Advantages of this design are lower dark current picoamp range and an increase in dynamic range. Confocal microscopes, spectrophotometers, and many high-end automatic camera exposure bodies utilize photomultipliers to gauge light intensity. Spectral sensitivity of the photomultiplier depends on the chemical composition of the photocathode with the best devices having gallium-arsenide elements, which are sensitive from to nanometers.
Photomultiplier photocathodes are not uniformly sensitive and typically the photons are spread over the entire entrance window rather than on one region. Because photomultipliers do not store charge and respond to changes in input light fluxes within a few nanoseconds, they can be used for the detection and recording of extremely fast events. Finally, the signal to noise ratio is very high in scientific grade photomultipliers because the dark current is extremely low it can be further reduced by cooling and the gain may be greater than one million.
Mortimer Abramowitz - Olympus America, Inc. Michael W. Microscopy Primer. Light and Color. Microscope Basics. Special Techniques. Digital Imaging. Confocal Microscopy. Live-Cell Imaging.
It is very extremely sensitive detectors of light in the ultraviolet, visible and near-infrared ranges of the electromagnetic spectrum. It is similar to phototubes which are consisting of the photocathode and a series of dynodes in an evacuated glass enclosure. How does a Photomultiplier tube work. When light enters the photocathode, then the photocathode emits photoelectrons into the vacuum and this emission of electrons is done by the photoelectric effect.
Then the photoelectrons are directed towards the electron multiplier by the help of electrode voltages. During the impact, each electron will liberate a number of secondary electrons which are in turn, electrostatically accelerated and focused on to the next dynode.
Due to the secondary emission multiplication, the photomultiplier tubes provide extremely high sensitivity and exceptionally low noise compared to the photosensitive devices. The photomultiplier tube features a fast time response and a choice of large photosensitive area. If light or electromagnetic waves with high frequency falls upon a metal surface, then electrons are emitted from the metal surface and this process is called the photoelectric effect and the electrons which are emitted in this manner are called photoelectrons.
Apparatus with a scintillating crystal, photomultiplier, and data acquisition components. Source: wikipedia. In general, a scintillation detector consists of: Scintillator. A scintillator generates photons in response to incident radiation. A sensitive photodetector usually a photomultiplier tube PMT , a charge-coupled device CCD camera, or a photodiode , which converts the light to an electrical signal and electronics to process this signal.
Components of Photomultiplier Tube The device consists of several components and these components are shown in the figure. Right after a thin entry window, is a photocathode, which is made of material in which the valence electrons are weakly bound and have a high cross section for converting photons to electrons via the photoelectric effect.
For example, Cs 3 Sb caesium-antimony may be used. As a result, the light created in the scintillator strikes the photocathode of a photomultiplier tube, releasing at most one photoelectron per photon.
Using a voltage potential, this group of primary electrons is electrostatically accelerated and focused so that they strike the first dynode with enough energy to release additional electrons. These electrodes are operated at ever increasing potential e. At the dynode the electrons are multiplied by secondary emission. The next dynode has a higher voltage which makes the electrons released from the first to accelerate towards it.
Typical operating voltages are in the range of to V. At the final dynode, sufficient electrons are available to produce a pulse of sufficient magnitude for further amplification. This pulse carries information about the energy of the original incident radiation.
The number of such pulses per unit time also gives information about the intensity of the radiation. Photomultiplier Tube — Principle of Operation The operation of scintillation counters and photomultiplier tubes is summarized in the following points: Scintillation Counter — Principle of Operation. KM3NeT, the neutrino telescope in the Mediterranean sea, has adopted a multi-PMT solution, where multiplexed PMTs improve angular uniformity of the response, increase photocathode coverage while keeping the time resolution excellent.
For the time being, PMTs are the only viable solution when large detection areas are required for instance for large volume neutrino detectors. On the other hand, the use of PMTs in cryogenic detectors is inconvenient and requires dedicated optimization. Moreover, some applications require reduced level of radioactivity.
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