Photocathodes can be configured to operate in either a reflection mode or a
transmission mode. Areflection mode photocathodeusually consists of a com-
pound semiconductor formed on a metal plate so that the photoelectrons travel in
the opposite direction as the incident light. In thetransmission mode photocathode
a compound semiconductor is deposited as a thinfilm on an optically transparent
glass plate. In this case the photoelectrons travel in the same direction as the
incident light.
During the PMT operation,first photons enter the PMT through a window or
faceplate in the vacuum tube and hit the photocathode. This process excites elec-
trons in the photocathode material thereby causing the emission of photoelectrons
into the vacuum. The photoelectrons then are accelerated and directed to thefirst
dynode by means of a focusing electrode that follows the photocathode. Upon
hitting thefirst dynode the photoelectrons are multiplied by means of secondary
emission. This secondary emission process then is repeated at each successive
dynode to generate additional electrons. At the end of the PMT an anode collects
and measures the secondary electrons emitted by the last dynode. This cascading
effect creates from 10^5 to 10^7 electrons for each photon hitting thefirst cathode. If a
PMT contains n dynode stages and each dynode has a secondary electron multi-
plication factor ofδ, then the gain of the PMT is given by
G¼dn ð 5 : 19 ÞNumerous PMT designs are available for a wide range of applications. The PMT
operation depends on the entrance window and photocathode materials, the number
of dynodes, the electrode configurations and their arrangement, and the voltages
applied to the electrodes. In general a photocathode is made from compound
semiconductors, which are materials made from two or more elements from dif-
ferent groups of the periodic table. Examples of popular materials for reflection
mode PMT photocathodes are the following:
- Cesium iodide (Cs-I) with a MgF 2 window for operation in the 115–200 nm
range - Bialkali (antimony-rubidium-cesium or Sb-Ru-Cs and antimony-potassium-
cesium or Sb-K-Cs) with a quartz or UV-transmitting glass window for the
185 – 750 nm spectral range - Multialkali (Na-K-Sb-Cs) with a UV-transmitting glass window for the
185 – 900 nm spectral range - InGaAsP-Cs (InGaAsP activated with cesium) for the 950–1400 nm spectral
range. - InGaAs-Cs (InGaAs activated with cesium) for the 950–1700 nm spectral
range.
5.5 Photomultiplier Tubes 137