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Rebuilding an x-ray tube.

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LAN_403

plazmatron

Wed Apr 04 2012, 06:21PM

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

As some of you know, I have done quite a bit of x-ray work on here, and I also produce x-ray prints on a fairly regular basis as a hobby.

For a lot of the work, I have been using the Russian BSV-25 tubes. These are x-ray diffraction tubes, and are excellent for low keV radiography, from 2-20 keV.

Surplus tubes however, are quite old, and I have not yet, out of any I have bought, managed to run them, for more than 6 months before they start to become unstable. After 10 months or so, they become unusable.

For an undetermined reason the vacuum becomes 'soft' over time, and eventually the tubes flash over internally. This softness does not seem to happen during their 30 years of storage, but happens during use. They have always been run within the specified parameters, and so I conclude it is either outgassing of the internal components, or leakage through the beryllium window seals, due to radiation damage.

These tubes were pretty expensive (Â£80 to Â£100), and for the last few months seem to be no longer available, so not wanting to be deprived of my x-rays, I set out to rebuild the tube.

First of all the tube was sawn open with a diamond saw. The glass was discarded, and the Anode, and heated cathode retrieved. (measurements of the cathode to anode distance, and others were taken here!)

The Anode was turned down on the lathe to mate with a brass NW-40 vacuum flange. It was designed to be a tight fit, and was hammered on, and then the joins, inside and out, were silver soldered. Silver solder is the method of choice here, as it has a much higher melting point than lead tin solder, and a much lower vapour pressure.
A 10 mm hole was also bored into the anode body, to accept a 10mm diameter copper tube, and an NW-10 flange for connection to the vacuum system.

A short length of Pyrex tubing was acquired for the tube body, and this was epoxied into NW-40 Aluminium flanges. Ordinary epoxy cannot be used. It must be vacuum compatible, and the choices are Torr-Seal or Hysol 1C. I used the latter since it is MUCH cheaper!

An NW-40 stainless steel flange was obtained for the Cathode end of the tube, into which was soldered a ceramic feedthrough for the heater. A turned copper ring was soldered to the inside face to provide support for the cathode assembly.

An aluminium tube was carefully machined, to push fit over the copper ring on the inside of the cathode flange, and the other end push fits into the stainless steel cathode jacket. A hole was machined into the side of the Aluminium support, to allow access to the heater connections.

NW flanges were chosen, since it makes assembly, and disassembly very convenient and easy.

Above are images of the cathode assembly mounted in the tube body, and a view of the filament itself.

And above is the assembled tube.

It should be noted that this re-built tube is designed to be permanently connected to a high vacuum system. I use a small Edwards EO50/60 diffusion pump, backed by a refrigeration service pump. The combination will reach operating pressure in around 7 minutes .

The biggest bonus here, is that the tube will essentially last forever. Being demountable, means I can clean the internal glass surfaces, re-finish the anode surface, and even replace the filament!

The tube has so far been on the vacuum rig to check for serious leaks, and is good. I have even produced x-rays with it at reduced voltage, and current. I am still awaiting some vacuum fittings, so it can be re-installed into its lead lined steel case.

Leslie Wright

Proud Mary

Wed Apr 04 2012, 07:11PM

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

Industrious and ingenious, and good to see an old horse shod with new shoes.

Thomas W

Wed Apr 04 2012, 07:18PM

Registered Member #3324 Joined: Sun Oct 17 2010, 06:57PM
Location:
Posts: 1276

that is VERY well made, looks epic :D

N_N

Thu Apr 05 2012, 10:43PM

Registered Member #4000 Joined: Wed Jul 13 2011, 06:27PM
Location: Russia, Moscow
Posts: 3

That's awesome

Nice job!

I have an experience in using such a tubes (BSV series) in x-ray diffractometers they are intended for. Normally the good ones work for years and usual reason of malfunctioning is than filament failure.
If the tube (usually at first run) flash over internally it is often helpful to run it without anode voltage with filament current close to maximal for a specific time (hours). Then the filament works as a getter.

plazmatron

Fri Apr 06 2012, 01:09PM

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

Thanks Guys and Gals!

Regarding the flashover, it never occurred at first run, and seemed to be a gradual thing.
I tried all the usual tricks including running the heater for hours, and applying anode voltage, at a very low duty cycle (1s on 30s off) for hours on end too, and whilst this improved the vacuum somewhat, it was only a temporary solution. Eventually the vacuum become so poor, that the tube would sustain a discharge with no heater voltage applied, and judging from the discharge, and the voltage that would initiate it, it looked by then to be at a pressure of around 1x10e-3 torr. My rotary pump can pull a better vacuum.

I even tried flashing the getter, though at this pressure it was doomed to fail. Yes the tube has a getter! If you remove the cathode connections, (one part unscrews, and the other parts must be unsoldered, and removed, as must the block of white silicone rubber (a very entertaining job getting you soldering iron down that end of the tube I might add!)) you will find a third lead that has been cut off, and this is for the getter.

If you click the image above an take a look in the rectangular window in the side, you can see the getter filaments (there are two in parallel).

Interestingly, if you look at the sixth picture in the above post at the cathode assembly, you will see it is in two parts. There is an outer stainless steel shell, and the plain steel cathode assembly. If you enlarge the picture you will note two brown stains on the surface of that steel assembly. It is rust! That was noted the moment it was removed from the tube, and the tube still had some vacuum, before I cracked it open!

So who knows, I could have just been unlucky, and gotten tubes from a bad batch, or maybe they had been stored in a cold wet warehouse somewhere (I have actually seen really badly corroded tubes show up on fleabay).

Ultimately, they would have died anyway, as you say from the filament going open circuit. As I recall from the datasheet the lifetime quoted was pretty low.

Proud Mary

Fri Apr 06 2012, 02:02PM

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

Les, you didn't mention at what voltage these flashovers occured, and with what anode elements, and I wondered how much thse variables might have to do with it.

I'm sure you will have done all the necessary graduated 'ageing' and such to re-absorb out-gassed gas, so perhaps you are right in thinking of slow leakage - permeability, porosity - of the windows or their seals over the years.

My own BSV-25 has a silver anode, and I have run it for five periods of 72 hours at 8kVDC without noticing any change of parameters - though this is hardly a stressful test, and few will be interested in this tube's very capable low energy performance.

I have also seen tubes offered for sale on ebay which may well have come from landfills, or ruined buildings, the plain brown cardboard of Soviet packaging buckled by wet, and black pits of corrosion visible on the tube windows.

plazmatron

Fri Apr 06 2012, 04:40PM

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

The flash overs on the last tube started at around started at around 20kv to begin with (you could actually hear it, long before the discharge became visible), and as the tube aged dropped to around 10kv, where is was no longer a flash over, but a sustained pale white discharge.

I lost one Fe anode tube, which I am quite annoyed about, and two Mo anode tubes. I'm annoyed about the Fe anode tube more because of the ~6keV peak, which was very useful. However, now I have the opportunity to use any anode material of my choice

One of the things I did note, which pointed me in the direction of Be windows, was that when the tubes were purchased, it was evident they had a coating on the Be windows (which fits with what I know from the patents). During the use of the tube, the coating changes from clear to brown, in the exact portions exposed to the x-rays. An image of the anode heel was quite visible on the windows. On the third tube I re-coated the windows with a substance called Vacseal by SPI. This is one of the coating materials used by modern tube manufacturers to coat the windows on x-ray tubes, as well as to bond camera tube faceplates etc etc. It was very expensive at Â£80 for 0.5 floz! And in the end didn't cure the problem, but it sure will come in handy in the future no doubt!.

The windows themselves seem to be bonded in with an epoxy, so it possible that they failed somehow.

These tubes, especially after the first one quit, were really well looked after by me. They were run in properly, cooled properly and powered appropriately, however I got far more than 72 hours use out of each. I have taken well over 400 good radiographs with them, with exposures ranging from 20 minutes, to over an hour and a half.

Yes, some did indeed look like they had been pulled from landfills!
Like I said, there is every possibility I just got unlucky!

Although losing tubes is pretty heartbreaking, I now have a much more useful and flexible tube.....

Proud Mary

Fri Apr 06 2012, 05:43PM

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

I've also noticed the state of the varnish or resinous coating on the windows, but have avoided poking it for obvious reasons. I've never been quite sure of its role - whether to add mechanical strength to the window, protect it from oxidation and corrosive gases, or add a secondary seal over the whole window assembly - or all of these things at once.

Yes, you will be able to sample the full Ã la carte anode menu with your new demountable tube. Even scandium!

plazmatron

Fri Apr 06 2012, 07:27PM

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

From what I have read it is described variously as either an environmental barrier, or to prevent gasses leaking through the windows.

I was just looking at Scandium earlier, from the Amptek emission line chart you once sent me! High melting point too!

Incidentally, the anodes in these tubes are coated somehow with the target material. It looks like metal foil of the material of choice has been fused to the heavy copper anode. It is maybe 0.5mm thick.

I will have to build some form of spring clip arrangement, that will allow me to mount thin metal samples over the original anode.

Proud Mary

Fri Apr 06 2012, 07:46PM

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

Effective thermal coupling between your target sample and the anode block might present a challenge, but I suppose it doesn't matter that much if the target pits and starts to vapourise, if you have a trap of some sort in the pump line.

I noted you are using a fridge pump for backing now. Did your Javac conk out?

PS: I see that Denys has sold out of everything except some doubtful looking BSM1-Fe and BH3-Ge, which I've never much liked the look of, so that's that for exotic anodes at affordable prices for the time being, at any rate.

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