Calibration Requirements for Contamination Monitors
Gem-5 User’s Manual 89
Calibrate Process
Calibration sources used should have sufficient activity to enable the collection of
good counting statistics in a reasonable data collection time and be traceable to a
nationally or internationally recognized standard/agency (like NIST). Using a
calibration source whose activity is greater than typical alarm limits usually leads to
better counting statistics and shorter count times.
A traceable Gamma point source with activity in the range of 3700 to 37000 Bq (100
to 1,000 nCi) is appropriate.
The contamination monitor should be calibrated with the radionuclide(s) that is most
likely to be of concern. The choice of radionuclide may have a significant effect on
count time and sensitivity and will also affect the High Voltage setting for Gamma
detectors (see page 78).
Gamma scintillation detectors contain only one detection zone.
A detection zone can be a single detector section or a sum zone consisting of 2 (pair),
3 (triple), 4 (quad), adjacent, or All detector sections. Normally only the single
detector zones require calibration and sum zones are automatically calculated from
these single detector efficiencies. If required, the sum zones can be calibrated with a
live source instead of using the derived efficiency.
Note: The source distance depends on detector location:
• Foot detector is 15 cm (6 in.).
• For all other detectors is 20 cm (8 in.).
Care should be taken when calibrating to ensure that the source is secured
preferably in a jig or is held at least at arms-length to minimize the effect of
self-shielding.
Note that the Self Shield Factor applied during calibration is settable in the
F3 – Calibration screen (see page 121). See Footnote 13 on page 215 for the
special case for gamma detectors.
A full calibration of some or all detection zones can be performed as follows:
Calibration Setup
1. Make sure that the detectors’ operating conditions have been optimized
(preferably using the FOM procedure described earlier).
As stated before this typically involves:
− Setting Discriminator Levels – Optional, applies to single detection
zones only; the factory default discriminator levels are usually adequate
and should not need to be adjusted for the life of the detectors; the
settings can be reviewed and adjusted if required using the
F8 – Discriminator Plot screen (on page 78).
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