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Maximise X-ray tube life

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aztec

Sat May 20 2017, 01:36PM

Registered Member #61568 Joined: Sat Apr 15 2017, 05:09PM
Location: Hungary
Posts: 2

Hello 4hv users, new guy here.
I've been experimenting with x-rays for some time, but recently i dug up an interesting tube from my collection, a Russian microfocus tube called BS7.
I made a few nice pictures with it, but because of its extremely low power rating the exposures take several minutes and i started to worry for its life.
Whats the best way to use something like this, close to the max rating to keep the exposures short as possible, or use lower current to reduce load on the target but then the filament may burn out faster?

Proud Mary

Sat May 20 2017, 11:45PM

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

Hello Mr Aztec,

I've had two Svetlana BS-7 microfocus transmission target tubes for about five years and both are still going strong.

I don't have the data sheet right in front of me, but the key parameters from memory are these:

Va = 4kV minimum, 15 kV absolute max for the Mk1 model, and 10kV for the Mk. 2.

Anode dissipation = 20mW

Vf= 0.4V minimum to 1.5V maximum
If = 1.5A. max

I recall a couple of cases online where people have got hold of BS-7s and destroyed them that same day by ignoring the parameters given in the data sheet.

The instructions for setting up the tube properly, including the conditioning process, are quite involved and I don't want to spend a few hours remembering them and typing them all out if not needed.

Anyway, to set it up properly you will need a 50Î¼A moving coil meter to measure the anode current, a 3A moving iron meter (or 1A etc meter that you can shunt out to cover 0 - 2A) to measure the filament current, and a variable voltage supply 0 -3V (or so) to power the filament.

Let us assume that you have gone through the conditioning process (which takes some hours) and the tube has passed A OK.

You are now ready to set up for experiment!

You hear that 9kV would be good for a first attempt at imaging a fly's wing. Power input must not exceed a nominal 20mW, so using Ohm's Law we see that this will correspond to an anode current of 2.2Î¼A.

Set anode voltage to 9kV.
Set filament current to 0A.
Slowly increase filament voltage until anode current is 2.2Î¼A.
Your tube is now in correct operation at the minimum end of its safe operating area.
Without adjusting any other controls, you may increase the filament voltage slowly until the anode current reaches 5Î¼A, which Svetlana give as the top end of the safe operating area. So long as you stay within the tube's safe operating area, it will run continuously for tens of hours without over-heating. .
(Svetlana say the minimum operational life of the tube is 400 hrs)

Some people may be surprised at the tiny amounts of power used by this device. It was originally designed for use in automated analytical laboratories aboard the Russian Lunakhod lunar rovers of 1969 - 1977 where power conservation was of the utmost.

Remember this is a microfocus tube. This means that immediately in front of the beryllium exit window, the entire output of the tube is concentrated in a dot less than 1mm across. If you scale this up in terms of kilowatts per square metre, you'll see that at the micro scale the tiny BS-7 can frighten off humungous tubes and their frightening power supplies.

Now think about the tube's beam divergence of less than 5Â° - from the very very rough geometry inside my head, this looks to mean that at one foot from the tube, your X-ray spot will be about one inch across.

I should have said 'those x-rays that can fight their way through one foot of air at atmospheric pressure for a distance of one foot' - because most of them won't make it. Using an old rule of thumb, two thirds of the output of an x-ray tube is in the lowest one third of energy distribution. If you have 10kV on the anode, two thirds of the x-ray output photons will have energies of 3.3keV and less - and having just worn themselves out getting through the beryllium exit window, the majority of these will give up the ghost after traveling from one devastating collision toanother, a total train wreck after just one or two centimetres.

But as a finale, let's get back to the fly's wing. Suppose you mount it 10mm in front of the exit window of your micro-focus tube where the focal spot is (notionally) 1mm diameter. At a distance of, say, 250mm, you find that beam divergence has turned your 1mm spot into a spot of, say, 25mm across. The spot has been magnified, that is, the spot and the information it contained since passing through the fly's wing has been magnified. You have made a simple x-ray microscope.

The best thing about BS-7 is that you can learn a very great deal from it, a thousand times more than producing just one more crappy radiograph of yawning mobile phones.

Have fun!

aztec

Sun May 21 2017, 06:46AM

Registered Member #61568 Joined: Sat Apr 15 2017, 05:09PM
Location: Hungary
Posts: 2

Wow, thanks for the detailed answer!

I've received the data sheet for the tube when i got it several years ago, but I wasn't able to decipher much more than the voltage and the current from it.
Well i didn't follow any break-in procedure, I just hooked it up to a Russian 13kv night vision power supply and slowly ramped up the filament until i measured 1.5uA current.
The 5Â° beam is interesting, my tube seems to cover the 35mm dental film at 60mm distance, the edges are less sharp but still usable.
I didn't had success with extreme enlargements, if there isn't enough contrast inside the small magnified part then the result is useless.
The real limitation is the film, i wrap into tinfoil but it reduces contrast and intensity somewhat.
I tried image intensifier too, but the resolution was pretty bad.
Here are a few examples, the pictures are edited

1495349117 61568 FT179845 Arn00540 800px

1495349117 61568 FT179845 Arn00616 800px

Proud Mary

Mon May 22 2017, 06:50PM

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

Svetlana BS-7 is a very interesting X-ray source and we are both lucky to have one.

It is the only tube I know of that was designed to work with as little as 4kV on the anode. I operate mine directly from a rectified 5kV-0-5kV neon sign transformer. In the European Union, ionising radiation devices - in this case, X-ray tubes - are not regulated or controlled by law when they operate with a potential difference of less than 5 kV across them - what could be simpler?

It's a strange fact that while there are many recent scientific papers on very low energy X-rays - like those produced by BS-7 - there is almost no interest among amateur scientists and hobbyists who always want more and more kilovolts to cook their DNA with :)

Edit added later: I have an idea for the perfect use of Ultra Low Energy Radiography - the imaging of bubbles - extremely thin layers of liquid separated by a gas. You could have bubbles consisting of liquid electrolyte through which a current is passing, bubbles of a dissolved chemical that react with chemicals dissolved in another bubble, different bubbles for different gases, dissolved radionuclides, the possibilities are endless.

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