Physics and Engineering of Radiation Detection

8.9. Electronics Noise 511

electron hole pairs is

N =

Edep

W

=

150 × 103

3. 6

(sinceW=3. 6 eV/charge pair for Si)

=4. 17 × 104 charge pairs.

The statistical fluctuations associated with the generation of charge pairs is

given by

σstat =

√

FN

=

√

(0.1) (4. 17 × 104 ) (sinceF≈ 0 .1forSi)

=64.5 charge pairs.

The electronics noise can be estimated from the expression for the signal to

noise ratio as follows.

S/N =

N

(

σstat^2 +σ^2 elec

) 1 / 2

⇒σelec =

[(

N

S/N

) 2

−σ^2 stat

] 1 / 2

=

[(

4. 17 × 104

300

) 2

− 64. 52

] 1 / 2

= 123 charge pairs

8.9.A TypesofElectronicsNoise....................

Broadly speaking, electronics noise can be divided into two categories: random and
non-random. The former follows Gaussian statistics while the latter generally has
1 /fspectral distribution (fis the frequency). In the following we will discuss some
of the most commonly encountered types of noise belonging to these two categories.

A.1 JohnsonNoise

The free charges in a conductor possess kinetic energy, which is a function of their
temperature. Due to this energy they constantly remain in random thermal motion.
Since motion of a charge carrier constitutes an electrical current therefore because
of this motion the electrical state of the system fluctuates even in the absence of
any current driving force. These fluctuations are referred to as Johnson, thermal,
or Nyquist noise. To elaborate on this further, let us suppose we have an electrical
conductor having contacts at both ends but not connected to any voltage source. If
we measure the voltage across these contacts using a voltmeter or an oscilloscope,
the average voltageV will be zero. However we will see fluctuations about this