CAPINTEC, INC. CAPRAC
®
-t
9-2 TESTS April 2021
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AUTO CALIBRATE
The CAPRAC
®
-t uses an automatic calibration procedure to adjust the gain to maintain the
correct relationship between energy and channel.
Auto Calibration should be performed on a daily basis.
Auto Calibration is performed using the same Cs137 rod source as in System Test and
Eu152. First, calibration is performed with Cs137 where the procedure makes use of its 662
keV and the 32 keV peaks to automatically adjust the gain and zero offset. Next, Eu152
provides multiple gammas over an energy range of 41 keV to 1,408 keV. The software
provides a point-by-point energy calibration using these gammas. This function corrects for
any non-linearity in response of the NaI detector and provides accurate identification of the
energy of the peaks. It is important to complete the calibration of your system using Eu152.
Unless an error message is displayed, the software will have succeeded in finding a set of
adjustments, which places the 662 keV gamma peak to the correct energy channel.
The Auto Calibration Test also calculates detector resolution by determining the FWHM (full
width-half maximum) area of the peak, and dividing by the peak centroid location. For peaks
such as the 662 keV peak of Cs137 whose shape is symmetrical or appears to follow a
Gaussian distribution, the FWHM is generally close to 2.35 σ. For well detectors, the
resolution should be ≤10 percent. Detectors do age with use, and this will result in a very
slight increase (1% to 3%) in resolution over a period of several years. A rapid increase in
percent resolution may indicate a problem with the detector assembly.
Over time, the amplification characteristics of a photomultiplier tube change. If the gain of the
detector is not adjusted occasionally, the resolution of the detector deteriorates significantly,
making it difficult to identify nuclides correctly. Therefore, it is important to check the gain of
the detector at least once daily.
Note: Monitoring the stability of the resolution over time is one of the best indications of the
overall system performance.
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