Hello everyone,

In my previous posts I've talked a lot about detecting or using ionizing radiation. But how do you actually obtain radiation sources? This post is going to show you how I made my own XRays. I briefly mentioned this in my 20kV power supply post but I will be going into more detail here.

Important note: Do not attempt to make your own XRays without a working radiation detector. Do not produce XRays anywhere near yourself. If you can, having multiple detectors is best since you can tell if one is faulty and measure dosages further away. Honestly just don't try to repeat anything in this post.

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Another note: This project involve high voltage. HV can be deadly. DO NOT attempt to use high voltage supplies unless you are very well aware of high voltage safety precautions.

And finally, there isn't a point to make this kind of radiation unless you are making XRay images or just want to test your detectors. Don't try to replicate these procedures unless you actually have a good use for your own low energy XRays. There is no use in irradiating yourself for no reason.

Once again, my apologies for the lack of original images. My parts and equipment are back at my apartment and I am away for the academic break.

Overview: What are XRays?

XRay radiation is just high frequency electromagnetic radiation. You can just think of it as very very very high frequency radio waves, or visible light photons with many magnitudes more energy (EM radiation starts being called XRays around 100 eV per photon, while visible light only has a few eV per photon). Most importantly, XRay radiation is ionizing, especially at the energies we will be discussing in this post.

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Below around 10-15 keV, most XRays don't make it past reasonably thick glass walls. I know this from experience, since my tritium tube doesn't register on my geiger counters. Since electrons have a charge of 1 e, you will typically need to produce a voltage above 15,000 volts to register any hits on a detector. That's a good thing: Lower energy XRays are actually very dangerous since they are so easily absorbed, so the fact that glass tubes already absorb these XRays is a major safety benefit.

XRays can be produced via several mechanisms but the two I'll be covering here are bremsstrahlung (braking radiation) and characteristic XRays. Braking XRays are produced when high-speed electrons are rapidly accelerated in some way, which in this case occurs when the electrons impact a nucleus head on. This results in a wide spread of different XRay photon energies, with a maximum energy equal to the maximum electron radiation energy. Characteristic XRays occurs when high-speed electrons knock out inner shell electrons from neutral metal atoms in the target electrode, causing another electron to drop down and fill this energy level, releasing an XRay. Characteristic XRays are emitted in sharp peaks around a certain photon energy because they are the result of electrons filling quantized energy levels in atoms.

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So, the short of it is that to produce XRays, you need a way to get electrons moving very, very fast: Up to 15 keV or more if you want to actually detect the XRays. I went the simple route and basically made an electrostatic electron accelerator, which sounds fancy and complicated until you realize that it's just a vacuum tube connected to a high voltage supply. Electrons accelerate across the electric field produced between the high voltage electrodes and impact the target electrode at a final speed/kinetic energy. You may wonder why we need the vacuum tube. Well, if you put two high voltage electrodes close together in the air, it will produce a plasma arc (see my previous post on this) but no XRays. This is because no electrons are actually reaching their maximum energy due to premature collisions with neutral gas atoms in the air. That's a good thing: If it wasn't true and electrons could accelerate to their full energy regardless of how much gas was present, stun guns, electric stoves, barbecue lighters and sticky tape would produce massive amounts of XRay radiation. So, in order to actually accelerate electrons to reasonable kinetic energies and observe XRays, we need to remove all of the air: Hence the vacuum tube.

Making XRays in my room: High Voltage source

The first step to accelerating electrons is a high voltage source. In a vacuum, every volt you apply across your electrodes corresponds to 1 electronvolt of kinetic energy gained by an electron that accelerates through a vacuum across the electrodes. Since we need approximately 15 keV (15,000 eV) to start seeing XRays, the minimum voltage is going to be around 15,000 volts.

My first high voltage supply was stungun I got off of Ebay for less than $10. It was rechargeable and claimed to output 10 Megavolts (10,000,000 volts). Quick stungun tip: If any seller claims to output more than 50,000 volts, they are massive liars that unfortunately won't be called out by Ebay or any other marketplace. That being said, if you divide the advertised voltage by 1000 you get a reasonable approximation of the real voltage. This one ended up being around 15-20 kV measured by me, although I think the peak voltage might be a little bit higher (I would need an oscilloscope to check this). It produces some pretty nice arcs. The internal circuitry is hidden inside a big block of epoxy to prevent people from reverse engineering the circuit, but I'm pretty sure it's a Cockcroft-Walton multiplier, which uses a string of capacitors and diodes to raise a voltage. The important thing is that it produces pulses of DC high voltage around 15 to 20 thousands volts each, several times per second. It looked a lot like this one, with different branding:

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My next supply was a 20,000 volt direct current power supply I built from a television set I found on the road and a florescent light ballast. I already did a pretty long writeup on that here, so click here if you'd like to read about that.

Another HV supply I was able to get purely by chance was a nice 35 kilovolt supply with a somewhat low current limiter. While trying to get permission with a friend at my university to salvage parts from a bunch of old computers that had been left out in the rain, I was given this machine, since it was about to be thrown away. The label was gone but I quickly realized after seeing the voltage multiplier circuit on the side that it was indeed a high voltage supply - how convenient. It even still worked - I just had to find a new 12V power supply to run it and attach a wire to the main output section. This is so far the highest voltage power supply I own.

I also have a bunch of smaller supplies, including $2 Chinese 10kV modules that run off of batteries, a dissected USB ionizer that produces a couple thousand volts off of the output wires I attached, and a modified bug swatter circuit that puts out about 1000 volts. None of these are high enough voltage to produce detectable XRays, and they really aren't great to begin with, having ridiculously tiny current outputs. Of the above, the 20 kV and 35 kV supplies are the only ones I'm really a bit afraid of.

Vacuum Tube #1: 2X2A

The 2X2A vacuum tube is a very old rectifier tube which was used in place of the diodes we use today. It consists of a filament that, when heated up with a low voltage, will emit low voltage electrons that can then be accelerated across the vacuum gap with a few dozen volts to complete the circuit. This only works in one direction, hence the diode-like capability. Conveniently, if you instead ignore the filament and just apply a really high voltage across the vacuum gap, you can cause electrons to tunnel out of one of the electrodes and accelerate into the other one. Since these vacuum tubes were built with a very good vacuum inside (essentially no air or gases), there is nothing to stop the electrons, and they accelerate to their maximum kinetic energy based on the applied voltage.

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After struggling to find decent pinouts online, I had some good luck using this to produce XRays. The stungun and 20kV supply were both able to produce enough XRays to cause the needle on my CDV-700 geiger counter to fly upwards. Since I was just doing this as proof that I could, and the 35 kV supply would massively increase the XRay output of the tube, I didn't use it with the 35kV supply, since there I already knew it would work and there was no need to put myself at risk.

Conclusion: The 2X2A is very good at accelerating electrons and producing XRays. It could be useful for making XRay photos if I had the film. However, it's a little potent for a Geiger counter check source, since the XRay emission is pretty high even at just 20 kV.

Vacuum Tube #2: Microwave Oven Light Bulb

This is the option if you'd like to find all of your parts of the side of the road. I had heard that the light bulbs inside microwave ovens sometimes contained a vacuum so I grabbed the bulb I had salvaged from a free microwave earlier to try it out. These bulbs are just incandescents, with two terminals that are joined by the tungsten filament that produces the light. By placing some aluminum foil over the top of the bulb and connecting a high voltage supply across the foil and the filament, I was sometimes able to get XRays. I didn't document this well so I can't remember if it was with the 20kV or the 35kV, but I definitely got XRay emissions from one of my bulbs. Note that not all of these bulbs will work and not all of them are vacuum insulated.

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Interestingly enough, the bulb will glow green when XRays are emitted. I'm not yet sure why. The emissions from this "tube" were still pretty high, but not anywhere near what the 2X2A was putting out if my geiger counter is to be believed.

Vacuum Tube #3: Homemade Tube

My last XRay producing tube was homemade. I currently own a really bad $40 vacuum pump used for refrigerator maintenance. It can theoretically get down to a couple Pa absolute pressure, but in practice this is very difficult, since containers outgas and have leaks. Because of this, typically smaller containers will result in lower pressures (better vacuums).

Running high voltage across my big vacuum chamber produced some cool low density plasma, but the air density inside was still way too high for significant acceleration and XRays. After a lot of attempts at making a smaller vacuum jar, I eventually just cut off a tiny piece of the tubing I was using to connect chambers, attached one end to the pump and the other end to a brass nozzle, put a piece of rubber on the other end of the nozzle, and ran 35kV across the tube. Sure enough, my geiger counter went off, indicating XRays. I'm more proud of this success, because it's the first and so far only time I've been able to accelerate electrons to >15 keV using only my own homemade vacuum tube. The reason it worked with the smaller tube is a combination of the electrodes being close together (high E-field means more acceleration) and the better vacuum. These two parameters allowed electrons to accelerate up to high energies and produce braking/characteristic XRays on impact. Here's an image of this XRay run:

(Image credit: Me)

Conclusion

I don't have an actual use for XRays (yet), so after proving that I could do it (and therefore proving that I could accelerate electrons to above the kinetic energy of tritium beta radiation), I stopped using the tubes to make XRays. It's still a decent test of the true capabilities of a high voltage supply if you can't measure the voltage some other way. If you have a less potent source that doesn't spew XRays when you connect it to a tube, you could use the concepts here to build a cheap electronic Geiger counter test source for testing your detectors - one that could be turned on or off at will, which is an advantage over radioisotopes like Am-241 that just decay no matter what you do or want. You can also make DIY XRay photos, but I haven't done that yet unfortunately. Something I wanted to do was make a USB XRay gun for checking radiaiton detectors with a lower power HV supply made from the guts of the stungun, but that fell apart when I broke the stungun HV supply and never got around to buying a new one. I may get around to that next year.

I was just pretty happy to have my own mini "particle accelerator", even if the "particle" in question was the electron which is by far the easiest particle to accelerate.

Thanks for reading! I hope you were able to learn something from this project writeup. If you have any questions, comment and I'll do my best to answer!

Tips/Upvotes appreciated!
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