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Grenadier's big thread of RÃ¶ntgen related shenanigans

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Adam Munich

Fri Apr 08 2011, 10:26AM

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

Hmm... I just found a bazillion 80K 2 watt resistors. If I put 250 in series that is a 20 meg 500W resistor (I think, morning math isn't so good). Looks like /i can in fact make a dummy tube.

I'll make a schematic today.

Proud Mary

Fri Apr 08 2011, 01:51PM

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

Grenadier wrote ...

Hmm... I just found a bazillion 80K 2 watt resistors. If I put 250 in series that is a 20 meg 500W resistor (I think, morning math isn't so good). Looks like /i can in fact make a dummy tube.

I'll make a schematic today.

It can be very helpful to run off a few large schematics that you can write comments on, and then tick off each connection in the construction only when you have physically checked it. You can note what voltage or current is supposed to be where, and in what coloured wire, and then check it off only when its presence has been verified. You can add number tags to each wire, which relate to numbered wires on the schematic. Many blunders can be avoided this way.

Adam Munich

Sun Apr 10 2011, 02:39AM

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

Well I made the resistor, it's a little under 20 megohms.

I measured the voltage and current across it and at 36V into the zvs I get 65kV and 2.5mA through the HV side. Now there is a huge issue with loading, it's like 30kV per mA. This is definatley not right and I think it is due to huge corona losses on the large and pointy resistor string. I'm going to put it inside aquarium tube with oil and see if that helps. I also need a bigger resistor to test loading at lower currents.

Pics some other time.

Ugh, tired, can't put words together right. :p

klugesmith

Mon Apr 11 2011, 07:17PM

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

Grenadier wrote ...
I measured the voltage and current across it and at 36V into the zvs I get 65kV and 2.5mA through the HV side. Now there is a huge issue with loading, it's like 30kV per mA.

Sounds like the multiplier doesn't need improving. Good work! Now go make x-ray pictures!

James

Mon Apr 11 2011, 08:42PM

Registered Member #3610 Joined: Thu Jan 13 2011, 03:29AM
Location: Seattle, WA
Posts: 506

Huge drops are typical of CW voltage multipliers. X ray heads that use these have closed loop regulation where the anode voltage is monitored through a high value resistor and used to control the power into the transformer to compensate for the loading.

You can get away with open loop if you're sure to always power the filament before turning on the HV. Fail to do that and it can easily shoot up over 100kV and arc.

Adam Munich

Mon Apr 11 2011, 11:58PM

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

meh... So I'm getting plenty of current, at 36V in I get 3mA out, yet only 60kV. Theoretically I should be getting only 1mA out with my 26 meg resistor but it would appear that I'm getting more.

It looks like the only way for me to see if it'll work is to hook it up to my coolidge tube and and see what happens. I have my 1cm thick pb box so that ought to stop near everything, so provided there is no rain I'll be hooking up the tube tonight.

I don't have an LDO regulator for my heater yet, so I'm just going to use a rheostat for the time being.

But first, I need a nap.

Adam Munich

Tue Apr 12 2011, 04:41AM

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

Alright 4HV, I present to you the latest and greatest in scientific prototyping ingenuity.

It works :D

As expected voltage is dependent on current and current on voltage, but with 1mA flowing through the tube the anode is at 65kV, while 24V is being fed into the device. That means a 32V lipo ought to work fine. Seeing as things are dependent on each other it might be hard to get consistent exposures, but that's what photoshop is for.

Now here's a problem I expected; scatter. Quite a lot actually. If I leave the GM tube outside, 12 feet behind the lead box it sounds like it's being held up to some fiestaware. Pressed right behind the lead box it clicks even faster. Concerned about this I took shelter behind my sliding glass door and the clicks dropped down dramatically; maybe a couple more per second over background. This means the scattered X-rays are soft, and the only explanation I have for that is bremsstrahlung. I still have enough lead sheet left to make a nice vest so I might be doing that in the future. Either that, or I could make it remote controlled. That kinda makes the meters pointless though... (unless I find a way to monitor kV/mA via an arduino, then I could simply tx the data to an LCD on the remote. I also managed to saturate the GM tube by putting it in the beam, that was pretty neat, and I fixed the unsteady-ness of the kilovoltmeter by putting the resistors under oil with the tube (pix to come)

And of course I couldn't call it a night without x-raying a few things.

A hard drive.

A pen I stole from my friend.

Proud Mary

Tue Apr 12 2011, 12:38PM

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

Grenadier wrote ...

Now here's a problem I expected; scatter. Quite a lot actually. If I leave the GM tube outside, 12 feet behind the lead box it sounds like it's being held up to some fiestaware. Pressed right behind the lead box it clicks even faster. Concerned about this I took shelter behind my sliding glass door and the clicks dropped down dramatically; maybe a couple more per second over background. This means the scattered X-rays are soft, and the only explanation I have for that is bremsstrahlung. I still have enough lead sheet left to make a nice vest so I might be doing that in the future. Either that, or I could make it remote controlled. That kinda makes the meters pointless though... (unless I find a way to monitor kV/mA via an arduino, then I could simply tx the data to an LCD on the remote. I also managed to saturate the GM tube by putting it in the beam, that was pretty neat, and I fixed the unsteady-ness of the kilovoltmeter by putting the resistors under oil with the tube (pix to come)

Remember that if you have Va = 65 kV then less that 1% of your photons will be 65 keV. Most of them will be between ~15 - 30 keV, because the number of photons actually produced increases as photon energy decreases. About 0.03% of 30 keV X-ray photons will survive a journey of 100 yards through the air, and be easily detected.

So far as I understand your setup, the entire output of your tube will be bremsstrahlung, because W K-shell electrons have a binding energy of 69.5 keV. To kick any of these out, your bombarding electrons must have energies greater than 69.5keV i.e. in practice, you'd have to stick about 72 kV and more on the anode - hence the standard 75 kV dental tube.

When you succed in kicking out a tungsten K-shell electron, its hole will immediately be filled by an electron from the L-shell - binding energy 10.2 keV. The difference between these energy states - 69.5 keV and 10.2 keV - gives us the W characteristic X-ray photon energy 69.5 - 10.2 = 59.3 keV

radhoo

Tue Apr 12 2011, 03:06PM

Registered Member #1938 Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 701

congrats on your first results!

for your meters, you might be interested in getting one of those cheap Chinese wireless cameras, that broadcasts in 2.4GHz. Then you can watch your setup remotely on a TV. Otherwise it might be difficult integrating some sensitive digital measuring equipment into your HV circuit.
Other options: use a camera to photograph your instruments, triggered remotely or with a timer.

Xray

Tue Apr 12 2011, 03:55PM

Registered Member #3429 Joined: Sun Nov 21 2010, 02:04AM
Location: Minnesota, USA
Posts: 288

X-rays do scatter. It's a nature of the beast, and there isn't much that you can do to mitigate all of it, short of lining your entire room with 10mm thick lead, and standing outside during exposure.

As you know, I repair dental X-ray heads for a living. I receive scattered X-rays to my body every time I energize a head for testing. When I first got into this business about 13 years ago, I bought a lead-lined vest, which I wore every time I needed to test a head. But I soon stopped wearing it because it was heavy and cumbersome, and the slight amount of scattered X-rays that I receive during each exposure is negligable (I of course avoid the main beam by standing behind the energized head). Using a standard GM Geiger Counter, I was measuring various intensities of radiation all around my shop (which is the size of a 2-car garage). No radiation extends beyond the walls, and so my neighbors have nothing to worry about. The bottom line is, most scattered radiation that I receive is low-energy, which probably doesn't penetrate my skin. Also, most test exposures last less than one second, and I typically make 3 or 4 test exposures (<1 second each) to make sure that the repaired head is operating within manufacturer's specifications. I check KVp, mA, and MR, as well as off-axis radiation.

In general, where safety is concerned, a person needs to be reasonably protected from the threat without being overly protected to the point where he/she can no longer function. So, should you wear a lead lined vest? Sure, so long as it doesn't become a burden, and if the threat of damage to your health is significant. If you are not certain about the risk, then it's better to be safe than sorry.

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