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Flash x-rays and phosphors

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Proud Mary

Sun Mar 30 2008, 11:30PM

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

Karol, most general purpose uncompensated GM tubes have a very big energy response peak stretching between about 40 and 90keV, with the maximum about 60keV, such that you can expect a reading ten times higher than would be given by a Cs-137 662keV calibration standard. On this basis, I would suggest that your actual X-ray fluence was more likely 6 or 7 Sv/hr.

c4r0

Mon Mar 31 2008, 09:36AM

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

I used the g-m counter only to determine the apron attenuation ratio. It's a proportion so calibration of the counter shouldn't affect on the result i think. Besides, measured attenuation agrees with Pb half-thickness calculations. Value of the dose rate i used in the body dose calculation comes from the Rad Pro Calculator not from my counter.

Proud Mary

Mon Mar 31 2008, 10:42AM

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

Oh, I see. Excellent!

plazmatron

Mon Mar 31 2008, 08:50PM

Registered Member #1134 Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351

No I didnt use an Al filter, when I X-rayed my hand.
The reason for that was, twofold, the first being to overexpose the background of the paper, since I often enlarge prints, which is difficult to do without a jet black background on the negative.
And secondly to get reasonable contrast on the plastic parts of the mouse, from the lower energy photons.
The x-ray was taken at 120kVp, which if i was using an Al filter, would have completely washed out the detail from the plastic body of the mouse.

Since this was a one off exposure, I figured going without an Al filter would`nt be too harmful

uzzors2k

Thu Apr 10 2008, 04:33PM

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

Nice x-ray photos guys. I've got paper and chemicals ready, so now I just need to make a setup. I spent some days thinking of materials to use for shielding, and then I remembered I had an unused MOT core. After some chopping and epoxy I was left with some ~1-2 cm thick slabs of steel. However when measuring radiation with the shield around the tube I didn't notice any difference than with no shielding. So I encased the Geiger counter, but to no avail. Radiation is still leaking through! Does anyone know the half-value thickness of steel at 150kV? Is it any use shielding around the tube, or does the entire thing have to be encased, with just a small viewing window?

plazmatron

Thu Apr 10 2008, 08:58PM

Registered Member #1134 Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351

Shielding high energy photons like this is a real pain. I use several layers of 3mm thick lead sheet, and some photons make their way through.
In the real world, "they" use Depleted Uranium as shielding for high energy x-ray tubes, but unfortunately for us, its not really common stuff!

The best advice here, is to distance yourself from the working tube, and take care of shielding yourself, and others, rather than the tube itself.

Leslie

Proud Mary

Thu Apr 10 2008, 09:18PM

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

If you go to Rad Pro Calculator v 2.62, open the X-ray devices screen, and tick the "Add Shielding" box, and choose "Iron" from the drop-down menu, you will get a fairly accurate answer to your question about steel shielding once you have entered your voltage, tube current, distance, and so on.

Remember that for a given tube at a given voltage, the majority of the radiation falls in the lower third of the energy spectrum. i.e. if you have a 100kV on the anode, most of the output energy will be under 33keV, with not very much at all actually at the maximum possible 100keV.

uzzors2k

Sat Apr 12 2008, 01:18PM

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

I played with some numbers in Rad Pro and it seems my shield will only half the radiation. At a distance of 5 meters assuming 150kV and 1mA of average current (I doubt my Marx is pumping even close to 150W) I should expect around 0.15 Sv/Hr. So what, about 4 hours near the tube before I need to worry about getting sick?

I haven't got any intensifying screens yet, should I bother trying an x-ray without them? How long would a x-ray on bare B/W photo paper take, or would it even work?

plazmatron

Sat Apr 12 2008, 06:42PM

Registered Member #1134 Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351

I would definately advise using intensifying screens!
Raw film/or paper, is really quite unresponsive to x-rays, and your exposure will take about an hour at 2 pulses/sec!

With blue intensifying screens, figure somewhere in the region of 90 sec @ 2 pps.

I burnt out a coolidge tube once, by not using screens, running at several 10`s of minutes per exposure.

However, just for fun, you could place say a washer or a bolt or something, on top of a black paper envelope containing your paper, expose it for 90 sec @ 2pps (or 180 flashes) , and then really over develop it? At the very least you should manage some kind of feint image.

Leslie

uzzors2k

Mon Apr 14 2008, 11:50AM

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

No point in burning things out already. Shipping intensifying screens costs unrealistic amounts, are they heavy or something? I've found sheets of phosphor which are cheaper to ship, would they work as a replacement?

He also sells more sensitive screens for x-rays, but they cost enough to pay for a real intensifying screen.

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