Physics and Engineering of Radiation Detection

214 Chapter 3. Gas Filled Detectors

Example:

If the detector of the previous example has a readout noise of 200 photons

and a quantum efficiency of 0.54, compute its output signal-to-noise ratio.

Solution:

The output signal is given by

Sout =(QE)(Sin)

=(0.54)

(

2. 83 × 106

)

=1. 53 × 106 photons.

The output noise can be computed from equation 3.8.15.

Nout =

[

Sout+σ^2 rout

] 1 / 2

=

[(

1. 53 × 106

)

+ (200)^2

] 1 / 2

=1. 25 × 103

The output signal-to-noise ratio is then given by

SNRout =

Sout

Nout

=

1. 53 × 106

1. 25 × 103

= 1224. (3.8.16)

In the previous section we learned about detective quantum efficiency. TheDQE
can be calculated from the square of the ratio of the output to input signal-to-noise
ratios, that is

DQE=

[

SNRout

SNRin

] 2

. (3.8.17)

Example:

Compute the detective quantum efficiency of the detector described in the

previous two examples.

Solution:

We use equation 3.8.17 to compute the detective quantum efficiency.

DQE =

[

SNRout

SNRin

] 2

=

[

1224

1683

] 2

=0. 53. (3.8.18)