Needing more than a spark test?

[RJSakowski]

As I recall, an X-ray tube emission is fairly broad spectrum with output energy determined by the accelerating voltage. x-ray technicians and medical physicists measure peak energy in Kvp. X ray machines have filters which filter out "soft" x-rays. This would be something like 1.5mm of aluminum.

I believe that the term x-rays was originally applied to electromagnetic radiation generated by impact of a target with an electron beam and gamma radiation was applied to electromagnetic radiation originating from nucleaar decay. Theraputic x-ray genera5tors use linear acclerators to generate x-ray beams up to 4 or 5 Mev. Cosmic radiation was radiation from space. The three specta do overlap. For our purpose, we can consider gamma and x-ray radiation to be interchangeable.

 

[graham-xrf]

Theraputic x-ray genera5tors use linear acclerators to generate x-ray beams up to 4 or 5 Mev. Cosmic radiation was radiation from space. The three specta do overlap. For our purpose, we can consider gamma and x-ray radiation to be interchangeable.

Thanks - that gets some perspective.
Years ago (too many), I worked on an auto-focus scheme for a image intensifier thing that allowed continuous viewing of one's innards, because of the very low level of X-rays used. To test it required access to a hospital X-ray machine, and I was given a 5 minutes speedy instruction on how to work the controls, and then the staff left me to it. I could not leave the room during on time. I had to be right close up to it while the beam was on. I was given a heavy white apron affair that covers front and back. I could see and adjust what I needed to if I sat on the treatment table, and had the beam go downwards past me, but right next the sit spot (so to speak).

I set the machine for the most feeble X-rays I could, I think it was around 25kV or maybe 30kV. The dial could go up to well beyond 120kV. The first try totally overwhelmed the imager, so I used a 150mm square x 1mm thick piece of copper sheet as a "neutral density filter", to cut the level down, though the beam still had to get past me before it encountered the copper.

It left a strong memory. This light you cannot see, that just goes through metal.

YouTube is absolutely full of videos of people get up to stupidly dangerous experments by applying high voltage to various old vacuum tubes, DY86 old TV power supply EHT rectifiers, or even any old light bulb. Amid them is a very interesting one by "glasslinger". I was more absorbed by his glass-working skills, but he makes up a X-Ray tube about the size of 3/4 of a finger. He turns up a piece of tungsten (where does one even get hold of that?), and puts the angle on it to direct the X-Rays. Most interesting is, with full voltage applied, you get zero X-rays, until you heat up the filament to supply electrons.

It seems the "amount" of X-Rays is controlled by the filament current. This would set an intensity of illumination, but from what you say about energy per photon, I guess the fewer photons (from low filament current) go just as far, and penetrate just as deep as the many (from higher filament current). The voltage sets the wavelength, and the filament current sets the intensity. Add to that, the timer sets the probability the X-rays will find and modify a DNA amid one's internals.

-->

 

[RJSakowski]

That's a great video. He has a great glass working shop as well.

Correct. The penetrating ability depends on the energy of the beam but the dose depends on the number of photons which are proportional to the current and to exposure tme. Actually, as diagnostic tool, it is the x-rays that get stopped which convey the useful information.

A product that we made was called a DAP or dose area product meter. It uses an in-line ionization chamber to monitor the x-ray beam aqnd does so by integrating the total beam over time. It was a required instrument on all diagnostic fluoroscopy machines in the EU.

Actually, an x-ray generator might not be a bad way to go. You wouldn't have a portable unit but the photon flux would be much higher. You would probably want to filter out the lower energy photons but that isn't too difficult. I happen to have an x-ray systen from the 1950's. It's never been fired up and I don't know if the tube is still good.

 

[graham-xrf]

Actually, an x-ray generator might not be a bad way to go. You wouldn't have a portable unit but the photon flux would be much higher. You would probably want to filter out the lower energy photons but that isn't too difficult. I happen to have an x-ray systen from the 1950's. It's never been fired up and I don't know if the tube is still good.

Wow! You kept some amazing kit! I would say that your misgivings about the tube be set aside. When would you get a better opportunity to try it again. A X-ray tube has the virtue of being constructed of solid basic stuff, unless yours has some sophisticated magnetics around it to control focus etc. In what ways could the tube become "no good". Filament coating gone? Getters used up from leak?

Applied to XRF, and allowing we lose some portability, it seems to me we do not need high filament current intensity. We just need enough events at high enough voltage. Nor need the voltage be continuous. It can be by "shots" of a limited number of X-Rays of short enough wavelength. I would say up to 70KeV, which gets no scatter return from lead shielding, but covers most other. Come to that, a clear return from the lead is OK. You know where it came from, and can calibrate on it.

I think turning the X-ray on as pulsed samplers, as short as possible consistent with provoking a response from the material, say 200nS pulses. Enough to fill up the histogram plot, perhaps 2000 to 5000 pulses. It all depends on how many come back from the material, and how fast. It begs the question as to what is the disintegration rate of Am241 delivering gamma photons.

I can't believe I am actually asking this, but will a 1/8" or 3mm thick lead roofing flashing material be enough to stop a 70KeV X-ray?

 

[homebrewed]

The inverse-square law applies to the intensity of the x-ray (or gamma-ray) flux. The photon energy does NOT change. If that was the case, a blue object would look redder and redder the further away you got from it. Doppler shift is a different thing altogether.

The lab where I used to work had a real-time X-Ray unit. It used a fluorescent screen and a CCD camera to image the x-rays. The x-rays were generated using a "tube" that was basically a scanning electron microscope without the scan coils. The electron beam was accelerated up to 120KEV and focused on a thin tungsten (IIRC) diaphragm. One side was at vacuum, the other side was at atmospheric pressure. Since the effective size of the x-ray source was basically a point source, the imager showed you the shadows cast by objects between it and the source. The closer the object got to the source, the higher the magnification. We used it to image gold bond wires attached to integrated circuits, and they are just a few mils in diameter. We placed the objects on a carbon-fiber tray that was remotely controlled for x,y,z, pitch and roll so we could adjust the object to get the exact view we needed -- in real time. A very cool device, but it had a very large footprint, and of course was very expensive. It had a mechanical pump and a turbo pump to get the "tube" down to its operating pressure. The pressure had to be low enough to stand off 120KV without arcing...not a trivial thing to accomplish!

For awhile I was interested in making my own fusor, a relatively simple type of fusion reactor. Since typical fusors operate in the 30-40KV range, one of the primary safety concerns is x-rays -- not from the fusion reaction, just plain old electrons hitting metal at high speed (although if you're successful you DO have to worry about neutrons, too). So if one has a turbo pump (or diffusion pump) and a mechanical roughing pump it's not outside the realm of possibility to make your own x-ray source. Not a trivial effort, and likely another hole to pour time and money into.

The attractive aspect of an XRF system using Americium is that most of the pieces can be gotten from ebay and the like. If anyone wants to try making their own x-ray tube I have some thoriated tungsten wire that was used to make old-style SEM "hairpins" for their electron gun. Free for the asking....

 

[RJSakowski]

Wow! You kept some amazing kit! I would say that your misgivings about the tube be set aside. When would you get a better opportunity to try it again. A X-ray tube has the virtue of being constructed of solid basic stuff, unless yours has some sophisticated magnetics around it to control focus etc. In what ways could the tube become "no good". Filament coating gone? Getters used up from leak?

Applied to XRF, and allowing we lose some portability, it seems to me we do not need high filament current intensity. We just need enough events at high enough voltage. Nor need the voltage be continuous. It can be by "shots" of a limited number of X-Rays of short enough wavelength. I would say up to 70KeV, which gets no scatter return from lead shielding, but covers most other. Come to that, a clear return from the lead is OK. You know where it came from, and can calibrate on it.

I think turning the X-ray on as pulsed samplers, as short as possible consistent with provoking a response from the material, say 200nS pulses. Enough to fill up the histogram plot, perhaps 2000 to 5000 pulses. It all depends on how many come back from the material, and how fast. It begs the question as to what is the disintegration rate of Am241 delivering gamma photons.

I can't believe I am actually asking this, but will a 1/8" or 3mm thick lead roofing flashing material be enough to stop a 70KeV X-ray?

The tube and power supply were new Korean War surplus from a MASH unit. The tube/power supply are red lined at 85Kv@15ma. The tube is well shielded and the beam is collimated.

My understanding is the x-ray tube will fail from loss of vacuum.

From a chart that I have, .28mm of lead will attenuate a 125 Kv x-ray beam by 50% so 3mm of lead will attenuate the 125 KV beam by 99.95%. .05mm of lead will attenuate a 40 Kv x-ray beam by 50% or 10e-18 of the beam passing through 3mm.

 

[graham-xrf]

The attractive aspect of an XRF system using Americium is that most of the pieces can be gotten from ebay and the like. If anyone wants to try making their own x-ray tube I have some thoriated tungsten wire that was used to make old-style SEM "hairpins" for their electron gun. Free for the asking....

Agreed on using Americium for cost and convenience and eBay availability. The thing is interesting enough without getting into making X-Ray tubes - for now.

Fusors have moved on, I think. Last I read of it, there were serious numbers of folk working a variant called "polywell", with some input from US Navy, and some private companies, to hit Boron-11 with hydrogen ion plasma confined magnetically. I think it has problems. This would be a different order of hobby machine, if not for "Final Nail"
(see https://en.wikipedia.org/wiki/Polywell)

Re: The chart. Is that an available PDF?

 

[homebrewed]

The Polywell is another one of those fusion projects that has been "just around the corner" for quite a few years now. This really is OT so no more on it. But if anyone is curious on what amateur "fusioneers" are up to, you can go here and look around. Plenty of opportunities for machining projects if you're so inclined. Also plenty of opportunities to spend money....electrocute yourself.....get radiation sickness...all in the name of some good fun!

 

[rwm]

Great explanations guys.
I mentioned 59 KeV since that is the highest energy of Am gamma. For identifying elements we are interested in the characteristic photon from the K-shell. This electron gets ejected by photoelectric interaction with the incoming gamma, and then emits the xray when the vacancy is re-filled. It requires a gamma of the same or higher energy to do this. So Am should not be able to eject a higher energy electron or create a higher energy xray.
My understanding of the difference between gamma and xray is as RJ describes. They are physically the same but gammas come from the nucleus and X-rays come from the electron interactions. I believe when Roentgen discovered this ray, he had no idea it was like light or gamma and therefore called it X. (It was also called the Roentgen ray.)
The xray tube I showed earlier failed due to vacuum leak. When that happens the filament burns out just like a light bulb. After much use the tungsten targets can actually wear out. So much erosion of the tungsten occurs that the geometry gets messed up.
I would love to mess with some of this stuff but quite honestly I am too e'scared of the high voltage!
Robert

 

[pontiac428]

Yeah - I just realized I must sound like HM's biggest $$[ tightwad ] $$.

So you shall be known as Her Majesty's biggest tightwad! Behold!

I think this thread has been a hoot to keep up with. I have very little buy-in for the undertaking, because I have years of inorganic analytical chemistry and field-portable XRF in my background. Knowing what's involved had me thinking it would be a fruitless effort, but I have enjoyed the progress you've made with the idea.

To provide some motivation, I've been working with 3M-Thermo-Fisher on obtaining an XRF for identification of unknown metals for one of the commands I serve. With the library and a spare battery, we're talking $35,000 per instrument here. At least they don't have all of the headaches of a Cd-109 source now that they've switched to photo tubes (I used to haul those across borders, fun!) Anyway, the technology has gotten amazing, and even further out of reach for us tinkerers. If you come up with something that works, people will follow. You know this is a masters' student's final project for physics or chemistry, and y'all are doing it in your garage in your spare time!