Appendix A Algorithms
290
only be 1 or 2 detection zones at the LLD, while the remaining zones may be still at
the ATP, i.e. providing full alarm sensitivity in those regions.
Equation 30 Count Time with T
b
term
2
bb
m
)cps(ALS
)cps(R)cps
(ALSK)cps(RK
)s(T
+×+×
=
βα
(30)
The equation above is used to check the effect of the Background Average Period
(T
b
) term on the High Count Time Out of Service Fault decision when set to
“AUTO” (discussed in Out of Service Causes).
Background FALSE Alarm Rate (FAR)
The K
α
(K Alpha) term, which is used in Equation 25 on page 286, is a statistical
term related to the false alarm rate (FAR) due to background radiation. The K Alpha
term determines the Lowest Level of Detection (LLD) for the monitor given the
acceptable false alarm rate. It is the system false alarm rate that is important, as
opposed to that for individual detection zones. The monitoring time for the system is
controlled by the “worst-case” zone and is dependent on the background count rate,
the zone efficiency, etc. As the worst-case zone sets the counting time, all other
zones are forced to count longer than needed and therefore have improved statistical
data for making a decision, i.e. they will have a lower FAR.
The system FAR is the sum of the FAR for each individual detection zone. If each
zone had identical conditions and independent count rates (which is not strictly true
for sum zones) and was set to an individual zone FAR of 1:370 (i.e. K Alpha = 3.0),
then the system FAR would be the sum of the individual probabilities (e.g. 1:53 for
an Argos-5AB Zeus monitor configured with 440 detection zones). See Figure 54 on
page 113.
False Alarm Rate Per Detector
1: 1 0.0
1: 2 0.5
1: 3 1.0
1: 7 1.5
1: 370 3.0
1: 2149 3.5
1: 15787 4.0
The value for the Bkg False Alarm (K Alpha) is set in the F1 – Zone Configuration
screen. The K Alpha value displayed in Figure 54 on page 113 is the sigma value
equivalent to the single detector probability [e.g. 3.000 (K Alpha) is for a probability
of 1:370]. That screen also displays the FAR value for the system when all detection
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