Efficiency Calculation (and Derived Efficiencies)
Gem-5 User’s Manual 291
zones are considered using the count time that will actually be used. A typical
system false alarm rate, e.g. 1:370, will be significantly better than the theoretical
worst case of 1:2378, as shown in Figure 54 on page 113.
Confidence of Alarm
Alarm Confidence (2 tailed) in % K Beta
75 1.151
80 1.282
85 1.440
95 1.960
96 2.054
97 2.170
98 2.326
99 2.575
The K Beta terms determine the confidence that the system will alarm for a true
“contaminated” condition. This is the statistical confidence that the alarm trip level
will be exceeded by the net count rate for the detection zone and is based on a
statistical calculation for a “2-tailed” Poisson distribution. The default K Beta is set
to 2.00 (95.45%) in the F1 – Zone Configuration screen.
Efficiency Calculation (and Derived Efficiencies)
See the appendix, Spare Parts Lists – Standard Models, starting on page 377 for
Factory Default information.
1. Individual detectors should be calibrated using a point calibration source
(usually Cs-137, 100-1000 nCi), source to be located as follows:
− For side and head detectors:
8” (20 cm) away from the detector surface at the centre of each detector,
6” (15 cm) away from the detector surface at its centre for the foot
detector.
− Expected efficiencies for Cs-137 are in the range 8-10% (default = 4%).
2. Efficiencies for the Sum Zones are calculated as per following formulas (E
n
is the efficiency of the n detector calibrated as described above):
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