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Cathode Ray Tube and Cloud Chamber Combined - Possible? Answered

Is it possible to build a cathode ray tube and observe the path of electrons in a combined (or linked) cloud chamber? I would want to apply a uniform magnetic field to the cloud chamber but not the CRT so that electrons travel in a straight line into the chamber and are subsequently deflected. Oh, and ideally using materials that were available in 1917! Thanks in advance.


Ok, here's the thing: if I wanted to observe an electron-positron pair, e.g. in a cloud chamber, they would need enough kinetic energy to ensure they were not immediately annihilated on creation. This implies high energy photons which I think means gamma or x-rays. I understand that this was not even theorised until the 1930s but hypothetically, and assuming safety was not an issue, would this have been possible circa 1917?

You really, REALLY, do not want to do that with any kind of source. If you build a multi-MeV particle accelerator (which is what you will need to get the >> 1 MeV gamma rays needed for pair production), you're going to be dealing very serious shielding issues. The level of your questions imply some lack of expertise in this area, which will lead to non-trivial hazards to yourself and to others.

If you want to observe pair production in a cloud chamber, you can get that reasonably well, at a low rate, just from cosmic rays. If I were doing it (the way we did in my undergrad HEP lab about 30 years ago), I'd use a muon hodoscope as a trigger to backlight the cloud chamber and shoot a digital camera image.

A hodoscope is a couple of plastic scintillator paddles with photodetectors, placed vertically, and with a stack of metal (lead or steel) to act as an energy filter. You connect the two (or more) photodetectors in a coincidence circuit. If both the upper and lower ones fire, then you know you have a particle going both in the right direction and with enough energy to do what you want. You can put another sheet of steel below the hodoscope to hep with gamma production.

Hi, thanks for the very helpful advice. Just to put your mind at rest (and sorry if the naivety of my questions caused any concern), I will not be doing anything like that! You are right about my lack of expertise, so I guess it would have been helpful if I had mentioned up front that I am an aspiring author who has a storyline (set in 1917) but insufficient knowledge of the science involved to know whether it is too far fetched to pursue. I have written to a number of university physics departments to ask for advice, but perhaps understandably have had no replies.

I was very keen to remain in the area of science-fiction rather than fantasy, but it appears from your comments that the sort of observations I had in mind would not have been even remotely possible back then, even for a genius scientist with a lab (partly inspired by Tesla). If you would like more details I can provide them but I have no wish to waste any more of your time. Once again, many thanks.

[ Sorry for the delay; I've been on vacation and offline... ]

Ah, that is much more reassuring! The Crookes tube was invented in the 1870s, so the technology was definitely there. In fact, it was exactly the fact that the apparent beam of light inside was deflected by a magnet which led Thompson to realize that they were little charged particles, and to measure the (negative) sign of that charge.

Cloud chambers are definitely within the bounds of that technology (you can make one at home with dry ice and alcohol).

Tesla coils and van de Graff generators can produce hundreds of megavolts of potential, which is easily enough to accelerate electrons to produce gamma rays, and to pair-produce.

I think you can definitely build a satisfactorily realistic scenario for your story using period technology. Just don't try building it at home!

You are not entirely correct about the time period and observing electrons moving.
Radioactive material does the same as your cathode tube experiment just on the safe side of things.
Alpha emitters can be obtained legally with little safety issues around the world, if not an old smoke detector with the typical warning label will contain what you need to create nice traces of electrons through a cloud chamber.
If the emitter is placed into a lead or tungsten tube (closed at one end) it will act like a flashlight with the electrons going out being in a more or less straight line.
That part would prove the concept of seeing the rays...
And for the cathode ray tube simply check when the first operational one was presented to the public ;)
With science fiction you can always upscale what they created to make ray gun or similar ;)

(If you're replying to Kelseymh, he will find out a lot more quickly if you hit the "Reply" button on the comment to which you are replying - replies like that generate an automatic alert via email and on your own profile.)

It sounds like, especially from your citation of "1917 materials" is that you want to build a Crookes' tube. This is similar to a CRT, in that the electrons are emitted and accelerated from a cathode, but it doesn't have magnets or electrostatic plates to produce or direct a beam.

I would recommend starting with the Wikipedia article, so you can see what one looks like and understand the general principles. You can then follow the links from there to get some guidance on building a simple one.

Ok, it seems that in theory what I want to do can be achieved with a Tesla open-ended unipolar vacuum tube which produces something called braking radiation, whereby electrons are accelerated by peaks of an electrical field produced by a high-voltage Tesla coil. I can then observe the tracks of these electrons in the cloud chamber. Does this sound realistic and what are the safety concerns?

Bremsstrahlung ("braking radiation" in German) is medium to high energy X-rays produced when electrons are suddenly decelerated. They're the main way that X-ray tubes work: accelerate electrons to high voltage and direct them onto a thin metal foil. The electrons stop, and X-rays come out the other side.

You're not going to see electrons, and you're going to have the safely issues of an unshielded X-ray source.

A cathode ray tube contains a [near] vacuum, but a cloud chamber is filled with with chilled methanol vapour, so, no, you can't combine them.

You can investigate the effect of magnetism on the electrons with a deflection tube.

They're expensive to buy:


But maybe you could adapt this instructable to include phosphorescent screens?


One warning; be careful with the voltage. Not only is there a significant risk of electrocution, if the accelerating voltage exceeds ~5kV, the anode or the glass itself starts to emit X-rays when struck by the electrons. Obviously, X-rays pose a cancer risk to you and those nearby (and internal walls are usually not dense enough to provide protection), and you have no easy way to detect their presence or strength.

Thanks Kiteman, will give it some thought and update on here later.