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Wolfram

Tue Dec 07 2010, 10:28PM

Registered Member #33 Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971

You could wrap some x-ray cassette screen around the tube, and photograph it from different angles in a dark room.

Here

is a link to a video with the best x-rays I've seen taken with a normal vacuum tube and a fluorescent screen. I would say it's absolutely worth a try. If you are serious about this hobby, much better results can be had with a proper x-ray tube though, example here

. Dental ones are practical in size, so I would suggest buying one if you find a good deal.

Anders M.

radhoo

Wed Dec 08 2010, 12:01AM

Registered Member #1938 Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 699

Thanks, Anders.

How about the flash x-ray tubes manufactured by Svetlana? Proud Mary posted a few pics in the Projects section for his X-ray spectrometer.

Wolfram

Wed Dec 08 2010, 12:24AM

Registered Member #33 Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971

Hello.

The tubes Mary posted in her spectrometer thread are not flash x-ray tubes, they are regular x-ray tubes with beryllium windows, made for x-ray fluorescence applications and such. Unfortunately these are not really that well suited for general radiography, as they are made for relatively low voltages and currents. They would work fine for radiography of flowers, insects and things like that, which can be very interesting.

I don't see any dental x-ray tubes on eBay right now, but there are ones for sale there regularly.

Anders M.

Proud Mary

Wed Dec 08 2010, 12:44AM

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

radhoo wrote ...

Thanks, Anders.

How about the flash x-ray tubes manufactured by Svetlana? Proud Mary posted a few pics in the Projects section for HER X-ray spectrometer.

These are not flash X-ray tubes, Radhu, as Anders has kindly pointed out. Flash X-rays tubes work by a special case of vacuum breakdown with heavy conduction and very fast rise time in the nanosecond regime.

To follow Ander's point, Svetlana do have a very stylish little X-ray tube in the dental industry standard ~70kV 7mA in this season's catalogue:

My beautiful analytical X-ray tubes work at very low power levels compared with a dental tube. The reason I am able to power them with an ignition transformer and a two stage full-wave Cockcroft and Walton multiplier at 50Hz is because these tubes have maximum anode currents of 600ÂµA. Notice that the two types I have selected for this series of experiments do not even have water cooling, yet they can run happily for 48 or 72 hours continuously, which can be necessary in the primitive form of X-ray crystallography possible with my simple equipment.

My main interest is the X-ray world below 10keV. At the moment I can both generate and detect X-rays of 4keV, but hope to get down to 2keV in the next year. This is where much of new X-ray research is taking place, so I shall be in good company!

I have answered all of your questions relating to the highly non-linear energy response curves of GM tubes so many times before that if you search Google Images for "GM tube energy response" you will be directed to some of my previous answers here on 4HV.org.

As for your GM pancake detector, it may have failed as you suggest, but the same symptoms are produced by combinations of HT+ too high, anode resistor too low, and too much capacitance in the anode circuit.

Remember it is the job of the anode resistor to pull down the voltage across the tube to below the conduction potential after each Townsend avalanche, so if the resistance is too low, the tube will not quench, and continuous conduction may occur.

You will see that excess capacitance in the anode circuit will also tend to prolong the current pulse during conduction events while the capacitor discharges through the tube- which is why it is always best practice to extract the signal from the cathode end, although this is rarely done in hobbyist designs. Excess capacitance at the anode is the biggest single cause of erratic performance, pulse pile-up, increased dead time, and reduced tube life in GM tubes.*

I would suggest increasing the anode resistor to 20M, and seeing if this cures the fault, though you may have to increase the supply voltage to make this work.

There are - or more correctly were - specialist X-ray detecting GM tubes containing Xe, chosen for its high atomic number compared with the other noble gases. These tubes were operated at a much higher pressure/counting gas density than is usual in GM tubes, to increase the chance of collisions.

Today X-rays are measured by ionisation chambers, proportional gas tubes, GEM and micro-channel detector arrays, by scintillation counters, and solid state detectors, of which there is an ever growing number of varieties - RADFETs, PIN photodiodes, germanium detectors, diamond detectors, and so on.

* Centronics GM Tube Handbook.

radhoo

Wed Dec 08 2010, 11:46AM

Registered Member #1938 Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 699

Proud Mary, sorry for the confusion, and congrats on the high level of knowledge you've achieved in this very technical field.

Time ago, I've seen some flash tubes on a webpage or (not sure) on Ebay (you might know the seller, denys2472

), and the shape was extremely similar to those you've pictured. So in a superficial, quick assumption, I concluded they were the same type.
Now doing a quick search, the differences are obvious:

Anyway, changing topic, I assume my SI14-B is dead. The electrodes seem corroded in a HV style . The seller said he tests all his tubes, but I don't know if this is good or bad, I assumed he just put too much voltage/current in it.
The tube is initially stable, at lets say 400V . Then when the first particles enter the tube, they trigger a continuous conduction. I need to follow your advice and see the results. Some more questions:
- what about the SI-12B? ( I hate the diameter difference, but this is all I have)
- assuming the SI-14B is not dead, is it usable for detecting/dosing x-rays? I've look over the energy response graphic, that you've posted before. I will need to read/understand more on the topic before I can comment.

I am aware of the scintillation counters, you've seen mine :(
Regarding ionization chambers, I tried building one, but at that time I didn't have a suitable resistor to convert the very tiny current to a measurable voltage, probably I should give it another try.
Photodiodes unless they are manufactured for this purpose, I don't think regular ones would be efficient.
As for Radfets they might be abundantly available the next few years, but currently they are not really an option for a hobbyist.

I'd like to know more about your detection mechanism , especially one that can go as low as 4keV! I assume you've built a ionization chamber?
I hope my scintillation probe is not dead. Despite the small crystal surface, would be a very good tool. I could also try to replace the crystal with a piece of Intensifying screen material from a X-ray cassette.

Proud Mary

Wed Dec 08 2010, 01:56PM

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

radhoo wrote ...

Time ago, I've seen some flash tubes on a webpage or (not sure) on Ebay (you might know the seller, denys2472

I've bought from Denys perhaps a dozen times in the last few years, but I've never known him to have any flash X-ray tubes for sale.

Les Wright - "Plazmatron" in this forum - is the only person I know of who has done any experiments with flash X-ray tubes. He really loves them in the way that you might like brÃ¢nzÄƒ de burduf with a glass of tuicÄƒ to wash it down.

radhoo wrote ...

Anyway, changing topic, I assume my SI14-B is dead. The electrodes seem corroded in a HV style .

Perhaps - but Soviet soldering can often look like that.

You should remember that the sensitivity of gas detectors like GM tubes is strongly related to the counting gas volume. A big tube is more sensitive than a small tube because there are more atoms of gas for particles to interact with. As there are more interactions, we have bigger samples, and so greater accuracy in any one counting period.

I've just taken this picture of my G26 GM tube to give you some idea of the size of a dedicated X-ray GM tube. (This tube is a relic of the Heroic Age of Atmospheric Atomic Tests, a technical fossil of those heady days when the World Superpowers shared out abundant radio-iodine and Sr-90 to the children of the world, so none would be left behind by technical progress.)

radhoo wrote ...

I am aware of the scintillation counters, you've seen mine :(

Don't worry if you've damaged the PMT. It's much easier to get a good cheap Russian PMT, than it is to get a decent piece of NaI(Tl)

radhoo wrote ...

Regarding ionization chambers, I tried building one, but at that time I didn't have a suitable resistor to convert the very tiny current to a measurable voltage, probably I should give it another try.

Don't. You need equipment you do not have to calibrate and make sense of them. You cannot use scientific toy ionisation chambers when you are working with real X-rays.

radhoo wrote ...

Photodiodes unless they are manufactured for this purpose, I don't think regular ones would be efficient.

You are not right to say that common PIN photodiodes are unsuitable for X-ray detection.

These three patents - issued to the UK National Radiological Protection Board - use the common PIN photodiode BPX65 as a radiation detector.

The quantum efficiency of PIN photodiodes increases with decreasing X-ray energy, so that at 1keV efficiency is almost 100%.

BPX65 has a plastic lens which must be removed for detection below about 10keV. I have bought BPX65 on ebay for a few Euros, and similar but unbranded PIN photodiodes for much less. Making the charge amplifier to use with PIN photodiodes in photon counting mode is not difficult, but requires a very careful and disciplined approach if it is to work well because of the microscopic currents involved. Here is an application note from Maxim giving a complete circuit for a PIN photodiode gamma detector.

With X-rays you can use the PIN photodiode in continuous mode (easy!) because you are not trying to detect individual tiny charges.

radhoo wrote ...

As for Radfets they might be abundantly available the next few years, but currently they are not really an option for a hobbyist.

Again, not true. A number of cheap p-MOS transistors will give good results, which is how it was discovered in the first place.

I have just designed a very simple but effective and reliable sensor for X-ray dosimetry, and am experimenting with it at the moment. I will give out details in a few days when I have fully tested it, and will in any case be sending a few out to friends in the forum as a small Christmas gift.

radhoo

Wed Dec 08 2010, 02:31PM

Registered Member #1938 Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 699

Off-topic, you seem to know a lot about RO ("Branza" and the other things you said in older posts). Most of us use US layout keyboards, so we usually skip the diacritics. Should I write it like you did , I would need to find the unicode/utf-8 character code. We also have "urdÄƒ", but we buy "Ricotta", because like all our other values, they fall without a strong spirit.

I also have this big tube (I.F.A. G64W ):

But didn't have the time to run tests on it, and a quick search revealed nothing.
Your arguments stand, I'd add that the bigger the tube, the more space is checked for radiation, meaning that I would prefer to use a big detector next to my source, then a small one that maybe skips some readings because of the field's distribution/repartition.

For the same reason I was delighted to see the big mica windows of the SI-14B. You've revived hope for this one, and there are chances it would still work. In some rare cases the avalanche discharge stopped, so a bigger resistor might actually transform this into a very stable detector.

It was a nice surprise to see the material on Maxim's site. Indeed this is what I would also call "innovation" (delivered). The attribute exotic might also fit in well :) . So all in one I will consider this approach, I must admit that I fell safer with so experienced people around, and seeing other experiments.

The BPX65 is currently unavailable on ebay, however this little photodiode reminds me of one that I took out of a floppy driver - disk present checker. I understand that the goal is to have the semiconductor crystal as exposed as possible, so removing any extra-plastic would help?

The last part of your reply, reminds me how I cut the metal case of germanium transistor, cleaned the white substance to get a photodiode. I'm sure that to some degree, the photoelectric effect would work with x-rays as well.
On the other hand a 2n3055 transistor has a very big Si crystal, totally exposed if you remove the metal cap (very easy). Would that work?
What about an array of such tiny semiconductor detectors, arranged like the photo-cells in a fly's eye?

What about using CCDs directly?

Proud Mary

Wed Dec 08 2010, 05:04PM

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

radhoo wrote ...

Off-topic, you seem to know a lot about RO ("Branza" and the other things you said in older posts). Most of us use US layout keyboards, so we usually skip the diacritics. Should I write it like you did , I would need to find the unicode/utf-8 character code. We also have "urdÄƒ", but we buy "Ricotta", because like all our other values, they fall without a strong spirit.

1291825255 543 FT0 Little Blue Horror Gm Tube 002

As you can see, these tubes have been made to a very low standard, probably to meet the production quota of a 5 Year Plan. I call them the Little Blue Horror because the glass is so thin and fragile that you can break them with your breath. But the thin glass makes them a very good detector despite the poor engineering of the base and its pins. They were all made in 1960 - 50 years ago.

1291825281 543 FT0 Little Blue Horror Gm Tube 003

The ageing paper wrapped around the tube is its data sheet, each one filled out by hand after measuring the characteristics of one particular tube, a time consuming labour that you no longer see today except for very expensive hand-built parts like some X-ray tubes, big hydrogen thyratrons, and so on.

1291825318 543 FT0 Little Blue Horror Gm Tube

radhoo wrote ...

I also have this big tube (I.F.A. G64W ):

But didn't have the time to run tests on it, and a quick search revealed nothing.

A very fine tube you have there, Radhu. I would guess it is a high voltage type - somewhere between 950V and 1150V - but that is only an educated guess. It will not be difficult to make a graph of its curve, but you must have a variable voltage supply 300V - 1200V.

radhoo wrote ...

For the same reason I was delighted to see the big mica windows of the SI-14B. You've revived hope for this one, and there are chances it would still work. In some rare cases the avalanche discharge stopped, so a bigger resistor might actually transform this into a very stable detector.

Yes, that is possible. Pancake GM devices are used mostly in close up contamination detectors - used for examining people and surfaces for nuclear bomb fall out ash in this case. They will detect some X-rays, but are not designed to do so. The distance through the counting gas between the front window and the back is very short, and there is not much metal there for X-rays to knock electrons out of - the main source of GM events for X and Gamma rays.

radhoo wrote ...

It was a nice surprise to see the material on Maxim's site. Indeed this is what I would also call "innovation" (delivered). The attribute exotic might also fit in well :) . So all in one I will consider this approach, I must admit that I fell safer with so experienced people around, and seeing other experiments.

I have other circuits for use with PIN photodiodes using cheap electrometer FETs in the critical first stage.

radhoo wrote ...

The BPX65 is currently unavailable on ebay, however this little photodiode reminds me of one that I took out of a floppy driver - disk present checker. I understand that the goal is to have the semiconductor crystal as exposed as possible, so removing any extra-plastic would help?

Very soft X-rays can only travel a short distance through the air, so even very thin plastic will block rays below about 2 or 3keV. Experiments must have both the X-ray source and the detector in the same vacuum space.

radhoo wrote ...

The last part of your reply, reminds me how I cut the metal case of germanium transistor, cleaned the white substance to get a photodiode.

I'm sure you know that big old transistors are often packed with beryllium oxide, a very toxic carcinogen.

Facts/SF301.pdf

radhoo wrote ...

I'm sure that to some degree, the photoelectric effect would work with x-rays as well.
On the other hand a 2n3055 transistor has a very big Si crystal, totally exposed if you remove the metal cap (very easy). Would that work?
What about an array of such tiny semiconductor detectors, arranged like the photo-cells in a fly's eye?

There are a thousand and one experiments you can do to try and create new detectors, or improve old ones. But I would encourage you to have at least two completely reliable systems for detecting and measuring X-rays before you go much further.

radhoo wrote ...

What about using CCDs directly?

CCDs are becoming more and more important in X-ray detection and imaging. For one example only:

That's enough for now. You have me all worn out with all these questions!

Good Stuff Added Later

I have found some references to your G64W GM tube!

Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment
Volume 339, Issues 1-2, 22 January 1994, Pages 329-333.

Measurements of the level of soil contamination using helicopters

D. M. Farcaiu, Maria Sahagia and Stefania Sahagia
Institute for Physics and Nuclear Engineering, P.O. Box MG-6, Bucharest, Romania

Here we learn that a "system type ASGM 25 which contained a battery of 72 GeigerMiiller counters type G64W (Romanian, organic gas filling, 32 mm diameter and 360 mm length)" has been installed in a helicopter to look for 'lost' (really!) Co-60 and I-131 in the Romanian countryside and forests.

Then in this paper:
Comparative Technology Transfer and Society, Volume 7, Number 1, April 2009, pp. 66-110 (Article)
Analysis of Soviet Technology Transfer in the Development of China's Nuclear Weapons
Liu Yanqiong
Liu Jifeng

we learn a bit more about the "ASGM 25" special equipment:

"Uranium prospecting: The Soviet Union hoped to obtain uranium from
China, while China wanted to acquire the Sovietâ€™s advanced technology and
equipment. Under Agreements 55.01 and 56.12, the Soviet Union agreed to
provide these for uranium exploration. Regarding prospecting equipment and
apparatus, the Soviets provided aircraft, comprehensive aerial-surveying instruments,
radiation devices, and logging measurements. Prior to Agreement
54.01, when Soviet expert Lazutekin assisted China in uranium exploration,
Geiger counters were usedâ€”the first radiation detector made in China, which
had been assembled by the Institute of Modern Physics (Liang, 2007, p. 25).
Under Agreement 55.01, the Soviet Union did provide some equipment and
technology, including an AN-2 aircraft for aerial surveying, a set of ASGM-25
aerial-surveying instruments, and a complete crew for aerial surveying. Prospecting
crew 309 then organized an air force survey team (Wen, 2005)."

The first statement that G64W has an 'organic gas filling' tells us that it is a high voltage GM tube. Organic quench gas - very often ethanol vapour -
is an older technology than halogen quenching, and is typically used with a so-called Penning gas mixture in high voltage tubes. The plateau length in organic quench tubes is usually 200-300V, so it would be safe to start the tube at 1050V - 1100V with a 20M anode resistor, and it will probably work straight away (if it it still in working order, of course!)

You will notice that G64W is described as having a length of 30cm - this its effective length - the length of the sensitive detecting part - rather than the mechanical length, including terminals etc, which is obviously longer.

radhoo

Wed Dec 08 2010, 06:43PM

Registered Member #1938 Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 699

Indeed :) Your time and valuable explanations are much appreciated!

Proud Mary

Wed Dec 08 2010, 06:51PM

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

radhoo wrote ...

Indeed :) Your time and valuable explanations are much appreciated!

Radhu, I added a bit about your big GM tube to my last post above while you were posting your reply, so you may not have seen it.

You'll understand that by using 72 GM tubes in 'Soviet apparatus ASGM 25' they have multiplied the detector volume of counting gas by 72, and so increased the system sensitivity to a very high degree.

Why can't I have Soviet apparatus ASGM 25?

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