Detailed explanation of the working principle of photomultiplier tubes

1、 Introduction

Photomultiplier Tube (PMT) is a photoelectric converter device based on the photoelectric effect and secondary electron emission effect, widely used in spectral analysis, photometry, nuclear physics, high-energy physics, astronomical observation and other fields. Its working principle involves multiple processes such as photon incidence, photoelectron emission, multiplication amplification, and signal output. This article will provide a detailed explanation of the working principle of photomultiplier tubes, including the role of external photoelectric effects, secondary electron emission, and electron optics theory.


2、 The basic structure of photomultiplier tubes

A photomultiplier tube typically consists of an incident window, a photocathode, a multiplier (including multiple multiplier electrodes), and an anode. The incident window is used to allow photons to enter the tube, while the photocathode is used to receive photons and generate photoelectrons. The multiplier electrode is composed of multiple electrodes and is used for amplifying photoelectrons. The anode is used to collect the doubled electrons and output an electrical signal.

3、 The working principle of photomultiplier tubes

Photon incidence

When photons enter the photomultiplier tube through the incident window, they interact with the material on the photocathode. This process is the starting point for the operation of photomultiplier tubes.
Optoelectron emission (external photoelectric effect)


When a photon interacts with a material on the photocathode, if the energy of the photon is greater than the escape work of the material, the photon will be absorbed and excited to produce photoelectrons. This process is called the external photoelectric effect. The number of photoelectrons is related to the energy and quantity of incident photons, as well as to the properties of the photocathode material.

In the external photoelectric effect, the energy of photons is converted into the kinetic energy and escape work of photoelectrons. The kinetic energy of photoelectrons determines their subsequent motion trajectory and velocity in the electric field.

Multiplication amplification (secondary electron emission)

Optoelectrons accelerate under the action of an electric field and collide with the material on the doubling pole. When the energy of photoelectrons is large enough, they will excite secondary electrons on the surface of the multiplier material. These secondary electrons continue to accelerate under the action of the electric field and collide with the next doubling pole, producing more secondary electrons. This process is constantly repeated, resulting in an exponential increase in the number of photoelectrons.


The efficiency of secondary electron emission is related to factors such as the properties of the doubling electrode material, electric field strength, photoelectron energy, and incident angle. In order to improve the efficiency of amplification, photomultiplier tubes usually use materials with high secondary electron emission coefficients and optimized electric field designs.

signal output 

After multiple times of amplification, the number of electrons significantly increased. These electrons eventually reach the anode and are collected. The anode is connected to the power supply to form a circuit. When electrons collide with the anode, they generate a current signal. The magnitude of this current signal is directly proportional to the number of incident photons, thus reflecting the intensity of the incident light.

In the process of signal output, it is also necessary to consider the influence of noise. The main sources of noise are thermal noise, shot noise, and dark counting. In order to reduce noise levels, photomultiplier tubes usually adopt low noise design and high-precision manufacturing processes.

4、 The role of electronic optics theory in the operation of photomultiplier tubes

Electronic optics theory is a discipline that studies the motion of electrons in electric and magnetic fields. In photomultiplier tubes, electron optics theory plays an important role in understanding the motion trajectory, velocity distribution, and multiplication process of photoelectrons.

Firstly, electronic optics theory can explain the acceleration and focusing process of photoelectrons in an electric field. By adjusting the intensity and distribution of the electric field, the trajectory and velocity distribution of photoelectrons can be controlled, thereby optimizing the efficiency of amplification.


Secondly, electronic optics theory can also explain the electron transfer process between doubling poles. Between the doubling poles, electrons need to overcome adverse factors such as space charge and scattering effects in order to successfully transfer to the next doubling pole. The theory of electronic optics can help us understand the mechanisms of these adverse factors and take corresponding measures to reduce their impact.

Finally, electronic optics theory can also guide the design and manufacturing of photomultiplier tubes. By means of theoretical calculations and simulation, the performance parameters and optimized design schemes of photomultiplier tubes can be predicted. This helps to improve the performance and reliability of photomultiplier tubes.

5、 Conclusion

Photomultiplier tube is a photoelectric converter device based on the photoelectric effect and secondary electron emission effect. Its working principle involves multiple processes such as photon incidence, photoelectron emission, multiplication amplification, and signal output. The external photoelectric effect is the foundation of the operation of photomultiplier tubes, and secondary electron emission is the key to achieving multiplication amplification. The theory of electronic optics plays an important role in the operation of photomultiplier tubes. It can help us understand the motion laws and multiplication process of photoelectrons, and guide the design and manufacturing of photomultiplier tubes. With the continuous development of technology, the performance and application fields of photomultiplier tubes will be further expanded.