372 Chapter 6. Scintillation Detectors and Photodetectors
A host of photocathode materials have been identified with varying characteristics
and spectral responses, some of which are listed below.
AgOCs: This is one of the most widely used photocathode materials. It has a
photoemission threshold of 1100nmand a peak quantum efficiency at around
800 nm. The working range of this material is in the near infrared range of the
photon spectrum. The main disadvantage ofAgOCsis its very low quantum
efficiency, which has a peak of less than 1%. It is mainly used as a transmission
photocathode.GaAs(Cs): This material has a spectral response that ranges from ultraviolet
to 930nm. This broadband response makes it suitable for used with a wide
range of scintillators without the need of a wavelength shifter. In most instances
it is used in the transmission mode. Since the quantum efficiency ofGaAs(Cs)
is temperature dependent with a peak at very low temperatures, it is sometimes
operated at very low temperatures.InGaAs(Cs): This material has greater sensitivity in the infrared range and
higher signal-to-noise ratio in 900-1000nmrange thanGaAs(Cs).SbCs 3 : This is one of the earliest used photocathode materials. It is still very
popular amongst manufacturers as its spectral response ranges from ultravio-
let to the visible region with a peak quantum efficiency of around 20%. The
photoemission threshold ofSbCs 3 lies at around 700nmand has a peak at ap-
proximately 400nm. Since it has very poor transmission capabilities therefore
it is generally used as a reflection photocathode.Bialkali Materials: The bialkali materials such asSbRbCsandSbKCsare
the most widely used of all photocathode materials due to their high sensitiv-
ities to blue light generated byNaIscintillators. The reader might recall that
NaIis the most popular scintillator for radiation detection. The sensitivity
to blue light is not the only reason for their popularity, though. These ma-
terials also have high quantum efficiencies with peaks of just less than 30%.
Another advantage is their good stability at elevated temperatures. Some bial-
kali materials can be used at a temperature as high as 175^0 C. A common
bialkali materialSbKCshas a photoemission threshold of about 700nmand
maximum quantum efficiency of 28% at around 400nm.Multialkali Materials:These materials have very wide spectral response ranging
from ultraviolet to near infrared, making them highly suitable to be used with
a number of different scintillators. Their main disadvantage is high thermionic
emission of electrons even at room temperature and therefore external cooling
is generally required.NaKSbCsis a common multialkali.CsTe,CsI: These materials are sensitive to photons in ultraviolet region only
and are therefore not very widely used.A.2 ElectronFocusingStructure..................
Since photocathodes have low quantum efficiencies therefore each electron produced
is important and should be collected by the first dynode. This requires the use of an