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What are "avalanche multiplication diodes," and how are they employed in photomultiplier tubes?

1 Answer

Avalanche multiplication diodes, also known as avalanche photodiodes (APDs), are semiconductor devices that can generate a much higher number of electron-hole pairs in response to a single photon compared to conventional photodiodes. This phenomenon is called avalanche multiplication.

In a regular photodiode, incident photons create electron-hole pairs within the semiconductor material, and these pairs are collected as photocurrent. However, in an avalanche photodiode, there is an additional region with a high electric field, known as the avalanche region or the multiplication region. When a photon generates an electron-hole pair in this region, the electric field accelerates the electrons, causing them to gain enough energy to create additional electron-hole pairs through impact ionization. These newly created carriers also gain energy and create more carriers through impact ionization, leading to an avalanche of carriers and a significant amplification of the initial photocurrent.

Avalanche photodiodes offer some advantages over conventional photodiodes:

High sensitivity: The avalanche multiplication process results in higher sensitivity to low light levels, making them more suitable for applications in low-light conditions.

Internal gain: APDs provide internal gain, reducing the need for external amplifiers, and simplifying the signal detection process.

Fast response: Avalanche photodiodes typically have faster response times, enabling them to detect fast-changing light signals.

Now, let's talk about how avalanche multiplication diodes are employed in photomultiplier tubes (PMTs):

Photomultiplier tubes (PMTs) are highly sensitive light detectors used in various applications, such as particle physics experiments, nuclear medicine, astronomy, and more. They are designed to detect extremely low levels of light and convert them into measurable electrical signals. PMTs consist of several stages, and one of the critical stages is the photocathode followed by a series of dynodes.

Photocathode: The process starts with the photocathode, a photosensitive material that emits electrons when photons strike it. These emitted electrons are typically in the order of a few electrons per incident photon.

Dynodes: The electrons generated by the photocathode are then accelerated and focused towards a series of metal dynodes through an electric field. Each dynode has a higher electric potential than the previous one. As the electrons hit the first dynode, they can dislodge multiple electrons from its surface, thanks to a phenomenon called secondary emission.

Avalanche multiplication diode (APD): To enhance the overall sensitivity and efficiency of PMTs, some designs incorporate an avalanche multiplication diode as the final stage. This APD acts as an electron multiplier, similar to standalone avalanche photodiodes. When the electrons from the previous dynodes strike the APD, they cause avalanche multiplication, resulting in a significant increase in the number of electrons, effectively amplifying the signal further.

By incorporating avalanche multiplication diodes into PMTs, the devices can achieve even higher gains and increased sensitivity, making them suitable for detecting extremely faint light signals that would be challenging to detect with other types of photodetectors.
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