3.8. Detector Efficiency 211

efficiency of the detector.

Solution:

The quantum efficiency of the detector can be calculated from equation 3.8.7.

However to use that equation, we need the value of the attenuation coefficient

μgfor dry air. For that we turn to the physical reference data made available

by the National Institute of Standards and Technology (12). We find

μm,air =40. 27 cm^2 g−^1

and ρair =1. 205 × 10 −^3 gcm−^3 ,

whereρm,airis the mass attenuation coefficient of dry air for 10keV photons

andρairis the density of dry air under standard conditions. The attenuation

coefficient is then given by

μair = μm,airρair

=(40.27)

(

1. 205 × 10 −^3

)

=4. 85 × 10 −^2 cm−^1.

—

We now substitute this and the length of the chamber into equation 3.8.7 to

get the quantum efficiency.

QE =1−e−μairxair.

=1−exp

[

−

(

4. 85 × 10 −^2

)

(5)

]

.

=0. 21

To compute the detective quantum efficiency, we make use of equation 3.8.8.

DQEint =

QE

1+ σ

(^2) m
(QE)(Nin)

0. 21

1+^150

2

(0.21)(10^5 )

=0. 10

This shows that the detective quantum efficiency of the detector is only 10%,

which may not be acceptable for most applications. However at higher incident

photon fluxes theDQEwill be higher and will eventually reach the quantum

efficiency.

The detector in the previous example did not have a good quantum efficiency.
But the good thing is that one has several options to increase the efficiency before
building the detector. One can customize a detector according to the application to
yield the maximum possible detective quantum efficiency. Perhaps the best way to
increase theDQEis to increase the quantum efficiency. This can be done in may
ways, such as by increasing the density of gas, using another gas, or by increasing
the size of the detector.