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Grenadier's big thread of RÃ¶ntgen related shenanigans

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Weazel

Sat Oct 16 2010, 06:49AM

Registered Member #3276 Joined: Tue Oct 05 2010, 11:25AM
Location:
Posts: 3

if u need a hand not speanding ur money i can help eg go get dud car batterys theres free lead etc

Adam Munich

Sat Oct 16 2010, 07:03AM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

Wouldn't worry about that... I have lead coming out the wazoo ATM.

Adam Munich

Wed Oct 20 2010, 05:20AM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

Hmm... I like the idea of an x-ray "gun". Simply put it outside and aim it 'twords the woods, place an object + cassette in front, adjust a variac for kVp, flip the filament battery switch, then go in the basement and plug it in (150ft extension cord). Unplug when exposure is done. This way you can take as big a picture as you want, size is only limited by your cassette size.

I could cast a small lead box ~ 5x5x6 in, half inch thick to put the tube in. Drill a hole in it for the aperture, then silicone a piece of glass or maybe thin aluminum over the aperture. Build a wooden holder for tube and put it inside, then fill with oil. Put a lead lid on and boom, directional source. There won't be any direct radiation anywhere but the aperture (.5inch lead for christ's sake). I'll still want to get away though since compton scattering would scatter some rays.

I'm thinking of portability. Maybe I could even make it battery powered with a ZVS, CW multiplier, and some lithium cells. Ignition coil maybe? Battery powered 30kVp x-ray source for taking radiographs of flowers in the woods? :D

Just carry around a backpack of loaded cassettes. If I make the "tube box" a separate entity from the HV power source, it could weigh a manageable 30lbs. Box would be very easy to make, since it's small a plaster of paris mold could be made for it. It should take maybe only 5 tries to get right. Since I'm not using the Pb sheet, that could be used as a lead apron when there is no basement to hide in.

edit: Half inch may be a little excessive. 1cm would do the trick. Plus the tube will have that lead ceramic covering. It's not like I'm going to be anywhere within 100 feet of this while it's on.

Proud Mary

Wed Oct 20 2010, 01:26PM

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

Grenadier wrote ...

you can take as big a picture as you want, size is only limited by your cassette size.

If you know the size of your focal spot, you can calculate the width of the unsharp image margin at any given distance by simple geometry.

Moreover, you are very likely to run into reciprocity law failure quite quickly as you extend the distance from tube to screen, see:

Seelentag WW, Panzer W, Reciprocity law failure with film/screen combinations for mammography British Journal of Radiology (1978) 51, 529-531

This excerpt was created in the absence of an abstract.

In radiology the validity of the reciprocity law is usually assumed: the film density for a given X-ray spectrum depends only on the tube exposure (the mAs), and not on the exposure time. This reciprocity law holds for non-screen films (Morgan, 1944). For screen/film combinations, where most of the film density is due to exposure to light from the screen, reciprocity law failure may occur. However, for materials used nowadays in general radiology the effect is negligible for exposure times from 0.005 to 1 second. During the course of measurements on film/screen combinations used in mammography, exposure times of up to 20 minutes had to be used (Panzer, et al., 1978). Therefore reciprocity law failure of these materials had to be investigated. It was found that even at an exposure time of 5 seconds the system speed is reduced by some 30% due to reciprocity law failure.

Reciprocity law failure is described in many textbooks, e.g. by Mees and James (1962) and Herz (1969). Photographic emulsions are most sensitive when the intensity of the exposing radiation is such that an exposure time between 0.01 and 10 seconds is required to produce a medium film density.

klugesmith

Wed Oct 20 2010, 01:55PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Grenadier wrote ...
...This way you can take as big a picture as you want, size is only limited by your cassette size.

and by how many seconds you can have the HV on before the anode focal spot get hot enough to emit electrons, or the whole anode gets hot enough to crack the glass seal, or (less likely) the XRT overheats somewhere. Oil immersion would help to cool the tube.

In regular service, dental tubeheads don't run for more than 1 second of any minute.

Proud Mary

Wed Oct 20 2010, 02:39PM

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

At the risk of stating the obvious, the problems of both reciprocity law failure, and anode over-heating with prolonged exposure, will be amplified with distance due to the inverse square law, thus:

Adam Munich

Wed Oct 20 2010, 08:20PM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

The tube will be oil immersed, that's why I said silicone-sealed aperture. I'm likely going to use oilve oil for the tube because it has a relatively high specific heat (1.97kJ/kg K). I'm going to put the x-former in vegetable oil mainly because vegetable oil is a lot cheaper. And yes, I'll have to spend an hour massaging all the bubbles out of the paper windings. As for overheating + Reciprocity law failure, multiple short exposures solve the problem. On the "mains end" (basement where I will be) I was thinking of an arduino controlling a SSR. For exposures longer than 1.5 seconds the arduino could break up the exposure giving time for heat to dissipate, as well as time for the cassette to "reset". The subjects won't be alive, so there's no need to worry that things will move.

I was also thinking of making the tube part separate, and having two power supplies. One with the giant x-former that plugs into the mains, and the other a flyback + CW generator for portable use.

For the mains transformer I could use ether a resistor on the primary or secondary side to limit the tube current to 750uA. At that current the heat will have time to dissipate. With the low current of a CW I need not worry about current when I use one.

The hardest part will be casting such a large amount of lead. Plaster of paris would likely be the best bet, and I'll have to heat the lead to about 900*F and pour it all once to prevent things from pre-solidifying. I'm only going to use 1cm because I did the math, and assuming a generous 3000 milliseiverts/hour of 75kVp off axis radiation, 10mm would reduce it to 0.376mSv/hr, or nearly nothing at all. Keep in mind this is directly in front of the shield, not 100 feet away. The overall dose of a 10 second exposure at 100 feet would be the same as me living for about a 15 seconds. This does not take into account compton scattering and reflection though, so it's still best to take cover in a basement or behind a car/big tree (for portable use).

Wolfram

Thu Oct 21 2010, 07:49AM

Registered Member #33 Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971

To safely work with x-rays requires a good understanding of the risks. It seems you don't really understand the risks that well, you over-estimate them greatly.

Why is this a problem? Well, it becomes a problem when you take other unnecessary risks to avoid imagined risks. Like when you give yourself lead poisoning because you wanted to reduce the radiation dose from negligible to unmeasurable.

Think about this. The tube you are using is a dental one. Dentists use these tubes many times a day, and their x-ray rooms are not shielded by 10mm of lead, and the operators are not 100 feet away. X-ray rooms are usually shielded with a couple of millimeters of lead (reference:

, first page, also see the very relevant table on page 5), and the operators are only a few feet away from the tube.

Sure, it would be an excellent idea if you already had a 10mm wall thickness lead box, but I can imagine making one would lead to quite a bit of lead exposure (unless you somehow manage to do it from 100 feet away in a completely sealed room), and that's going to do significantly more harm to you than an almost immeasurable quantity of extra x-rays.

Also, aren't you buying a complete x-ray head? In that case, the simplest solution is just to use it as it is, no need to worry about air bubbles getting into the transformer windings, oil leaks, etc.

Anders M.

Adam Munich

Thu Oct 21 2010, 10:43AM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

Well I know I'm overestimating the risks, but my philosophy is "better safe than sorry". I still want the lead box so I don't need to wear an apron. I'm not going to get exposed to too much Pb because unlike my other adventure hammering welding and melting that huge box together, this one will just be cast as a single piece. Of course it'll be done outside with a breeze to get rid of fumes. I'll spend all of ten minutes pouring the stuff, and likely the hardest part would be making a suitable mold. The only material I have to work with is plaster of paris. Irregardless, the exposure would be nearly non-significant; less than a plumber making a lead drain pipe.

Once the box is done, it'll be drilled, not too hard. Then It'll be filled with oil + tube and a lid made. Then somehow I'll seal the lid to the box, likely with silicone. The whole thing will be put in a bigger box, so I'll never need to see it again.

The reason I'm taking apart the head is because I want to have the kVp adjustable. The problem with leaving it together and just using a variac is that if I use a variac, the heater voltage would vary too. If I want 40kVp rays the heater would nearly be cold. Plus I'd like to put at least some craftsmanship into this.

Proud Mary

Thu Oct 21 2010, 03:38PM

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

I think it important to understand the basic physics of radiological protection, and what is considered to be best practice in your jurisdiction, and then apply those principles to your own endeavours.

There is no longer believed to be any 'safe' minimum dose, as there was claimed to be in the bad heyday of atmospheric testing. The ALARA principle - As Low As Reasonably Achievable - is a good benchmark to work by.

Of course, lawyers and ambulance chasers can argue themselves a living out of what 'Reasonably' means in ALARA, but I would say that it meant that no radiation should be detectable above background outside the master enclosure of the X-ray system - be that master enclosure a lead box the size of a house brick, or a lead-lined room.

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