Physics and Engineering of Radiation Detection

456 Chapter 7. Position Sensitive Detection and Imaging

and thus degrades its performance over time due to damage caused by the

radiation.

The active area of direct detection CCDs is small and is not suitable for most

imaging requirements.

7.3.E IndirectImaging

The disadvantages of direct detection can be overcome by converting x-ray photons
into visible light photons, for which the standard CCD sensors have the highest
sensitivity. The trick, of course, is to ensure linearity of the conversion process
and make it as efficient as possible. Different types of phosphors and scintillators
have been identified as efficient photon converters. Phosphors are granular solids
while scintillators are found in polycrystalline powder form. Generally phosphors
are used in CCD cameras. The efficiency of the conversion process can be optimized
by changing the chemical composition and grain size of the phosphors. Sometimes
additional mirror coating is also provided to increase the x-ray interaction probability
in the material.
Fig.7.3.4 shows the sketch of the main components of a typical imaging device
based on CCD sensors. The x-rays get absorbed in the phosphor, which promptly
emit light. These light photons are guided through an optical coupling, such as fiber
optic, to the photocathode of an image intensifier. The role of this intensifier is to
amplify the light coming from the initial phosphor. The photocathode converts the
light photons into electrons, which are multiplied by passing them through multiple
stages of a photomultiplier tube. These electrons are then converted into light
photons by another layer of phosphor or scintillator. These light photons are made
to pass through a fiber optic guide before entering the CCD depletion region to
produce electron-hole pairs. Such a multi-stage process ensures that the x-rays do
not reach the sensitive CCD sensors, prolonging the life of the detector. Also such
a device can be used for a broad range of x-ray energies as compared to the direct
imaging systems, which become essentially insensitive even at moderate energies.

Photons Electrons

x−rays

Phosphor Optic Guide

Photocathode Optic Guide

CCD

Phosphor

PMT

Figure 7.3.4: Sketch of an indirect CCD for x-ray imaging.