Physics and Engineering of Radiation Detection

6.4. Transfer of Scintillation Photons 361

The simplest way to do this is to directly attach a scintillator to a photodetector
having an area greater than the scintillator to avoid loss of photons. However, the
geometries of scintillators and photodetectors normally do not fulfill this require-
ment and the situation is the other way round. That is the scintillators have areas
larger than the photodetectors. Hence building a detector in such a configuration is
generally not possible. A more practical approach is to use alight guideto connect
the photon emitting surface to the photodetector such that the scintillation photons
reach the photodetector with minimal loss.
A light guide can be constructed in different configurations but all of them are
based on the principle of reflection. Their main purpose is to guide the photons
from the scintillator to the photodetector, hence the termlight guide. Light guides
can be constructed to work on simple reflection as well as total internal reflection.
Since simple reflection is associated with loss of light intensity due to refraction and
absorption therefore the detectors based on such a device lack good precision and
sensitivity. On the other hand the devices based on total internal reflection offer
minimal photon loss and consequently better sensitivity and precision. However it
should be noted that it is impossible to build a light guide that guarantees total
internal reflection at all angles. Therefore sometimes another material is used in
conjunction with such a light guide to guide the refracted photons towards the
photodetector.
A number of materials have been identified with good photon transmission prop-
erties, some of which are listed below.

Glass and plexiglass

Fiberglass

Clear plastic

Liquid

All these materials have been used to construct light guides but optical fiber is the
most popular and, in most instances, the best choice. The good thing about optical
fiber is that it offers a fair bit of flexibility in arranging the photodetector with
respect to the scintillator.

6.4.A TypesofLightGuides......................

The basic function of a light guide is to transmit scintillation photons to the pho-
todetector with minimum loss. A device that performs this function, no matter on
what principle it is based, can be called a light guide. The task of designing and con-
structing such a device can be fairly involved. This holds specially if the geometries
of the scintillator and the photodetector differ considerably from each other.
In the following sections we will discuss different types of light guides that are
commonly used in radiation detectors.

A.1 SimpleReflectionType

A simple reflection type of light guide can be constructed such that both scintilla-
tor and photodetector are enclosed in a vacuumed container whose inside surface is