The structure, working principle, and similarities and differences with photomultiplier tubes

1、 Introduction



Photomultiplier Tube (PMT) and Phototube are two photoelectric converter devices based on the photoelectric effect, which have wide applications in fields such as photodetection, spectral analysis, and image processing. This article will provide a detailed introduction to the structure and working principle of photomultiplier tubes, and compare their similarities and differences with phototubes, in order to better understand and apply these two devices.
2、 The structure of photomultiplier tubes
The photomultiplier tube is mainly composed of a light incidence window, a photocathode, an electron optical input system, a secondary emission multiplication system, and an anode. Among them, the light incidence window is the entrance of light, allowing photons to enter the interior of the device; A photocathode is the emission source of photoelectrons, and when photons are irradiated on its surface, they can excite photoelectrons; The electronic optical input system is responsible for focusing and guiding photoelectrons to the first doubling pole; The secondary emission doubling system consists of multiple doubling electrodes, each of which can generate more secondary electrons than the number of incident electrons, thereby achieving the doubling of photoelectrons; The anode is responsible for collecting the doubled secondary electrons and outputting the corresponding electrical signal.

3、 The working principle of photomultiplier tubes

The working principle of photomultiplier tubes is based on the photoelectric effect and secondary electron emission effect. When photons are irradiated onto the photocathode through the light incidence window, the electrons of the photocathode are excited by the photons and leave the surface to emit into the vacuum. These photoelectrons are accelerated and focused on the first doubling pole under the action of an electric field. At the first multiplier, photoelectrons collide with the multiplier material and excite more secondary electrons. These secondary electrons continue to be accelerated and focused on the next doubling pole, producing more secondary electrons again. After multiple levels of multiplication, the number of photoelectrons has been greatly amplified. Finally, the doubled secondary electrons are collected by the anode and form an anode photocurrent, which in turn generates a signal voltage on the load.

4、 The structure and working principle of photoelectric tubes

Phototubes are mainly composed of semiconductor materials, PN junctions, electrodes, windows, and packaging. Its working principle is based on the photoelectric effect, which is the process in which photons interact with matter to release electrons from atoms, forming free electrons and holes. When light shines on the window of the photoelectric tube, photons enter the semiconductor material and are absorbed. If the energy of photons is greater than the bandgap width of the material, electrons can obtain sufficient energy to transition from the valence band to the conduction band, forming free electron and hole pairs. In the depletion region of the PN junction, due to the presence of an built-in electric field, free electrons are pulled towards the N-type region and holes are pulled towards the P-type region, achieving effective separation of charge carriers. The separated electrons and holes move in opposite directions to form photocurrent, which is amplified and processed by external circuits and ultimately converted into electrical signals for output.


5、 Differences and similarities between photomultiplier tubes and phototubes

Structural similarities and differences

The main difference in structure between photomultiplier tubes and phototubes is the setting of the multiplier electrode. A photomultiplier tube is equipped with multiple electrodes (called a dynode) between the photocathode and anode, which gradually increase the potential and generate secondary electrons. These dynodes play a crucial role in the doubling process of photoelectrons. However, phototubes do not have such a multiplier structure, and the generation of photocurrent mainly depends on the photoelectric effect and the carrier separation effect of the PN junction.

Differences and similarities in working principles

The main difference between photomultiplier tubes and phototubes in terms of working principle lies in the multiplication process of photoelectrons. The photomultiplier tube achieves maximum amplification of photoelectrons through the action of multi-level multiplier electrodes, resulting in higher sensitivity than the photoelectric tube. The photoelectric tube, on the other hand, mainly relies on the photoelectric effect and the carrier separation effect of the PN junction to generate photocurrent, and its sensitivity is relatively low.

Application similarities and differences

Due to its higher sensitivity and faster response speed, photomultiplier tubes are more widely used in situations that require high-precision measurement and fast response. For example, photomultiplier tubes have important applications in fields such as astronomical observation, high-energy physics experiments, and medical imaging. Phototubes are mainly used in situations where sensitivity is not high, such as photoelectric switches, light intensity measurements, etc.



6、 Conclusion

In summary, photomultiplier tubes and phototubes are two types of photoelectric converter devices based on the photoelectric effect. They have certain differences in structure and working principles. The photomultiplier tube achieves maximum amplification of photoelectrons through the action of multi-level multiplier electrodes, with higher sensitivity and faster response speed. Therefore, it is more widely used in situations that require high-precision measurement and fast response. Phototubes, on the other hand, mainly rely on the photoelectric effect and the carrier separation effect of the PN junction to generate photocurrent. Their sensitivity is relatively low but their cost is low, and they are suitable for some situations where sensitivity is not high. In practical applications, appropriate devices should be selected based on specific needs to achieve optimal performance and cost-effectiveness.