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Flash x-rays and phosphors

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uzzors2k

Sun Mar 16 2008, 09:38AM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

I tried a flash x-ray setup like Leslie has done here. It produces x-rays alright, and my Geiger counter clicks with each spark, even at a distance of 1 meter. (probably further away too.) The problem is the ZnS screens I'm using aren't fluorescing, they might do better in the dark but I had hoped some fluorescence would be visible in the daylight. The ZnS was harvested from some monitors, so it is doped, but the persistence time is much too short for the screens to remain lit between shots. (2.5 per second) Has anyone fluoresced monitor ZnS with x-rays before to see how they respond, and are there any other household sources of phosphor that I can try? Even if my ZnS screens are working, would I be able to see the fluorescence real-time, or would I need to take a long term exposure with a camera/photographic paper? Would a professional x-ray screen be much better?

Proud Mary

Sun Mar 16 2008, 01:47PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Have you thought of strontium aluminate doped with europium? You can buy it easily on the internet in paints, powders, tapes, and even impregnated computer printer paper from a British company called Glowtec.

This certainly fluoresces under both UV and gamma radiation, but I don't know how it will do with the relatively soft X-rays lying between the two. It has a very long glow persistence half life compared to ZnS phosphors.

Calcium tungstate and cadmium tungstate are the simplest (old fashioned!) X-ray phosphors that can be prepared in the home workshop, though the need to fire them at high temperatures (800 - 1000 Â°C) for hours at a time to activate them needs a pottery or ceramics kiln to do properly.

I believe that the main commerical X-ray phosphors used today are
Gd2O2S:Tb (Green) Gd2O2S:Eu (red) Gd2O2S:Pr (green again)
La2O2S:Eu (red) La2O2S:Tb (green) Y2O2S:Tb (blue-white) (Zn,Cd)S:Ag (red) CsI:Na (blue) CsI:Tl (yellow-white)

uzzors2k

Sun Mar 16 2008, 08:16PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

I have some silly putty which is glow in the dark, if I can get it to fluoresce I'll buy some of those glow in the dark sheets. That way I can do long term exposures, and just photograph the sheet afterwards. I tested the current setup in the dark tonight and I couldn't even see a hint of fluorescence. I'll try pushing 100kV from a single flyback like jmartis did, which is enough to run the tube. That will provide a more or less continuous supply which should make any fluorescence easier to see.

c4r0

Mon Mar 17 2008, 07:20PM

Registered Member #151 Joined: Sun Feb 12 2006, 02:53PM
Location: Poland
Posts: 153

Just remember - do not look directly on the x-ray tube when operating

Steve Conner

Tue Mar 18 2008, 10:46AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Let me get this straight, you've burnt your face with X-rays? That's more

than

Bjørn

Tue Mar 18 2008, 11:35AM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

That is just a average evening out in Gdansk, come to think of it in Glasgow too,

What was it I was going to post? Ah, yes, we are supposed to stay on topic which is Flash x-rays and phosphors.

c4r0

Tue Mar 18 2008, 03:21PM

Registered Member #151 Joined: Sun Feb 12 2006, 02:53PM
Location: Poland
Posts: 153

Just kidding guys

I've been one week in mountains without sunblocking cream. Maybe somebody should delete these offtopic posts?

uzzors2k

Thu Mar 20 2008, 07:27PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

Lol you tricked me, c4r0.

I took a picture of the tube while operating and tested the x-ray range. My brother said the counter would click outside of my room with the door closed, which is about 5 meters away. And that isn't even in the direction which most of the x-rays should radiate! The glass in the tube fluoresces strongly. Notice the pixels around the tube in the photo. The spark to the left is from the Marx generator.

1206037914 95 FT41218 Glass Fluorescence

Somewhat eerie, and given the energy of the x-rays I'm starting to feel a bit unsafe. Despite x-rays apparently being present, so far neither the silly putty nor the ZnS will fluoresce.

What I don't get is why the silly putty doesn't fluoresce, it's phosphor's persistence time is more than long enough for it to remain lit between each time the Marx fires. Is it possible that the monitor ZnS and silly putty phosphor aren't excited by x-rays? I've also tried a floro tube, but it would flash without the "x-ray tube" in place making it unreliable. (even static will light a foro tube...) Is photographic film worth trying?

Proud Mary

Thu Mar 20 2008, 08:45PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

The phosophors from a CRT tube should certainly fluoresce under X-ray radiation, as it is in fact X-rays that make them glow.

The screen of CRT tubes is covered with a layer of a fluorescent phosphor such as ZnS:Ag or ZnCdS:Ag and rare earth oxysulphides. A layer of 25 to 50 nm of Aluminum is deposited over the phosphor to evacuate the static charges. About 3 keV are necessary for the electron beam to go through the aluminum film generating Bremstrahlung X-rays that excite the phosphor and are stopped by the glass face of the tube.The electrons in the beam have typically an energy of 25 keV, thus 22 keV are available for exciting the phosphor. Since the typical energy of excitation is of the order of 3 eV (360 nm), numerous phosphor molecules are excited by the same electron (or X-ray) in a quick succession causing an entanglement of said excited phosphor molecules. The molecules then phosphoresce together emitting entangled photons.

Remember that the decay time of screen phosphors is necessarily very short, so you should only expect a very brief pulse of light following exposure.

Try irradiating your phosphor with infrared after the X-ray exposure, as this will often release any stored energy.

uzzors2k

Sun Mar 23 2008, 10:42AM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

I always thought it was the electrons themselves which caused the fluorescence, but if it's actually x-rays then the screens must be fluorescing, just not bright/long enough to see.

I have no experience whatsoever with photography, what kind of paper/film should I get and where? Won't I need some chemicals to expose the paper as well?

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