Physics and Engineering of Radiation Detection

454 Chapter 7. Position Sensitive Detection and Imaging

There is another quantity, which is also sometimes referred to as the quantum

efficiency, that actually determines the number of electrons generatedNe,genper

incident photonNγ,inc. To avoid confusion, we will call it conversion efficiency

CE.

CE=

Ne,gen

Nγ,inc

(7.3.2)

The ratio of these two quantities gives the effective quantum yieldηof the

CCD, that is, the number of charge pairs generated per absorbed photon.

η=

CE

QE

=

Ne,gen

Nγ,abs

(7.3.3)

Dynamic Range: The dynamic range of any detection system determines

the smallest and the largest signals detectable by the device. It is conven-

tionally determined by dividing the largest detectable signal by the smallest.

For a CCD, the dynamic range depends on three factors: resolution of the

readout electronics, system gain and the energy of the incident radiation. The

electronics resolution is determined by the resolution of the analog to digital

converter. The sytem gainGis calculated by taking the ratio of the number

of electron-hole pairs generatedNehto the number of ADC countsNc

G=

Neh

Nc

. (7.3.4)

The numerator in the above equation is simply determined by dividing the

energy of the incident radiation by thewvalue of the semiconductor material,

that is,Neh=E/w. Hence above equation can also be written as

G =

E

wNc

or Nc =

E

wG

. (7.3.5)

IfGandEare known, we can easily determine the minimum detectable signal

Ncfrom the above equation. The dynamic rangeDwould then be simply given

by

D=

Nmax

Nc

, (7.3.6)

whereNmaxis the maximum number of measurable counts and is determined

by the ADC resolution (see example below).

Example:

Plot the dynamic range of a silicon based CCD having a gain of 10 photoelec-

trons per ADC count in the photon energy range of 50eV to 20keV. Assume

that the ADC has 15 bit resolution (It is generally safe to assume that a

system having a commercially available 16-bit ADC has a net resolution of

15 bits.).