An important character that is required for the anode is that an adequate potential difference must be established between the last dynode and the anode in order to prevent space charge effects and to obtain a large output current. What are the factors that must be considered while choosing a photomultiplier tube. There are some important factors for selecting a photomultiplier tube, they are window material, photocathode spectral response, number of dynodes and type of dynode.
A photomultiplier must be selected which can detect the incident light and also select the optimum circuit conditions that match the application. Photomultiplier tube must be selected according to the operating conditions and circuit configurations according to the incident light wavelength, intensity, beam size and the speed of optical phenomenon.
The photomultiplier tubes have high sensitivity, so there is a chance that it could detect extraneous light other than the light to be measured. This decreases the signal to noise ratio, so an external housing is needed for the external light shielding.
Photo-multiplier tube characteristics may vary with external electromagnetic fields, ambient temperature, humidity, or mechanical stress applied to the photomultiplier tube and because of this, a magnetic or electric shield is required to protect the photomultiplier tube. So all of this must be checked while selecting a photomultiplier tube.
The photomultiplier tube performed many functions — in astronomy, WWII radar jamming, medicine, and theoretical probes into the mysterious world of quantum mechanics. Its inner workings are straightforward:. The photoelectric effect, also called photoemission, arises when a body, struck by light, emits electrons. Moreover, alterations in the intensity or wavelength can result in changes in the rate of emission of the electron stream. Strangely, electrons are only emitted when the incident light exceeds a minimum frequency, regardless of the intensity or duration of the light.
Albert Einstein explained this odd phenomenon by theorizing that the beam of light really consists of discrete quantized packets of photons. This, in conjunction with groundwork previously laid by Max Planck, was the intellectual event that gave rise to quantum mechanics. This is one of the underlying principles of the photomultiplier tube, the other being secondary emission: When electrons of sufficient energy strike a surface or pass through a conductive body, secondary particles are emitted into the surrounding space.
In a photomultiplier tube, electrons are emitted from a photocathode and accelerated in a beam that strikes the dynode, which is a polished metal electrode suitably biased. When this happens, through the miracle of secondary emission, a greater number of electrons, perhaps in the ratio of five to one, get released. They strike a second dynode, where again the number of emitted electrons is multiplied by the design factor.
By stacking a chosen number of dynodes in a photomultiplier tube and connecting the output to an auto-ranging oscilloscope, it is possible to see a measurable signal based on a single photon at the input.
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. At the final dynode , sufficient electrons are available to produce a pulse of sufficient magnitude for further amplification. Quantum Efficiency The sensitivity of a photocathode is usually quoted in terms of the quantum efficiency.
The quantum efficiency of the photocathode is defined as the probability for the conversion of incident photons to an electrical signal and is defined as: The quantum efficiency of any photosensitive device is a strong function of wavelength of the incident light, and an effort is made to match the spectral response of the photocathode to the emission spectrum of the scintillator in use.
Radiation Protection: Knoll, Glenn F. ISBN Stabin, Michael G. Martin, James E. Department of Energy, Instrumantation and Control. June Nuclear and Reactor Physics: J. Lamarsh, Introduction to Nuclear Reactor Theory, 2nd ed. Lamarsh, A. Baratta, Introduction to Nuclear Engineering, 3d ed. Glasstone, Sesonske. Nuclear and Particle Physics. Physics of Nuclear Kinetics. Addison-Wesley Pub. January
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