Channel electron multiplier as an electron source

In summary: The setup is a commercial channeltron, so it may have a built-in resistor. The voltage must be high enough to overcome the resistor. Is it possible that the emissive surface is not located at the entrance cone?The emissive surface is located at the entrance cone.
  • #1
Christoffer B
TL;DR Summary
Channel electron multiplier as an electron source

I'm playing around with a type of electron (and by electron/molecule interactions: ion) source that was briefly touched upon in the 1960's.

The basic idea is to use a channel electron multiplier "channeltron" with the anode removed; similar to a single MCP channel. The burst of electrons out will then be observable as a pulse on the output end of the channel, similar to extracting the dynode signal from a PMT.

The initiator of the burst can be anything from a light source producing photoelectrons, to a beta source like Sr-90. I went for the former.

I have modified a commercial channeltron by removing its anode cap, and provided negative bias at the cone, but no pulses are observed.

The channel has a built-in resistor to make sure the electrons don't reach zero potential within the channel, before they hit the anode. In my setup this can be shorted if needed.

However both with Rint shorted or not, I see no output pulses, regardless of light strenght (a quartz window provides light to the cone)

Please see attached image and schematic of the setup. I am unsure if it's an electronics problem or a fundamental issue.

Pressure is ~5E-7 Torr, bias is approx. -3.5kV.

The signal has been observed both directly on oscilloscope, through a preamplifier, and through a pulse inverter and preamplifier. same disappointing result.

Thank you for the interest!



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  • #2
It looks as if the device is de-mountable, so have you the necessary high vacuum?
Also are you using very short pulses. You will need to arrange the output circuit/load resistor so high speed is obtainable.
Does the light pulse fall on a photo emitter?
  • #3
Thank you for your reply!

I should have explained, one of the flanges goes to a turbomolecular pump and ion gauge - working pressure is about 10E-6-7 mbar.

The plan is not to use pulses but to use so low light flux that the photoelectrons generated are in the range of 100's /second. The emissive surface is the entrance cone of the multiplier.

I have tested this previously with an unmodified CEM, showing decent control over count rate by adjusting light. This one however is a different model, and modified as mentioned. Perhaps it is treated to avoid photoelectron generation, as that would just be noise in many applications.

I was also concerned photoelectrons would be repulsed by the large negative voltage, and be attracted to the nearby chamber wall out of the CEM, instead of into it, so I tried with reverse polarity; ground at the cone and +4kV at the output. No difference.

Related to Channel electron multiplier as an electron source

1. What is a channel electron multiplier (CEM)?

A channel electron multiplier is a device used to amplify the number of electrons in an electron beam. It consists of a series of dynodes, or electrodes, that are arranged in a channel. When an electron beam enters the channel, it strikes the dynodes, causing them to release additional electrons. These electrons are then accelerated towards the next dynode, causing a chain reaction of electron multiplication.

2. How does a CEM work as an electron source?

A CEM works as an electron source by using the amplified electron beam to generate a larger and more focused electron beam. The amplified electrons are collected at the end of the channel and focused into a beam, which can then be used for various applications such as electron microscopy and particle accelerators.

3. What are the advantages of using a CEM as an electron source?

There are several advantages to using a CEM as an electron source. Firstly, it allows for the production of a highly focused and intense electron beam, which is essential for many scientific experiments and applications. Additionally, CEMs have a high efficiency and can be used with a wide range of electron energies, making them versatile for different research needs.

4. What are the limitations of using a CEM as an electron source?

While CEMs have many advantages, there are also some limitations to using them as an electron source. One limitation is that they are susceptible to damage from high energy electrons, which can cause the dynodes to degrade over time. Additionally, CEMs are sensitive to magnetic fields, which can affect the trajectory of the electron beam and reduce its accuracy.

5. How is a CEM different from other types of electron sources?

CEMs are different from other types of electron sources, such as thermionic and field emission sources, in that they use a different mechanism to generate and amplify the electron beam. Thermionic sources use heat to release electrons, while field emission sources use high electric fields. CEMs, on the other hand, use a cascade of dynodes to amplify the electron beam, allowing for a more intense and focused beam compared to other sources.

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