212 Chapter 3. Gas Filled Detectors
Example:
A GM detector having a quantum efficiency of 0.34 and dead time of 50μs
is used in a low radiation environment. If within a measurement time of 1
s, 1000 photons enter the detector’s active volume, compute its detective
quantum efficiency. What will be theDQEif the number of photons increases
to 10^5 within the same measurement time?Solution:
Since a GM detector is a quantum detector, we can use equation 3.8.9 to
compute theDQE.DQEquant = QE[
1 −
(QE)(τ)(Nin)
t] 2
exp(
−
(QE)(τ)(Nin)
t)
=0. 34
[
1 −
(0.34)
(
50 × 10 −^6
)
(1000)
1
] 2
×
exp(
−
(0.34)
(
50 × 10 −^6
)
(1000)
1
)
=0. 32
Next we have to determine the detective quantum efficiency for the case when
the same detector is used in a much more hostile radiation environment. Since
all other parameters remain the same, we substituteNin=10^5 in the above
equation to get
DQEquant=0. 2.3.8.A Signal-to-NoiseRatio.......................
Signal-to-Noise ratio, generally represented bySNRorS/Ris one of the most widely
used parameters to characterize detector response. As the name suggests, it is given
by the ratio of the signal to noise, that is
SNR=
S
N
(3.8.10)
whereSandN represent signal and noise respectively. There are generally two
types ofSNRs that are associated with detectors: inputSNRand outputSNR.
The inputSNRtells us what we should expect to see from the detector, while the
outputSNRrepresents the actual situation. Let us take the example of a photon
detector. If we know the flux of photons incident on the detector window, we can
calculate the number of photons expected inside the detector. This will be ourinput
signalSin. The noise of this signal is given by the statistical fluctuations in the
number of photons, which can be calculated from
Nin=√
Sin. (3.8.11)