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Photomultiplier Tube (PMT)

Introduction

Photomultiplier tubes (PMTs) convert photons to an electrical signal. They have a high internal gain and are sensitive detectors for low-intensity applications such as fluorescence spectroscopy.


Design

A PMT consists of a photocathode and a series of dynodes in an evacuated glass enclosure. When a photon of sufficient energy strikes the photocathode, it ejects a photoelectron due to the photoelectric effect. The photocathode material is usually a mixture of alkali metals, which make the PMT sensitive to photons throughout the visible region of the electromagnetic spectrum. The photocathode is at a high negative voltage, typically -500 to -1500 volts. The photoelectron is accelerated towards a series of additional electrodes called dynodes. These electrodes are each maintained at successively less negative potentials. Additional electrons are generated at each dynode. This cascading effect creates 105 to 107 electrons for each photoelectron that is ejected from the photocathode. The amplification depends on the number of dynodes and the accelerating voltage. This amplified electrical signal is collected at an anode at ground potential, which can be measured.

Phototubes are similar to PMTs, but consist of only a photocathode and anode. Since phototubes do not have a dynode chain to provide internal amplification, they are used in less sensitive applications such as absorption spectrometers.

Schematic of a PMT


Typical specifications

Wavelength range:1 110-1100 nm
Quantum efficiency (Q.E.):2 1-10%
Response time: 1-20 ns
1 Wavelength sensitivity depends on wavelength. UV-sensitive PMTs must have UV-transmitting windows, see optical materials.
2 The Q.E. is the (number of electrons ejected by the photocathode / number of incident photons).

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