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Aer 5400
9. 2. Dynamic gamma compensation, dose rate measurement
9. 2. 1. Dynamic gamma compensation
Instrument versions with the option "G" are equipped with a second detector with identical
geometry and orientation for dynamic compensation of the gamma background radiation. An
aluminium absorber between the detectors avoids the detection of beta activity by the
compensation detector which has been collected on the filter. The beta count rates generated by
gamma background are expected to be similar for both detectors. Because the measurement
detector counts the sum of real collected beta activity and gamma induced beta activity while the
compensation detector counts only gamma related activity, the collected filter activity can be
obtained by subtraction of both detector signals.
Directed or in-homogeneous radiation fields (e.g. sources very close to the detectors) result in
differences between the gamma related count rates of both detectors. For incidence angles more
than ±30° related to the detector surface a significant over- or under-compensation will appear. This
will result either in an increase of the detection limit or in the detection of a not really present beta
activity. The configuration of the instrument allows the compensation of this difference by an
additional factor:
•
R
N beta
•
R
MD
•
R
CD
•
K
To check if such conditions are present and to determine the correction factor K, a test measurement
without filter activity (R
carried out. The count sums per interval are available in the spectrum view of dVISION after reading
them from the instrument ("Total Lo-E" = measurement detector, "Gamma background" =
compensation detector). The correction factor K can be determined by the simple formula
To obtain a sufficient statistical uncertainty, the count sum of each detector should be more than
10,000 counts (can be achieved also by adding of a few intervals). The calculated correction factor
(called as "Ratio Gamma Filter/Compensation" in the parameter table of the spectrometer
configuration) can be transferred to the instrument (see chapter "User specific settings").
After delivery this factor can be slightly different from one due to slight differences of the beta pulse
threshold of both detectors.
9. 2. 2. Measurement of local dose rate
The count rate of the compensation detector is simultaneously used to determine the local dose rate.
The counter channel (CNT1) is configured in the manner that an average dose rate will be calculated
beginning at the start of the current interval. Thus, the statistical variance will be reduced with
advancing interval.
16
R
N beta
count rate of real beta activity collected on the filter
gross count rate of the measurement detector
count rate of the compensation detector
correction factor
= 0; new filter, pump off by disconnected mains power plug) should be
N beta
K = N
Manual_Aer 5400_EN_27-04-2023.docx
= R
– K * R
MD
CD
/ N
MD
CD
SARAD GmbH 2023
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