Secondary Emission: A Phenomenon of Electron Emission

Secondary emission is a phenomenon where a material emits secondary particles, like electrons or ions, when bombarded by high-energy particles such as electrons, ions, or photons. When the emitted particles are electrons, it is called secondary electron emission.

Secondary emission plays a crucial role in devices like photomultiplier tubes, electron multipliers, image intensifiers, and radiation detectors. However, it can also cause problems in electronic devices like vacuum tubes and klystron tubes, leading to parasitic oscillation and negative resistance.

What is Secondary Electron Emission?

Secondary electron emission is defined as the emission of electrons from a material when high-energy particles hit it. The primary particles transfer their kinetic energy to surface electrons, helping them overcome the potential barrier created by the electrostatic attraction of the positive nuclei in the material.

The number of secondary electrons emitted per incident particle is called the secondary emission yield. The secondary emission yield depends on several factors, such as:

• The material and the surface properties of the emitter
• The energy and the angle of incidence of the primary particles
• The electric field and the temperature of the emitter
• The type and the charge of the primary and secondary particles.

The secondary emission yield can vary from less than one to more than ten, depending on these factors. For example, cesium oxide has a high secondary emission yield of about 10 for low-energy electrons, while metals have lower yields of less than one for high-energy electrons.

How Does Secondary Electron Emission Work?

Secondary electron emission occurs through two main mechanisms: elastic scattering and inelastic scattering.

Elastic Scattering

Elastic scattering occurs when a primary particle collides with an electron on the surface and transfers some of its kinetic energy to it without losing any energy itself. The scattered electron then escapes from the surface if its energy is greater than the potential barrier. This mechanism is more dominant for low-energy primary particles (less than 50 eV) and for materials with low work functions (the minimum energy required to remove an electron from a surface).

Inelastic Scattering

Inelastic scattering occurs when a primary particle collides with an electron inside the material and loses some of its kinetic energy to it. The excited electron then moves toward the surface and escapes if its energy is greater than the potential barrier. This mechanism is more dominant for high-energy primary particles (greater than 50 eV) and for materials with high work functions. In this case, multiple secondary electrons can be emitted from one primary particle, as the excited electron can collide with other electrons on its way to the surface and transfer some of its energy to them.

What are Some Applications of Secondary Electron Emission?

Secondary electron emission is used in various devices that rely on amplifying or detecting weak electric currents or radiation. Some examples are:

• Photomultiplier tubes: These are devices that convert light into electric current by using a photocathode that emits primary electrons when exposed to light and a series of electrodes called dynodes that emit secondary electrons when struck by primary electrons. The secondary electrons are then collected by an anode at the end of the tube. Each dynode produces multiple secondary electrons for each incident electron, resulting in an overall gain or multiplication of the current by a factor of about one million. Photomultiplier tubes are used in applications such as spectroscopy, scintillation detectors, night vision devices, and medical imaging.
• Electron multipliers: These are devices that amplify electric currents by using a series of electrodes that emit secondary electrons when struck by primary electrons or ions. The electrodes are arranged in a funnel-like shape that focuses the secondary electrons toward the next electrode. Electron multipliers are used in applications such as mass spectrometry, ion detectors, and electron microscopes.
• Image intensifiers: These are devices that produce a bright image of a faintly illuminated object by using a photocathode that emits primary electrons when exposed to light, an electron multiplier that amplifies the current of primary electrons, and a phosphor screen that converts the secondary electrons into visible light. Image intensifiers are used in applications such as night vision goggles, medical imaging, and astronomy.

What are Some Challenges of Secondary Electron Emission?

Secondary electron emission can also cause problems in some electronic devices that operate in vacuum conditions, such as vacuum tubes and klystron tubes. Some examples are:

• Parasitic oscillation: This occurs when secondary electrons emitted from an anode or a collector electrode return to a cathode or an emitter electrode due to an electric field or a magnetic field inside the device. This creates a feedback loop that causes unwanted oscillations or noise in the output signal. Parasitic oscillation can be reduced by using grids or shields between electrodes, coating electrodes with materials with low secondary emission yields, or applying negative bias voltages to electrodes.
• Negative resistance: This occurs when secondary electrons emitted from an anode or a collector electrode reduce the current flowing through it due to their opposite charge. This creates a negative slope in the voltage-current characteristic curve of the device, which can lead to instability or oscillation in some circuits. Negative resistance can be avoided by using grids or shields between electrodes, coating electrodes with materials with high secondary emission yields, or applying positive bias voltages to electrodes.

Conclusion

Secondary emission is a phenomenon that occurs when high-energy particles strike a surface or a material and induce the emission of secondary particles, such as electrons or ions.

Secondary electron emission is a type of secondary emission that involves the emission of electrons from a surface or a material due to the impact of high-energy electrons or other charged particles.

Secondary electron emission can be explained by two main mechanisms: elastic scattering and inelastic scattering.

Secondary electron emission is used in various devices that rely on amplifying or detecting weak electric currents or radiation.

Secondary electron emission can also cause problems in some electronic devices that operate in vacuum conditions.