Physics and Engineering of Radiation Detection

362 Chapter 6. Scintillation Detectors and Photodetectors

highly smooth and polished (see Fig.6.4.1). The photons that travel straight from
the scintillator to the photodetector do not undergo any attenuation. Since a scin-
tillator can emit photons in all directions therefore some of them will hit the walls
of the container and get reflected. The smaller the angle of incidence the higher the
probability that the photon will again hit the opposite wall of the container. After
one or more such reflections the photons reach the photodetector and get counted.
In principle, such a light guide should work quite efficiently. However, in reality
these devices are quite inefficient. There are several reasons why simple reflection
type light guides do not work well. First of all it is practically extremely difficult
to ensure a very high degree of smoothness as any deviation would let the photons
wander around more and even get absorbed by the material. Secondly the sim-
ple reflection is always associated with some degree of absorption by the material.
Therefore the intensity of light emitted by the scintillator is bound to decrease as
the light travels down the light guide.

Scintillation Photons

Radiation

Incident

Light Guide

Scintillator Photodetector

Figure 6.4.1: Transmission of scintillation photons through a simple reflection

type light guide to a photodetector.

A.2 TotalInternalReflectionType

The light guides based on total internal reflection are the most popular amongst
radiation detector technologists since they offer lower loss of light intensity and hence
better sensitivity and superior resolution of the detector. The basic idea behind such
a device is to ensure that the process of total internal reflection is guaranteed at all
angles of incidence. The basic condition for total internal reflection to occur is

sinθi≥

nm

n

, (6.4.1)

whereθiis the angle of incidence (see Fig.6.4.2),nmis the refractive index of the
medium outside the waveguide (usually air), andnis the refractive index of the light
guide. If the detector is kept in air thennm= 1, which reduces the above condition
to

sinθi≥

1

n

. (6.4.2)