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Uzzors' Coolidge X-ray Machine

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uzzors2k

Fri Jan 30 2009, 07:46PM

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

It's about time I tried using a Coolidge X-ray tube I bought on Ebay last summer. I was given no information on it other than it's a dentist tube, so everything will need to be determined empirically. The x-ray tube is 30cm long and 6cm in diameter. The filament has a resistance of about 0.2 - 0.3 Ohms, and is wound "Benson" style, or as a helical coil. So where do I start? I'll need to know an approximate power rating of the tube, and maximum voltage. Since the x-ray intensity is proportional to the filament temperature, I thought I could measure the anode current at very low filament temps, and assuming the anode current rises linearly with filament current plot a graph. A suitable filament current range can then be determined.

I don't even have ballpark figures for this tube, so your guess is as good as mine! Tube power and tube voltage.

Download

Fri Jan 30 2009, 10:38PM

Registered Member #561 Joined: Sat Mar 03 2007, 02:46AM
Location: Adelaide Australia
Posts: 230

So it's got no numbers or markings at all?

uzzors2k

Fri Jan 30 2009, 10:59PM

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

Whoops, thanks for the reminder.

It says:

Ritter
CYB
Made in USA

There's also a serial number stamped on the anode: F35182.

rp181

Fri Jan 30 2009, 11:44PM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529

You can try emailing the company, or finding a similar sized tube.
Also, congratulations on the 1k post =)

Proud Mary

Sat Jan 31 2009, 01:43AM

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

Dental X-ray tubes almost always have anode voltages in the range 50kV - 80kV (with 70kV being much the most common) and anode currents of 5 - 10mA, with 10mA being the most common.

Remember that dental X-ray tubes are designed for short exposures - e.g. 1/15th second - and so will rapidly overheat and destroy themselves if run for any length of time at full power.

uzzors2k

Sun Feb 01 2009, 07:43PM

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

Thanks for the tips. I sent an email to some Ritter repairman, so I'll see if I get an answer. For future refernece, I found some info here on using coolidge tubes of unknown ratings.

W007! 1000 posts!

c4r0

Sun Feb 01 2009, 09:23PM

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

Hello!

Take a look at these tubes:

Maybe you will find one that looks similar to yours - if it looks similar it should have similar parameters too. You can find also some useful characteristics on that site, and a FAQ section with explanations what each parameter of a tube means:

Harry wrote ...
Remember that dental X-ray tubes are designed for short exposures - e.g. 1/15th second

I've had a broken Orix-70 dental x-ray machine and the control unit had exposure time settings up to 3s or so. But maybe such long times was available only at lower anode current settings, i don't know.

Proud Mary

Mon Feb 02 2009, 02:28AM

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

The reason dental X-ray tubes have very standard ratings is because of the type of work they do - the amount of radiation that must be absorbed by teeth to produce an image is very standard, as are the legal amount of radiation that may be absorbed by the patient, so you won't find much variation from tube to tube.

Assume that the tube is intended to operate at 70kV/5-10mA and you won't be far wrong.

Do you have the RadPro radiation safety freeware calculator, Uzzors? This will calculate the dose in Sieverts for a given tube at a given range, and the amount of radiation shielding required and so on.

If you choose the X-ray menu, you can enter 70kV anode volts, and see how increasing the current increases the dose at any given distance, how filters of different metals will reduce it, and how different shielding materials will attenuate the rays.

uzzors2k

Mon Feb 02 2009, 07:36PM

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

I'll run the tube at low current (1mA range), since I'll be using photo-paper anyway. It will make controlling the exposure time much easier too. I've made a shield calculator using mass attenuation coefficients, so I can use the data at physics.nist.gov. My spreadsheet gives the I/I0 factor, and I have a list of radiation doses so I know what the safe range is. All I need to know is the worst (best

) case output from the tube. Does 30-100mSv/hr sound reasonable for a dental x-ray? I found some numbers for x-ray exposure related to medical x-rays, and the average dental x-ray dose is 0.1mSv, per image I assume. Currently I've finished epoxying MOT laminations together into two 3-4cm thick slabs, I'll make exact measurements when the epoxy dries. They should provide more than adequate shielding for 70keV x-rays, and with some distance I'll be safe from backscatter which I doubt is a problem with such low energy.

I have a copy of RadPro, but I'm not sure what some of the inputs mean. Is distance the distance from a pin-point x-ray source to the subject? When I put a tiny number in for the distance the radiation dose is in the order of 1*10^70 mSv/hr! Is that true? It seems the dose approaches an infinite value as the distance approaches 0, regardless of voltage/current. Is the shield placed at the tube or at the subject, or does it matter?

Oh well, assuming 100 mSv/hr exposure rate, and a 3cm iron shield providing near 180 degree protection, the radiation will be well below background radiation levels.

Proud Mary

Tue Feb 03 2009, 01:36AM

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

RadPro is a well-accepted practical dosimetry application, so does not concern itself with theoretical pin-point sources and infinite numbers.

If you enter 70kV anode voltage and 1mA current, you can rely on RadPro's distance, filter, and shield calculations for all practical purposes with a real world tube such as yours - and see how very few seconds it takes for you to absorb a hazardous dose if you expose your body to the rays at short distances, as I am sure you know.

As for the size of your anode spot, you may image it easily enough by making your own X-ray pinhole camera, if you have what it takes to drill very small diameter holes in a metal sheet. There is a lot about X-ray pin hole cameras on the web to guide you.

Post Script: the best way to control the anode current is by having a variable heater voltage.

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