Physics and Engineering of Radiation Detection

6.5. Photodetectors 371

Wavelength ( )nm

200 300 400 500 600 700 800

10

20

30

QE (%)

Figure 6.5.2: Quantum ef-

ficiency of a typical pho-

tocathode as a function of

wavelength of incident pho-

tons.

Photocathodes can be used in essentially two different modes: transmission and
reflection. In transmission mode the photocathode is semitransparent to allow trans-
mission of photoelectrons (see Fig.6.5.3(a)). Such a photocathode is constructed by
depositing a very thin layer of the material on the inside of the photon entrance
window. Since most of the photoelectrons are emitted in the direction of travel of
the incident photons therefore it is called transmission photocathode. Most PMTs
are constructed with this type of photocathodes. There are also some photocathode
materials that have high quantum efficiencies but very poor transmission properties.
Theseopaquematerials are used to construct the so called reflection photocathodes
as shown in Fig.6.5.3(b). A reflection photocathode is made by depositing a thin
layer of the material on a metal electrode inside the PMT.

Photocathode

Photons
Dynode

Dynode

Photoelectrons

Photoelectrons

Photons

Photocathode

(a)

(b)

Figure 6.5.3: (a) Semitransparent photocathode

used as a transmission photoemission device in

a head-on type PMT. (b) Reflection type photo-

cathode. Such photocathodes are generally used

in circular type PMTs.