Photomultiplier tube

A photomultiplier tube is a vacuum electronic device that converts weak light signals into electrical signals. Photomultiplier tubes are used in optical measurement instruments and spectral analysis instruments. It can measure extremely weak radiation power at wavelengths of 200-1200 nanometers in low-energy photometry and spectroscopy. The emergence of scintillation counters has expanded the application range of photomultiplier tubes. The development of laser detection instruments is closely related to the use of photomultiplier tubes as effective receivers. The emission and image transmission of television and movies also rely on photomultiplier tubes. Photomultiplier tubes are widely used in fields such as metallurgy, electronics, machinery, chemical engineering, geology, medicine, nuclear industry, astronomy, and space research. (Encyclopedia of China: Electronics and Computers)


principle

The photomultiplier tube is based on the theory of external photoelectric effect, secondary electron emission, and electron optics, combined with characteristics such as high gain, low noise, high frequency response, and large signal receiving area. It is a photosensitive vacuum device with extremely high sensitivity and ultra fast time response, which can work in the spectral region of ultraviolet, visible, and near-infrared. The day blind ultraviolet photomultiplier tube is insensitive to visible light, near ultraviolet and other spectral radiation outside the day blind ultraviolet region, and has the characteristics of low noise (dark current less than 1nA), fast response, and large receiving area.

process

When light shines on the photocathode, it excites photoelectrons into the vacuum. These photoelectrons enter the doubling system according to the focusing electrode electric field and are amplified through further secondary emission. Then collect the amplified electrons using an anode as a signal output. Due to the use of a secondary emission multiplication system, photomultiplier tubes have extremely high sensitivity and low noise in photodetectors that detect radiation energy in the ultraviolet, visible, and near-infrared regions. In addition, photomultiplier tubes also have advantages such as fast response, low cost, and large cathode area.
Electronic vacuum devices based on external photoelectric effect and secondary electron emission effect. It utilizes secondary electron emission to double the escaped photoelectrons, achieving a sensitivity far higher than that of a photoelectric tube, and can measure weak light signals. The photomultiplier tube consists of two parts: a cathode chamber and a secondary emission multiplication system composed of several dynodes (see figure). The structure of the cathode chamber is related to the size and shape of the photocathode K, and its function is to focus the electrons generated by the external photoelectric effect (see photoelectric sensor) of the cathode under illumination on the surface of the first dynode D1, which has a smaller area than the photocathode. The secondary emission multiplication system is the most complex part. Dana electrodes are mainly made of materials that have high sensitivity and secondary emission coefficient at lower incident electron energy. Commonly used electrode materials for tanning include cesium antimonide, oxidized silver magnesium alloy, and oxidized copper beryllium alloy. The shape of the electrode should facilitate the collection of electrons emitted from the previous stage to the next electrode. A gradually increasing positive voltage is applied to each of the anode A, D1, D2, D3, etc., and the voltage difference between adjacent poles should ensure that the secondary emission coefficient is greater than 1. In this way, the electrons emitted by the photocathode are directed towards the Dana electrode D1 at high speed under the action of the D1 electric field, generating more secondary emission electrons, which then fly towards D2 under the action of the D2 electric field. Continuing like this, each photoelectron will excite a exponentially increasing number of secondary emission electrons, which will eventually be collected by the anode. There are two types of electron multiplication systems: focused and unfocused. The focused dynode doubles the electrons from the previous stage and focuses them on the next stage, which may result in the crossing of electron beam trajectories between the two poles. Non focusing types are further divided into circular tile type (i.e. squirrel cage type), linear tile type, box grid type, and louver type.


A photomultiplier tube is a device made based on the principles of photoelectron emission, secondary electron emission, and electron optics, with special electrodes inside a transparent vacuum shell. The photocathode emits electrons under the action of photons, which are accelerated by an external electric field (or magnetic field) and focused on the first pole. These electrons that impact the secondary pole can cause the secondary pole to release more electrons, which are then focused on the second pole. In this way, after more than ten times of multiplication, the magnification can reach 108-1010. Finally, amplified photocurrent was collected at the high potential anode. The output current is proportional to the number of incident photons. The entire process takes about 10-8 seconds. There is also a compact multiplier tube that utilizes the secondary electron emission inside the bent lead glass tube itself. The photomultiplier tube will also output a weak current under all dark conditions when working voltage is applied, which is called dark current. It mainly comes from cathode thermionic emission. Photomultiplier tubes have two drawbacks: ① Sensitivity decreases due to strong light irradiation or prolonged irradiation time, and partially recovers after stopping irradiation, which is called "fatigue"; ② The sensitivity of each point on the surface of the photocathode is uneven.

Segment doubling method

The photomultiplier tube has two types of multiplication methods: dynode and MCP.

Dana pole type

The dynode type photomultiplier tube consists of a photocathode, a multiplier stage, and an anode, which are encapsulated in glass and have a high vacuum inside. The multiplier stage is composed of a series of multiplier electrodes, each of which operates at a higher voltage in the preceding stage. There are two types of receiving light for the dynode photomultiplier tube: end window and side window.
The working principle of a dynode type photomultiplier tube: Photons collide with the photocathode material, overcoming the work function of the photocathode and generating photoelectrons. After being accelerated and focused by an electric field, they collide with the first stage photomultiplier tube with higher energy, emitting more low-energy electrons. These electrons are sequentially accelerated and collided with the lower level photomultiplier electrode, resulting in a series of geometric level multiplications. Finally, the electrons reach the anode, and the sharp current pulse formed by the accumulation of charges can characterize the input photons.
MCP type

MCP type photomultiplier tubes are all end window photomultiplier tubes, suitable for applications with large illuminated areas. The typical composition of MCP photomultiplier tubes includes an input window, a photocathode, an electron multiplier, and an electron collector (anode).

Operating characteristics

1. Stability

The stability of photomultiplier tubes is determined by various factors such as the characteristics of the device itself, operating conditions, and environmental conditions. There are many situations where the output of pipes is unstable during the working process, mainly including:
a. The unstable jumping phenomenon caused by poor electrode welding, loose structure, poor contact of cathode shrapnel, discharge at the tip of the electrode, and jumping fire inside the pipe, resulting in fluctuating signals.
b. The instability of continuity and fatigue caused by excessive anode output current.
c. The impact of environmental conditions on stability. As the ambient temperature increases, the sensitivity of the tube decreases.
d. Wet environments cause leakage between pins, leading to an increase in dark current and instability.
e. Environmental electromagnetic interference causes unstable operation.
2. Maximum operating voltage
The maximum working voltage refers to the upper limit of the voltage allowed to be applied by the tube. Above this voltage, the tube may experience discharge or even breakdown. [1]
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Due to its high gain and short response time, as well as its output current being proportional to the number of incident photons, photomultiplier tubes are widely used in astrophotometry and astrophotometry measurements. Its advantages are: high measurement accuracy, the ability to measure relatively faint celestial bodies, and the ability to measure rapid changes in celestial luminosity. In astronomical photometry, the most commonly used is the multiplier tube of antimony cesium photocathode, such as RCA1P21. The maximum quantum efficiency of this photomultiplier tube is around 4200 angstroms, which is about 20%. There is also a photomultiplier tube with a dual alkali photocathode, such as GDB-53. Its signal-to-noise ratio is one order of magnitude higher than RCA1P21, and the undercurrent is very low. To observe the near-infrared region, photomultiplier tubes with multi alkali photocathodes and gallium arsenide cathodes are commonly used, with the latter having a maximum quantum efficiency of up to 50%.
Ordinary photomultiplier tubes can only measure one information at a time, that is, the number of channels is 1. Matrix. Due to the limitation of the number of channels by the thin metal wire at the anode end, only hundreds of channels can be achieved.

component

The photomultiplier tube can be divided into four main parts, namely: photocathode, electron optical input system, electron multiplication system, and anode.

advantage

Electric multiplier tube is a photoelectric converter device that further improves the sensitivity of photoelectric tubes. In addition to the photocathode and anode, multiple tile shaped multiplier electrodes are also placed between the two electrodes inside the tube. When in use, voltage is applied between adjacent doubling electrodes to accelerate electrons. After being illuminated by light, the photocathode releases photoelectrons, which are directed towards the first doubling electrode under the action of an electric field, causing secondary emission of electrons and exciting more electrons. Then, under the action of an electric field, they fly towards the next doubling electrode and exciting more electrons. By continuously doubling the number of electrons, the number of electrons collected by the anode can increase by 10 ^ 4~10 ^ 8 times, making the sensitivity of photomultiplier tubes much higher than that of ordinary photomultiplier tubes, which can be used to detect weak light signals. The high sensitivity and low noise characteristics of photomultiplier tubes make them widely used in optical measurement.


size

The photomultiplier tube has different sizes according to different applications, and currently has the largest amount of light in the world
20 inch photomultiplier tube
20 inch photomultiplier tube
The photomultiplier tube is a 20 inch tube developed and produced by Hamamatsu, Japan. It was originally used in Masayoshi Kobayashi's Super Kamioka detector and contained 11200 particles, ultimately detecting neutrinos in the universe. Masayoshi Kobayashi was awarded the Nobel Prize in Physics in 2002, and the 20 inch photomultiplier tube was awarded the "IEEE milestone" in 2014.