Physics and Engineering of Radiation Detection

7.3. Imaging Devices 455

Solution:

The maximum counts that can be recorded by a 15-bit ADC areNmax=2^15 =

32768 counts. Hence according to equation 7.3.6, the dynamic range is

D =

32768

Nc

=

(32768)(wG)

E

= (32768)(3.62)(10)

1

E

,

wherew=3. 62 eV is the ionization energy for silicon. This variation of

dynamic range with energy has been plotted in a semilogarithmic graph (see

Fig.7.3.3).

E (keV)

0 2 4 6 8 101214161820

Log(D)

4

5

6

7

8

9

10

11

Figure 7.3.3: Semilogarithmic graph of variation of dynamic range with inci-

dent photon energy in a silicon based CCD having a gain of 10 photoelectrons

per ADC count and 15-bit ADC resolution.

D.2 DisadvantagesofDirectImaging

This direct detection method works quite well for x-rays having high interaction
probability in the depletion region. However it suffers from several disadvantages,
which make it less desirable for x-ray imaging, some of which are stated below.

Production of electron-hole pairs in the depletion region depends on the energy

of incident x-ray photons. At energies higher than 20keV, the interaction

probability becomes so low that direct detection method does not produce

reasonable signal-to-noise ratio.

Like all semiconductor detectors, CCDs also get damaged by instantaneous

and integrated radiation doses. Direct detection exposes the CCD to radiation