Physics and Engineering of Radiation Detection

65

Chapter 2

Interaction of Radiation with Matter

Whenever we want to detect or measure radiation we have to make it interact with

some material and then study the resulting change in the system configuration.

According to our present understanding, it is not possible to detect radiation or

measure its properties without letting it interact with a measuring device. In fact,

this can be stated as a universal rule for any kind of measurement. Even our five

senses are no exception. For example, our eyes sense photons that strike cells on its

retina after being reflected from the objects. Without such interactions we would

not be able to see anything^1. The same is true for radiation detectors, which use

some form of radiation interaction to generate a measurable signal. This signal is

then used toreverse engineerthe properties of the radiation. For example, when x-

ray photons after passing through an object strike a photographic plate, theystain^2

the plate. The photons, which can not pass through the highly absorbing materials

in the object, do not reach the photographic plate. The varying intensity, by which

the photographic plate is fogged, creates a two dimensional image of the object and

in this way the photographic plate acts as a position sensitive detector. Although

such a method of radiation detection is still widely used, it is neither very sensitive

nor very accurate for measuring properties of radiation, such as its energy and flux.

Thesocalledelectronic detectorsprovide an alternative and much better means of

detecting and measuring radiation. An electronic detector uses a detection medium,

such as a gas, to generate an electrical signal when radiation passes through it.

This electrical signal can be used to characterize the radiation and its properties.

Therefore, a necessary step in building a detector is to understand the interaction

mechanisms of radiation in the detection medium. This chapter is devoted to exactly

this task. We will look at different types of radiation detectors in the subsequent

chapters.

2.1 SomeBasicConceptsandTerminologies

Every particle carries some energy with it and is, therefore, capable of exerting force

on other particles through processes we call particle interactions. These particle in-

teractions may or may not change the states and properties of the particles involved.

The way particles interact with matter depends not only on the types of incident

and target particles but also their properties, such as energy and momentum. A

lot of work has gone into building theoretical foundations of different interaction

(^1) In this sense we can say that our eyes are actually photon detectors
(^2) The stains become visible after the plate has been developed through appropriate chemicals.