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Registered Member #1134
Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351
Proud Mary wrote ...
This paper looks a bit dodgy to me, Les.
"DC electric potential of from 500 V to 2 kV were applied, ... (and)... X-rays were observed by a Geiger-Müller counter." I suppose some of the 2 keV photons will have have transited the Kapton window, and heroically struggled through what would have to be a very short air gap - but then they would have to get through the end window or wall of the GM tube, and still have enough puff left to do some ionising when once they got inside. It just doesn't look right, does it? Elsewhere in the paper, and in the abstract, mention is made of 5 kV, but even here no one familiar with the ineffable mystery of the ultra-soft rays would ever think of using a GM tube to detect them. It is, of course, a paper translated from the Japanese, and the translator may have misused the term "Geiger-Müller counter" as a generic for all radiation detectors, but it certainly raises a question mark, doesn't it?
The problem with papers like this, it that they are little more than synopses, and no specific detection equipment is listed. I would say, regarding GM x-ray photon detection that, 5keV is likely, 2keV is possible under certain circumstances, and 500eV is a real stretch, even with a very long integration period, and large window (i.e. pancake tubes). Either way, as you and I know, GM tubes aren't really suited for this kind of work, even tubes of special design.
More likely, as you say, it is a translation error. It wouldn't be the first time I have heard of large area PIN diodes run in avalanche mode, being referred to as running in 'Geiger mode'. I'm pretty sure too, that Japanese is one of those languages where the logogram, can be interpreted in many ways, depending on its placement.
It may be that the x-ray flux, is of such great intensity, that even at 500eV, enough photons make it through a GM end window. In which case, I want one!
Ultimately, the only way to find out for sure, is to try it out......
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Here are a few interim results from an afternoon's tinkering that give some idea of the problems of measurement I have encountered at these low energies.
At 8 kV, all was Nil except (a) the ZP1430 on a Mini-Instruments Mini-Monitor GM Meter Type 510, which went full scale (b) the Fluke-Victoreen Low Energy X-ray Survey Meter (ionization chamber) which went out of range i.e. > 1000 μSv/hr.
I then resorted to the high reading Victoreen Rad-Check Plus, which produced an answer equivalent to ~40 mSv/hr averaged over three five minute periods, which seems a credible figure for a flea-power tube.
I have not worked up the ionization chamber results as to the volume actually irradiated, as I haven't figured out how to factor in a significant percentage of low energy photons not reaching the full depth of the chamber yet.
From this small experiment, it seems I shall have to work on making some new measuring instruments if I am to go much further below 8 keV.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
I looked at yesterday's tinkering again today, and realized that the tube's sudden enthusiasm at 8 kV, was in fact the W Lα1 line at 8.4 keV, which knocks the rather feeble continuous spectrum into a cocked hat! I think I may give it all up for knitting and furtive sips of gin with my afternoon tea.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
plazmatron wrote ...
Interesting results! It cant have been a geiger muller tube that was used then!
Due to the fact that 100% of the photons energy would be lost in a relatively small volume of air, a thimble sized ionization chamber would be ideal.
The other issue with the article(s) is its planned application. what exactly does one hope to sterilise at 1keV.......?
Les
I'll consider your last observation first. 1 keV would be more germicidal than UV in surface sterilization, but as for other uses proposed for the glow discharge sources, I've found an up-and-coming soft X-ray application from Hamamatsu that has some useful ideas around it.
Hamamatsu have just 4.9 kV/200 μA on their anode* in another version of their photoionizer:
These devices solve the problem of 'adhesive' charge build-up when pouring polystyrene foam beads through a plastic funnel into a polythene bag, and similar industrial operations.
I must go out and enjoy the seaside while the weather is so wonderfully good, but when rain returns next week, I am going to look into all this further, as we can't really hope to get anywhere below 5 keV until the problem is solved.
Now, as to the GM tubes: I will try to find out where between 3 keV and 8 keV, the ZP1430 end window becomes effectively opaque. As you suggest, the authors might have used an avalanche device in 'Geiger mode,' which was then badly translated, but I don't see how any mica end window GM tube could work at the low energy levels claimed.
As to ionization chambers, this seems to me to be the way ahead, in spite of the burden of corrections that must be made at low energies. But I don't think a thimble Farmer-type chamber would have much in the way of useable/measurable ion current at low fluences.
The difficulties in designing - dimensioning - a chamber for use below 10 keV are revealed by the plot below of transmission through 10 cm of air at atmospheric pressure:
At the moment I am thinking of a parallel plate design, where one plate is a sheet of 400 mesh copper (which I have to hand) rather than a sheet of beryllium which I do not. Obviously the mesh will block more photons than it lets through, but its open area is easily calculable, which can't be said for other options I have thought of, such as polyethylene coated with colloidal graphite.
I'll keep you posted as I learn more.
Stella.
* Why 4.9 kV? Perhaps because regulation stops at 5 kV in the EC?
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Results:
Set-up: Svetlana BS-7 (W transmission target) Vf 1.2V, Va 3 - 4.99 kV variable.
Detector: ZP1430, Mini Monitor type 510.
Distance: BS-7 Be window to ZP1430 mica window = 1 cm.
Here is a picture of what is going on in the 1 cm air gap:
There was no obvious increase over background until 4 kV, whereafter the count rate began to climb steadily.
At 4.99 kV ( ) the Mini Monitor was logging about 30 c.p.s. - mildly lively, one might say - and, of course, a purely relative reading - but a long way short of saturation. I could have perked up the fluence (and hence the counting rate) by increasing Vf to 1.5V, but this seemed an unnecessary extravagance, since I knew it could be done.
Conclusion: At this stage, I can't say whether the W transmission target and Be window of the BS-7 is not allowing anything below ~ 4 keV to pass, or whether the ZP1430 mica window is blocking the photons that get through the Be and make it across the air gap, or very likely a combination of attenuation by Be, air, and mica together.
In any event, I am 99% satisfied that a GM tube could not have detected the ultra-soft rays reported in the Japanese paper, so to increase certainty I will give it another go with a different non-transmission target tube to see if I can get the ZP1430 to detect anything below 4 keV. Looks like a case for the BSM-1.
PS. Would you like me to move all this stuff into another thread, as it has meandered some way from your project?
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Results with Svetlana BSM-1-Ni: picture
It looks as though I shall have to eat my hat.
I set up BSM-1-Ni with Vf 2.0V and the same 1 cm air gap between the Be window and the mica window of ZP1430.
Counts rose above background at 1.8 kV,* and the GM tube was saturated before it reached the magic 4.999 kV where my experiments stop.
This means that the Japanese could have detected much softer rays than I had supposed in the BS-7 experiment,
Here is a graph for the 1 cm air gap losses at 0 - 2 keV:
Given that at Va 1.8 kV the peak emission must be somewhere between ~0.8 - 1.3 keV - where there is significant loss due to the air alone - I will revise my guesstime down to about 1.5 keV for the minimum photon energy to get through the mica window into the GM tube.
Thanks to the proverbial loose screw, I also erred in not thinking through the consequences of using a fleapower 50 mW tube like BS-7, as against a stout 4 W source like BSM-1 - the tube traditionally matured for thirty years in a landfill site before being offered for sale.
I made a total bodge of the first experiment, but it is still impossible that any photons were detected with a GM tube with only 500 V on the source as reported in the Japanese experiment.
*Update Sunday morning: using a very much more accurate variable PSU (Bertan Model 313A) counting significantly above background does not begin until 2.25 kV then:
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
plazmatron wrote ...
If I had real money, I would have had a glass section made with moulded NW-40 ends on it, as a single piece.
In fact, thinking about it, I might ask for a quote on a bespoke glass section.....
Put me down for a few if you do get a quote. I could use ceramics for my application, but glass would be nice.
One of my other projects will require quartz glass, as opposed to pyrex, but I was planning on using NW25 fittings for that, but I may be able to use NW40.
Registered Member #1134
Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351
Ash Small wrote ...
Put me down for a few if you do get a quote. I could use ceramics for my application, but glass would be nice.
One of my other projects will require quartz glass, as opposed to pyrex, but I was planning on using NW25 fittings for that, but I may be able to use NW40.
Unfortunately I had already ordered the piece a while back, from Adams and Chittenden here:
The specific dimensions of my ordered piece were:
Double ended NW-40 (single tooled piece) Duran glass Length including flanges ~114mm
The costs in USD were (approx) For the tube itself $60.24 Pair of Teflon chain clamps, and Teflon centering rings with seals $95.85 Post is around $45
They can do a less expensive version of the tube, by simply joining two standard glass flanges together, but I didn't want a visible join for my tube!
If you are interested in glass pieces of the size above, Tom Adams, made about a dozen of them, since the tooling had to be set up anyway, so he has some of that same spec in stock now.
They are very helpful folk, and can create just about anything you like, and will happily supply one offs.
Registered Member #1134
Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351
So an update to this project.
My glass-work, clamps and seals arrived today, so this project has made a little leap towards completion. The glass piece is beautifully made, and will certainly fare far better than the original.
Since I could have this piece made to my specifications, it is a much larger diameter than the original, which is good for two reasons. Firstly the glass will be much less prone to the heat radiated from the cathode. Secondly there will be a much smaller chance of any vacuum breakdown at the high voltages used. In x-ray tubes, charges build up on the surfaces of the glass, and contribute to vacuum breakdown. X-ray tubes love internal space.
Below, is a photo of all the parts of the tube, as it now stands:
The tube partially assembled:
I have just ordered Aluminium tubing from which to build a heat shield, for the anode end of the tube. This will consist of a short length of tubing, pressed into the anode flange, and protruding slightly into the glass section, to prevent damage to both the seal and mitigate stress on the glass, from the radiated heat of the cathode, scattered x-rays and electrons.
I am tempted to come up with a better clamping arrangement than using the chain clamps. The tube will be mounted horizontally, and I fear the cathode assembly is just a bit on the heavy side. I would much rather have bolts take the strain, than the glass. Secondly there are metal parts within the chains, which may encourage corona formation around the highly negative cathode.
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Rebuilding an x-ray tube.
) the Mini Monitor was logging about 30 c.p.s. - mildly lively, one might say - and, of course, a purely relative reading - but a long way short of saturation. I could have perked up the fluence (and hence the counting rate) by increasing Vf to 1.5V, but this seemed an unnecessary extravagance, since I knew it could be done. 
