Chapter 4 Unit Setup
88
Calibration Requirements for Contamination Monitors
GEM-5 monitors are manufactured to strict tolerances. In addition, all units are
calibrated at the factory for normal background conditions. Although the GEM-5
monitor will work reasonably well with the factory calibration settings, it is
recommended that all detectors be calibrated during commissioning using the isotope
of concern at the facility and calibrations should be repeated at least once per year
after that, at the customer site. In addition, a calibration should be performed when a
replacement detector or photomultiplier tube (PMT) is installed.
Factory optimization of high voltage and efficiency calibration settings for
scintillation detectors will result in safe operation of the equipment but do not
guarantee best performance. Therefore, high voltage optimization should be done at
the final site location to take into account insitu background conditions.
Additionally, if the high voltage(s) is/are changed for any reason the affected
scintillation detector(s) must be re-calibrated!
To supplement periodic full calibrations, an Alarm Test (see page 69) should be
performed at least monthly (weekly is recommended) to verify that the system is
performing satisfactorily.
Why is Calibration Necessary?
Contamination monitors have to be calibrated to a known isotope to provide a
benchmark for how accurately they can monitor to prescribed limits. This is
necessary because detector response is different to various types of isotopes. In
addition, not all isotopes decay with a yield of 100% of measurable decay products.
All contamination monitors are calibrated at Mirion Technologies (Canberra CA)
Ltd. site by using the following radioactive isotopes:
• Cs-137 Gamma point source.
• Co-60 Gamma point source for European market
Note that check and calibration sources shall be selected to meet local regulatory
requirements.
Additional information can be found through organizations such as Electric Power
Research Institute (EPRI), Institute of Nuclear Power Operations (INPO) and
International Electrotechnical Commission (IEC) in their published documents as
appropriate.
Some examples of these are: “EPRI Radioactive Material Monitoring & Control
Guideline” - December 2009, “EPRI Alpha Monitoring Guidelines for Operating
Nuclear Power Stations”- December 2009, “INPO RP 1-π, Contamination Control
and Monitoring Evaluator How To” and “IEC 61098” respectively.
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