Physics and Engineering of Radiation Detection

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Chapter 5

Solid State Detectors

We visited some of the most widely used gas filled detectors in the previous section.

Although such systems have proved to be extremely useful in many applications, still

their usage is somewhat limited due to a number of reasons. For example the small

number of electron-ion pairs that can be generated in a gas is a serious problem

for high resolution systems in low radiation environments. One of the reasons of

this inefficiency lies in the number of target atoms per unit volume in the gas that

the incident radiation sees. This implies that if we use liquids or solids instead, the

probability of production of charge pairs would increase. However, as it turns out,

the mechanism of charge pair production depends on many factors and not just the

density. Nevertheless one type of solid has been found to have far superior charge

pair production properties than gases. These so calledsemiconductorshave electrical

conduction properties between conductors and insulators. Diamond is another solid

that has been found to have very good charge pair production capabilities. All of the

detectors that use solids as active detection media are collectively called solid state

detectors, a term that is sometimes exclusively used for semiconductor detectors. In

this chapter we will discuss the mechanism of radiation detection and measurement

using solids as active detection media. We will also survey some of the most widely

used solid state detectors.

5.1 Semiconductor Detectors

Semiconductors are basically crystalline solids in which atoms are held together by

covalent bonds. They are called semiconductors because their electrical conduction

properties lie between those of insulators and conductors. Germanium (Ge)and

Silicon (Si) are two of the most commonly used semiconductor materials. Up till

now the majority of the semiconductor detectors have been made with silicon, a trend

that may change in future as the search for more radiation tolerant semiconductors

continues.GaAsis one of the materials that have shown to be very promising as an

alternative to silicon, although at a much higher cost. GaAshas another property

that makes it very desirable is its larger band gap. We will explore this further later

in the chapter. In the following sections we will discuss the important characteristics

of semiconductors and look at how they are employed as active media for detection

of ionizing radiation.