Physics and Engineering of Radiation Detection

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Chapter 4

Liquid Filled Detectors

There is no reason why a liquid can not be used as an ionizing medium for detection

of radiation. When radiation passes through a liquid, it produces charge pairs,

which can be directed towards electrodes for generation of a pulse. If the liquid

assures good proportionality between the energy deposited and the number of charge

pairs generated, the height of the pulse would give a good measure of the energy

deposited. As it turns out, there are a number of liquids that have fairly good

proportionality and therefore can be used as detection media. Now, one would

expect the charge recombination probability in a liquid to be much higher that in

a typical gas. This is certainly true but we should also remember that the higher

density ensures production of larger number of charge pairs as well. We will discuss

these two competing factors later in the chapter, but the point to consider is that,

in principle, liquids can be used as ionizing media to detect and measure radiation.

Apart from a section onbubble chambers, in this chapter we will concentrate

on different types of electronic detectors that use liquids as detection media. The

bubble chambers, as we will see later, do not work like conventional electronic detec-

tors in which the voltage or current is measured at the readout electrode. Instead,

the particles passing though them produce bubbles that are photographed and then

visually inspected. There is also a class of detectors, calledliquid scintillation detec-

tors, in which the liquids produce light when their molecules are excited by incident

radiation. Such devices will be discussed in the chapter on scintillation detectors.

The reason for devoting a whole chapter on this topic is that the use of liquids

as active detection media is now gaining momentum in different fields, including

medical imaging and high energy physics, which have traditionally relied on gas

filled and solid state detectors.

4.1 PropertiesofLiquids...........................

Before we go on to specific types of detectors, let us first discuss some properties of

liquids that are important with respect to their use in detectors.

4.1.A Charge Pair Generation and Recombination

The principle mechanism of a liquid filled detector is the same as a gas filled detector:

a charge pair is created by the incident radiation and the resulting change in current

or voltage across the electrodes is measured. Therefore the first thing to investigate

is whether the generation of charge pairs in liquids and gases are analogous or not.