Physics and Engineering of Radiation Detection

510 Chapter 8. Signal Processing

medium, implying that the deposited energy is 90keV. The number of electron hole
pairs generated by this photon will be

N =

90 × 103

3. 6

=2. 5 × 104 electron hole pairs

The statistical fluctuation in this number with the Fano factorF=0.1 will be

σstat =

√

FN

=

√

0. 1 × 25000

= 50electronholepairs

The presently available electronics for semiconductor detectors can have a noise
level of as low as 10 electron hole pairs. However such a noise level is difficult and
expensive to achieve. Most of the systems have an inherent noise of several hundred
electrons. Even if we assume that the electronics noise levelσelecis 100 electrons, it
has a significant effect on the signal to noise ratio as shown below.

S/N =

N

σstat

(with only statistical fluctuations)

=

2. 5 × 104

50

= 500

S/N =

N

(

σ^2 stat+σ^2 elec

) 1 / 2 (with statistical and electronics fluctuation)

=

2. 5 × 104

(50^2 + 100^2 )^1 /^2

≈ 224

(8.9.1)

This clearly shows how large an impact does the electronics noise have on the signal
to noise ratio. In this particular case, therefore, the strategy should be to decrease
the electronics noise as much as possible.

Example:

A silicon detector is used to measure the intensity of a 150keVphoton beam.

The signal to noise ratio is found to be 300. Determine the noise introduced

by the associated electronics.

Solution:

We first compute the statistical noise level. To do that we must first estimate

the number of charge pairs generated. Assuming that all of the incident energy

is absorbed in the active volume of the detector, the number of generated