false alarms, if possible, you need to set the threshold so low
that with the selected measurement time the threshold is, for
example, 6 standard deviations below the mean background
value. Then the probability that the background rate at the end
of the fail time is lower is less than 1.0E-06 and then there is vir-
tually no false alarm.
Example:
Background Ro = 0.5 cps; set measuring time Ta = 600 s; then
it holds for standard deviation S:
=0.029 cps
6 standard deviations = 0.173 cps
Fail threshold = Ro – 6 · S = 0.33 cps
At Ta = 300 s, the fail threshold would be 0.255 cps.
If you get a negative fail threshold, you have to increase the fail
time. However, you can always increase the number of stand-
ard deviations to be quite sure that you do not get any false
alarms.
Dead time Each detector has its own dead time which depends on the de-
tector properties and the subsequent amplifier and discriminator
electronics. With gas-filled counter tubes and NaI crystals this is
typically 1 to 3 µs. The effect of the dead time is that count rates
with increasing size do not increase linearly any more at some
point, but go into saturation. This effect is corrected to a certain
degree by the dead-time correction and the curve is linearized
again. The dead times for the individual detectors are deter-
mined and preset at the factory.
The formula used is:
Rcorr = Rmeas / ( 1 – tau * Rmeas)
Where
Rmeas: measured count rate in cps
Tau: Dead time in s
Rcorr: dead-time corrected count rate
Alpha only / Beta only mode Three modes can be used in a detector with simultaneous al-
pha/beta measurement:
Alpha/Beta simultaneously
Only alpha device
Only beta device
If you select alpha only or beta only, the device behaves as if it
is measuring only alphas or only betas. The results of the other
type of radiation appear never and nowhere. With simultaneous
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