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Electron Multipliers Information

Figure 1: Secondary electron multiplier, venetian blind type. Source: Michael Schmid/CC BY-SA 4.0

Electron multipliers work just as their name suggests. A single electron or other charged particle enters into the multiplier and then a mix of physics and electromagnetism is used to induce more charged particles to move. Each stage of an electron multiplier can generate an additional one to three electrons, resulting in incredible amplification after just a few stages.

Theory of Operation

An electron multiplier is a device used in many scientific instruments to detect and amplify signals. It operates on the principle of secondary emission, where the impact of an energetic particle (in this case, an electron) causes the emission of multiple secondary electrons from a surface. Electron multipliers typically work well with any type of charged particle.

Figure 2: A simplified diagram of an electron multiplier. Source: Time3000/CC BY-SA 3.0

Here's a simplified explanation of how electron multipliers work:

Incident Electron

The process begins with an incident electron, which can be a signal from a scientific instrument such as a mass spectrometer or a particle detector. The incident electron is what starts the entire process.

First Dynode

The incident electron strikes the first dynode (a type of electrode). The dynode is made of a material that emits secondary electrons when struck by an energetic electron. The number of secondary electrons is typically more than one, which is where the amplification begins. Each strike typically generates one to three more electrons.

Secondary Emission and Amplification

The secondary electrons are then accelerated toward the next dynode, where they each cause the emission of more secondary electrons. This process is repeated across multiple dynodes, each time increasing the number of electrons.

Signal Collection

Finally, the electrons are collected at the anode, where they create a current that can be measured. The current is proportional to the number of electrons, and thus to the original signal, but greatly amplified.

The exact number of secondary electrons emitted per incident electron depends on the material of the dynodes and the energy of the incident electron. However, the amplification can be very large, often on the order of 10^6 or more.

 

Figure 3: The Continuous Electron Multiplier. Source: Egmason/CC BY-SA 3.0

Specifications

The specifications for electron multipliers can vary widely depending on the specific application and design of the device. However, here are some common specifications that you might find:

Gain

This is the amplification factor of the electron multiplier. It's the ratio of the output signal to the input signal. High-gain electron multipliers can amplify signals by a factor of 10^6 or more.

Dynode Stages

The number of dynode stages can affect the gain and the linearity of the electron multiplier. More stages can lead to higher gain but may also increase the size and complexity of the device.

Dark Current

This is the current that flows through the electron multiplier when there is no input signal. It's essentially the noise level of the device. Lower dark current is generally better.

Lifetime

The lifetime of an electron multiplier is how long it can operate before the gain decreases to a certain level (often half the initial gain). This can be affected by factors such as the material of the dynodes and the operating conditions.

Voltage

The operating voltage of the electron multiplier. Higher voltages can lead to higher gain but may also increase the risk of damage to the device.

Size and Shape

The physical dimensions of the electron multiplier can be important, especially for applications where space is limited. The shape of the electron multiplier (e.g., linear, circular) can also affect its performance.

Response Time

This is the time it takes for the electron multiplier to respond to an input signal. Faster response times can be important for applications where the signal changes rapidly.

Operating Temperature

The temperature range in which the electron multiplier can operate effectively.

Types

There are several types of electron multipliers, each with its own unique design and application. Here are a few of the most common types:

Continuous Dynode Electron Multiplier (Channel Electron Multiplier, CEM)

This type of electron multiplier is made from a single piece of glass or ceramic that is shaped into a funnel or a curved channel. The inside of the channel is coated with a material that emits secondary electrons. As an electron travels down the channel, it causes a cascade of secondary electrons, amplifying the signal. CEMs are often used in mass spectrometry and other applications where high gain and low noise are required.

Figure 4: Continuous dynode electron multiplier detector. Source: Kkmurray/CC BY-SA 3.0

Discrete Dynode Electron Multiplier

This type of electron multiplier has a series of separate dynodes, each of which emits secondary electrons when struck by an electron. The dynodes are typically arranged in a linear or circular pattern. Discrete dynode electron multipliers can provide very high gain and are often used in photomultiplier tubes and other applications where extreme sensitivity is required.

Figure 5: A diagram of discrete and continuous electron multiplier. Source: Nikob7/CC BY-SA 4.0

Microchannel Plate (MCP) Electron Multiplier

This type of electron multiplier consists of a plate with millions of tiny parallel channels. Each channel acts like a mini continuous dynode electron multiplier. MCPs can provide high gain and fast response times, and they are often used in imaging applications, such as night vision devices and certain types of telescopes.

Photomultiplier Tubes (PMTs)

While technically a device that uses discrete dynode electron multipliers, PMTs deserve a separate mention due to their widespread use. They are extremely sensitive detectors of light in the ultraviolet, visible, and near-infrared ranges. The light photons enter the PMT and strike the photocathode, which emits electrons that are then multiplied by the dynodes.

Figure 6: Schematic view of a photomultiplier coupled to a scintillator, illustrating detection of gamma rays. Source: Qwerty123uiop/CC BY-SA 3.0

Features

Electron multipliers have several key features that make them useful in a variety of scientific and technical applications. Here are some of the most important features:

High Gain

Electron multipliers can amplify signals by a factor of 10^6 or more. This makes them ideal for detecting weak signals that would otherwise be difficult to measure.

Fast Response Time

Electron multipliers can respond to changes in the input signal very quickly, often in the nanosecond range. This makes them useful for applications where the signal changes rapidly.

Low Noise

The process of secondary electron emission used in electron multipliers is inherently low noise, especially in continuous dynode electron multipliers. This means that the signal can be amplified without adding a lot of extra noise.

Wide Dynamic Range

Electron multipliers can handle a wide range of input signal levels. This makes them versatile and suitable for a variety of applications.

Compact Size

Many types of electron multipliers, especially continuous dynode and microchannel plate types, are relatively small and lightweight. This makes them suitable for applications where space is limited.

Durability

Electron multipliers are typically made of robust materials and can withstand harsh operating conditions. This makes them suitable for use in a variety of environments, from laboratory instruments to space probes.

Versatility

Electron multipliers can be used to detect a variety of signals, from light photons to charged particles. This makes them useful in a wide range of scientific and technical fields, including physics, chemistry, astronomy, and engineering.

Sensitivity

Electron multipliers, especially when used in devices like photomultiplier tubes, can detect very low levels of light, making them extremely sensitive detectors.

Manufacture

The manufacturing process of electron multipliers can vary depending on the type of multiplier (e.g., continuous dynode, discrete dynode, or microchannel plate) and the specific design requirements. However, here's a general overview of the process:

Material Selection

The first step is to select the appropriate materials. The dynodes (or channels, in the case of a continuous dynode or microchannel plate multiplier) are typically made from a material that emits secondary electrons when struck by an energetic electron. The material must also have sufficiently high resistance. This can be a type of glass or ceramic or a metal such as copper or nickel.

Shaping

The material is then shaped into the desired form. A discrete dynode multiplier involves forming individual dynodes and arranging them in a linear or circular pattern. For a continuous dynode or microchannel plate multiplier, this involves forming a single piece of material into a funnel or channel shape, or a plate with many tiny parallel channels.

Coating

The dynodes or channels are then coated with a secondary electron emissive material. This is often a type of semiconductor material, such as cesium or beryllium oxide. The coating process can involve techniques such as sputtering or chemical vapor deposition.

Assembly

The dynodes or channels are then assembled into the final electron multiplier device. This involves placing them in a vacuum tube and attaching the necessary electrical connections. In the case of a discrete dynode multiplier, the dynodes are typically connected in a series with resistors.

Testing

The final step is to test the electron multiplier to ensure it meets the required specifications. This involves applying a voltage to the multiplier and measuring the gain, noise level, and other performance characteristics.

Sealing

Once the electron multiplier passes the tests, it is sealed to maintain the vacuum and protect the internal components.

This is a simplified overview of the process, and the exact steps can vary depending on the specific design and manufacturing techniques. It's also worth noting that manufacturing electron multipliers requires a high degree of precision and quality control, as even small defects can significantly affect the performance of the device.

Applications

Electron multipliers are used in various applications due to their ability to detect and amplify weak signals. Here are some of the most common applications:

Mass Spectrometry

In mass spectrometry, electron multipliers are used to detect and amplify the signals from ions that have been separated by their mass-to-charge ratio. This allows for the identification and quantification of the components of a sample

Figure 7: Mass spectrometry. Source: Marsupium Photography/ CC BY-SA 2.0

Particle Detectors

Electron multipliers are used in various types of particle detectors, such as those used in nuclear and particle physics. They can detect and amplify the signals from charged particles, allowing for the detection of even very rare events.

Telescopes and Astronomy

Electron multipliers, particularly microchannel plate (MCP) types, are used in some types of telescopes to detect and amplify the light signals from distant stars and galaxies. They can also be used in spectrometers to analyze the light from these objects.

Night Vision Devices

MCP electron multipliers are used in night vision devices to amplify the weak light signals that are available at night. This allows the creation of a visible image even in near-total darkness.

Figure 8: A Royal Air Force Loadmaster makes full use of his night vision goggles as his Puma helicopter lifts off from Baghdad, Iraq. Source: Cpl Ralph Merry RAF/MOD/Open Government License Version 1.0

Photomultiplier Tubes

These devices, which use discrete dynode electron multipliers, are extremely sensitive detectors of light in the ultraviolet, visible, and near-infrared ranges. They are used in a wide range of applications, from medical imaging to particle physics.

Scanning Electron Microscopy (SEM)

In SEM, an electron multiplier can be used to detect secondary electrons that are emitted from a sample when it is scanned by a focused beam of electrons. This allows for the creation of high-resolution images of the sample surface.

Figure 9: Laboratory of electron microscopy (SEM) at J. Heyrovsky Institute of Physical Chemistry of the CAoS. Source: Tadeáš Bednarz/CC BY-SA 4.0

Space Probes and Satellites

Electron multipliers are used in instruments on space probes and satellites to detect and measure particles and radiation in space. They are chosen for their sensitivity, durability, and ability to operate in the harsh conditions of space.

References

ChemEurope.com—Electron Multiplier

Restek—Electron Multipliers for Mass Spectrometry

Mass Spec Pro—Electron Multiplier

ETP Ion Detect—How a Multiplier Works

Shimadzu—Structure of Electron Multiplier (Discrete-Dynode Type)

Matsusada Precision—Electron Multiplier (EM)

Related Information

Electronics360—Addressing Phase Noise Challenges in Radar and Communication Systems


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