The photomultiplier tube converts weak light signals into electrical signalsVacuum electronic device。Photomultiplier tubes are used in optical measuring instruments andSpectrum analyzerIn.It can measure extremely weak radiation power with wavelength of 200~1200 nm in low-energy photometry and spectroscopy.Scintillation counterThe appearance of photomultiplier tube has expanded the application range of photomultiplier tube.The development of laser detection instruments is closely related to the use of photomultiplier tubes as effective receivers.The transmission and image transmission of television and film can not be separated from photomultiplier tubes.Photomultiplier tubes are widely used in metallurgy, electronics, machinery, chemical industry, geology, medical treatment, nuclear industry, astronomy, space research and other fields.Leonid Kubersky invented the world's first photomultiplier tube.[2]
The photomultiplier tube is built onExternal photoelectric effectBased on the secondary electron emission and electron optics theory, combined with the characteristics of high gain, low noise, high frequency response and large signal receiving area, it is a kind of photosensitive electric vacuum device with high sensitivity and ultrafast time response, which can work in the spectral region of ultraviolet, visible and near-infrared regions.The solar blind UV photomultiplier is insensitive to spectral radiation such as visible light and near UV outside the solar blind UV region, and has the characteristics of low noise (dark current less than 1nA), fast response, and large receiving area[1]。
process
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When the light shines on the photocathode, the photocathode is excited to vacuumphotoelectron。These photoelectrons enter the multiplier system according to the electric field of the focusing electrode, and are amplified by further secondary emission.Then the amplified electrons are collected with anode as signal output.Because of the use of a secondary emission multiplication system, the photomultiplier tube can detectRadiant energyOfPhotodetectorMedium, high sensitivity and low noise.In addition, the photomultiplier tube has the advantages of fast response, low cost, large cathode area, etc.
Based on external photoelectric effect andSecondary electronElectron vacuum device with emission effect.It uses secondary electron emission to multiply the escaped photoelectrons, obtaining much higher thanPhotocellIt can measure weak light signal.The photomultiplier tube consists of a cathode chamber and a secondary emission multiplication system composed of a number of electrodes (see figure).The structure of the cathode chamber is related to the size and shape of the photocathode K, which is used to make the cathode photoelectric effect (seePhotoelectric sensor）The generated electrons are focused on the surface of the first dynode D1 with an area smaller than that of the photocathode.The secondary emission multiplication system is the most complex part.The dynode is mainly made of materials that can have high sensitivity and secondary emission coefficient at low incident electron energy.Common tap electrode materials include cesium antimonide, oxidized silver magnesium alloy and oxidized copper beryllium alloy.The shape of the dynode should be conducive to collecting the electrons emitted from the previous stage to the next pole.Gradually increasingPositive voltageAnd the voltage difference between two adjacent poles should make the secondary emission coefficient greater than 1.In this way, the electrons emitted by the photocathode shoot at a high speed to the dynode D1 under the effect of the D1 electric field, producing more secondary emitted electrons, and then these electrons fly to D2 under the effect of the D2 electric field.If this continues, each photoelectron will excite a multiplied number of secondary emission electrons, which will be finally collected by the anode.There are two types of electron multiplication systems: focused and unfocused.Focused electrodes focus electrons from the previous stage to the next stage after multiplication, and the cross of electron beam tracks may occur between the two poles.The unfocused type can be divided into circular tile type (i.e. squirrel cage type), linear tile type, box grid type and shutter type.
Photomultiplier tube is based on photoelectron emission, secondary electron emission andElectron opticsThe transparent vacuum shell is made of the principle of, and is equipped with a special electrode device.PhotocathodephotonUnder the action, electrons are emitted, which are accelerated by the external electric field (or magnetic field) and focused on the first pole.These electrons that impact the secondary pole can make the secondary pole release more electrons, and they are focused on the secondary pole again.In this way, the magnification can reach 108~1010 after more than ten times of multiplication.Finally, the amplified photocurrent is collected at the high potential anode.The output current is proportional to the number of incident photons.The whole process takes about 10-8 seconds.Another kind uses the secondary electron emission inside the curved lead glass tube itself to form a small multiplier tube.The photomultiplier tube will also output weak current, called dark current, when the operating voltage is applied under the full dark condition.It mainly comes from cathodeThermionic emission。The photomultiplier tube has two disadvantages: ① the sensitivity decreases due to strong light irradiation or too long irradiation time, and then partially recovers after stopping irradiation, which is called "fatigue"; ②The sensitivity of each point on the photocathode surface is uneven.
Multiplication mode
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There are two modes of photomultiplier tube multiplication, namely, dynode and MCP.
Dana polar type
The dynode photomultiplier tube is composed ofPhotocathode、MultiplierIt is composed of a series of multipliers, each of which operates at a higher voltage of the previous stage.There are two kinds of light receiving modes of the dynode photomultiplier: end window and side window.
Working principle of the dynode photomultiplier tube: photons hit the photocathode material, overcome the work function of the photocathode, and produce photoelectrons. After being accelerated and focused by the electric field, they hit the first stage photomultiplier tube with higher energy, and emit more low-energy electrons. These electrons are accelerated and hit the lower level photomultiplier in turn, resulting in a series of geometric level multipliers,Finally, the electrons arrive at the anode, and the sharp current pulse formed by the accumulated charge can characterize the input photons.
MCP type
MCP photomultiplier tubes are all end window photomultiplier tubes, which are suitable for applications with large exposed areas.The typical MCP photomultiplier tube consists of an input window, a photocathode, an electron multiplier and an electron collector (anode).
Operating characteristics
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1. Stability
The stability of photomultiplier tubes is determined by many factors, such as device characteristics, working conditions and environmental conditions.There are many cases of unstable output of pipes during operation, mainly including:
a. Jumping instability caused by poor welding of electrodes in the pipe, loose structure, poor contact of cathode shrapnel, discharge at the tip between electrodes, fire jump, etc., and the signal changes from big to small.
b. The instability of continuity and fatigue caused by too large anode output current.
c. Influence of environmental conditions on stability.When the ambient temperature rises, the sensitivity of the pipe decreases.
d. Leakage between pins caused by wet environmentdark currentEnlargement and instability.
e. Environmental electromagnetic field interference causes unstable operation.
2. Limiting operating voltage
The limiting operating voltage refers to the upper limit of the voltage allowed to be applied by the pipe.Above this voltage, the tube will discharge or even break down.
application
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As the photomultiplier tube has high gain and short response time, and its output current is proportional to the number of incident photons, it is widely used inAstrophotometryandCelestial spectrophotometryMedium.Its advantages are: high measurement accuracy, can measure relatively dim celestial bodies, but also can measure the rapid changes in celestial luminosity.In astronomical photometry, the multiplier tube of antimony cesium photocathode is widely used, such as RCA1P21.The maximum quantum efficiency of this photomultiplier tube is around 4200 angstroms, about 20%.There is also a double alkali photocathode photomultiplier tube, such asGDB-53。Its signal-to-noise ratio is one order of magnitude higher than RCA1P21, and the undercurrent is very low.In order to observe the near-infrared regionMultialkali photocathodeandGallium arsenidePhotomultiplier tube with cathode, the maximum quantum efficiency of the latter can reach 50%.
Ordinary photomultiplier tubes can only measure one information at a time, that is, the number of channels is 1.matrix。As the number of channels is limited by the thin wire at the anode end, only hundreds of channels can be achieved.
other
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component
The photomultiplier tube can be divided into four main parts: photocathode, electron optical input system, electron multiplication system and anode.
advantage
The photomultiplier tube is a photoelectric converter to further improve the sensitivity of the photoelectric tube.In addition to the photocathode and anode, a plurality of tile shaped multiplier electrodes are placed between the two electrodes in the tube.When in use, a voltage is applied between two adjacent multiplier electrodes to accelerate electrons.The photocathode releases photoelectrons after being illuminated, and under the action of electric field, it shoots at the first multiplier electrode, causing the secondary emission of electrons, which excites more electrons, and then under the action of electric field, it flies to the next multiplier electrode, which excites more electrons.In this way, the number of electrons continues to multiply, and the electrons finally collected by the anode can increase by 10 ^ 4~10 ^ 8 times, which makes the sensitivity of the photomultiplier tube much higher than that of the ordinary photoelectric tube, which can be used to detect weak light signals.The characteristics of high sensitivity and low noise of photomultiplier tubes make them widely used in light measurement.
size
20 inch photomultiplier tube
The photomultiplier tube has different sizes according to different applications. At present, the largest photomultiplier tube in the world is 20 inches（hamamatsu）Development and production, initially used forMasatoshi Koshiba OfSuper Kamioka detector11200 neutrinos were installed and finally detected, so Masahiro Kochai won the award of 2002The nobel prize in physicsAs a result, the 20 inch photomultiplier tube won the "IEEE milestone" in 2014.
