Radiation Instruments,” which will be held at AIHce 2012 in Indianapolis,
features a brief review of the fundamentals of radiation to give
attendees a better understanding of how instruments work
and what they measure. A variety of radiation instruments will
be made available at the PDC for attendees to see how they
respond to several exempt quantity radioactive sources. The
mode of instruction will be primarily show-and-tell, including
a laboratory exercise for participants to observe which instruments are fast or slow, stable or erratic, more or less sensitive, and analog versus digital readouts.
For more information about AIHce 2012, visit www.
aihce2012.org.
from the face of the detector. Therefore, all readings with this
detector should be taken at a distance of ¼ inch. According to
the inverse square law for a point source of gamma radiation—
doubling the distance reduces the signal by a factor of four—a
reading taken at ½ inch is in error by 400 percent.
Verifying Instrument Response
When asked whether an instrument is responding properly,
people will often say that the instrument clicks when it’s turned
on or the calibration sticker shows that it was recently calibrated. Neither of these answers verifies that the instrument is
responding the same way it did at the time of calibration.
The best way to verify response is to take a reading from a
check source at the time of calibration; ideally, this is a source
that will remain constant over a period of years. This check source
reading should be recorded on the instrument calibration sticker.
The normal steps to take before using a radiation instrument are:
1. check visually for any signs of damage to the detector,
cables or meter housing
2. verify the battery condition
3. take a check source reading
The normal practice is to accept readings within 20 percent
of the original reading.
Interpretation of Measurements
After choosing the proper instrument, verifying that it is working properly and is being used the same way it was calibrated,
and learning its limitations, you may then acquire measurement
data. When interpreting these measurements, it may be helpful
to know that there are no absolute measurements for radiation.
Since radiation is random in time and direction, repeated measurements will always differ, and all radiation measurements
are only best estimates.
Some safety decisions require higher quality radiation data
than others. When evaluating radiation data, you must carefully
consider the quality necessary to make a particular decision.
Standard practice assumes that portable radiation instruments
should be accurate within 20 percent of the true value. If this
seems high, consider the discussion above, which describes
several factors that could cause an instrument to be in error by
several hundred percent.
The basic rules before making an expensive decision based
on portable radiation instruments are to ask lots of questions
about how the measurements were made, and at a minimum,
confirm the measurement by repeating it, ideally with a different instrument or different people. This last caution can be
problematic in an emergency where the first tendency is often
to take quick action.
Ray Johnson, PE, CHP, is vice president of Dade Moeller and Associates. He can
be reached at (301) 990-6006 or ray.johnson@moellerinc.com.
Judson Kenoyer, CIH, CHP, is manager of the Academy Division at Dade Moeller
and Associates. He can be reached at (865) 481-6050 x.3408 or jkenoyer@
moellerinc.com.
