Physics and Engineering of Radiation Detection

368 Chapter 6. Scintillation Detectors and Photodetectors

Dynode

Vacuumed Enclosure

Photons

Photocathode

Electrons

1R

1R

1R

1R

1R

HV

Figure 6.5.1: Working principle

of a typical side-on type PMT.

The light photons (e.g. com-

ing from a scintillator) produce

electrons through the photoe-

mission process in the photo-

cathode. These electrons are

focused on to the first dynode

where they produce secondary

electrons, which then move to-

wards the second dynode and

produce more secondary elec-

trons and so on. An actual

PMT may contain 10 or more

such dynodes. The amplified

signal is measured at the final

dynode or anode.

whereλis the wavelength of the incident light and Φ is the energy threshold or
work function of the material. For a given material, this expression can be used to
determine the maximum detectable wavelength (see example below).

Example:

A material having a work function of 2eV is used to convert photons into

electrons, which are then detected by a photomultiplier tube. Compute the

maximum wavelength of the photons it can convert into electrons.

Solution:

The maximum wavelength can be obtained by settingEe=0inequation

6.5.1, which simply means that all of the incident energy has been used in the

conversion process and the electron has not taken away any of the photon’s