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Grenadier's big thread of RÃ¶ntgen related shenanigans

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Adam Munich

Sun Oct 24 2010, 11:54PM

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

The CDV-138 is perfect for my purposes, as I'm not stupid enough to stand in the beam path. I found one for $7, and a charger for $17. How will this work kludge? Should I just send you money, or would you accept a trade for something I have?

Proud Mary

Mon Oct 25 2010, 01:17PM

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

The 200mR FSD of the antique CDV-138 QFD is roughly equivalent to 1.75mSv - about 8% of the UK HSE annual adult occupational dose limit of 20mSv.

I see from the Oak Ridge Universities site that these were manufactured between 1956 and 1974.

The seller's enlarged photo shows significant corrosion, so let's hope their keeping qualities are better than they look!

Adam Munich

Mon Oct 25 2010, 08:35PM

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

Autodesk drawings are done for the head module. You can DL them here.

1288038025 2893 FT90619 New Bitmap Image 2

Box is 3/4" wood with a hinge top, 10" square. I'll put a handle and locking snap on it too. Inside is the lead box, which is 20 cm wide, 12cm long, 16cm tall. I don't think I'll cast it all as one anymore, because things will likely go wrong somehow. Instead I'll cast plates and solder them together with plumbing solder + a pencil torch. Lead sheet will cover the edges since they'll be weak spots. There's a small piece of glass siliconed over the aperture hole, and the pb lid will be siliconed on too. It'll be filled with some sort of oil.

On back there's a 5 ohm rheostat and a switch. The switch turns on the filament and the 5 ohm rheostat will adjust the filament current. (with the HV removed of course) one can see the glow through the aperture to make sure things are OK. Power is supplied by two D cells. I'm going to design a little shutter to cover the aperature in case the glow causes a problem. I haven't designed the HV feedthroughs yet, but I'll likely just use park plugs. Since the tube has quite a low resistance I don't think insulation breakdown will be too much of a problem.

I've decided to build the tube module before I buy the x-ray head so I have an option to give up if things go awry.

uzzors2k

Wed Oct 27 2010, 01:00PM

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

Hey Grenadier, wait on buying from this guy

I purchased one as soon as I saw the link, forgetting to check his feedback. I haven't gotten any response from him yet, nor did the last guy. He's probably forgotten about the auction.

Adam Munich

Wed Oct 27 2010, 07:34PM

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

Okay. Besides, I'm going to build all the hardware before I mess with the electronics, let alone radiation. I plan on making plaster molds for the lead slabs today. They should be completely dry by friday then.

With the current size of the Pb box, it weighs 30 kg! I may use either 8mm lead or shrink the box a little.

klugesmith

Fri Oct 29 2010, 07:15PM

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

Grenadier wrote ...
With the current size of the Pb box, it weighs 30 kg! I may use either 8mm lead or shrink the box a little.

You are wasting lead and fuel to melt it.
4mm of Pb would attenuate 60 keV monochromatic radiation by 8,000,000,000 (lin atten 57 cm^-1)
and 80 keV monochromatic radiation by 50,000 (lin atten 27 cm^-1).
As you know, the x-ray spectrum from a Coolidge tube peaks at a small fraction of the peak kV,
even running on DC (and lower still, running self rectified).

If you can come up with a dosimeter charger, I will give you a tested CDV-138 dosimeter.
Yesterday at the dentist (for a wisdom tooth extraction

), the good fellow was happy to zap
a couple of the dosimeters with a normal diagnostic exposure. They registered 10 and 11 mR,
or roughly 0.1 mSv -- the dose to about 1 cubic inch of your body when your tooth is x-rayed.
A whole-body exposure at that level, repeated daily, would be way over the safety limits.

[edit] There is a standard way to compute the full-body equivalent dose when only part of the body is exposed.
Not simply proportional to exposed volume or mass, it accounts for the relative sensitivity of different tissues.
There's a table of organs and weighting factors which add up to 100% of the body.
I think the gonads get about 20% of the total, which is disproportionate even in my case.

Proud Mary

Sat Oct 30 2010, 12:28PM

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

It can be handy in the long run to have a good stock of kilogram lead ingots, so you can build shields to suit a particular experiment.

Adam Munich

Sat Oct 30 2010, 08:17PM

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

Alright, enough procrastination, I've started to build the box. I decided on these dimensions: 14cm * 20cm* 11cm with 6mm thick walls. That makes the box a manageable 20lbs, yet still provides adequate protection.

I decided to use plasticine clay to make a positive and use plaster to make a negative. Instead of using words, I decided to use pictures for this part of the build.

Rolling out the clay. (1cm thick guide sticks)

Clay's all rolled nicely.

It's like silly putty!

Making the template.

(20x14)

Cutting the clay to size.

That'll do just nicely.

Plaster time!

A minor inconvenience.

Clay placed in pan.

And plaster poured in pan. I had to use half the carton. I also realized just now it'll be tough to get out of there, but it's set now so I can't move it anyway. I'll get it out somehow.

I'm probably going to need another carton of plaster, but other than that things are going good.

Kludge, I'd love to take you up on that offer. However I won't need it for maybe a month because I want to build all the hardware before I mess with the electronics. As for your numbers, that's the exposure directly in front of the tube, and I'll be far from the aperture when this thing is on.

And proud mary I have 120lbs of lead muffins. I'd say that's a good stock. They aren't really good for stacking though, maybe sometime I'll turn them into bricks.

Bored Chemist

Sun Oct 31 2010, 02:18PM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

I still have a scar on my hand from pouring molten lead into a plaster mould.

The hot lead boils the water trapped in the plaster and the steam spatters the molten metal.

Adam Munich

Sun Oct 31 2010, 02:26PM

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

I learned that the hard way before too. That's why you're supposed to let it dry out for a few days. I plan on leaving it in front of my fireplace for a while, that should get all that water out.

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