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Registered Member #33
Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971
Proud Mary wrote ...
I've just noticed a used dental tube on fleabay UK, a buy-it-now for £20.
It looks a bit forlorn and sorry for itself, but the seller says the filament is good.
It's not my kind of thing at all, but it might be good as a starter tube for someone here, but as ever with ebay, caveat emptor!
That tube is not in bad condition, the glass is a bit dirty and a pin is bent, but aside from that it looks fine. It actually looks to be of the biased filament cup variety, as it has three pins with solder on them.
Registered Member #33
Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971
radhoo wrote ...
Anders, from what book is the page you've scanned? How should we interpret the pinhole image results in the case of the 2x2?
The scan is from "The Essential Physics of Medical Imaging, Second Edition" , this is an excellent book about these kinds of things. It even contains energy response curves for (Gd2O2S-based) fluorescent screens and all sorts of other information that's hard to find anywhere else. I've read a few medical books on radiography, and most of them concentrate on patient positioning and image interpretation, they usually contain very little useful information, but this book is an exception to that rule, and I can recommend it to anyone with an interest in x-rays.
When we're on the subject of good x-ray books, I can also recommend "X-ray Studies" from General Electric, which is freely available on archive.org . It's a collection of papers on hot-cathode x-ray tubes and related subjects. It includes W.D. Coolidge's original paper on the hot-cathode x-ray tube and some follow-up papers, and a lot of other interesting documents.
I'm not sure how I would interpret your 2X2 pinhole images. As you say it seems very paradoxical. Maybe Plazmatron's experiments will clear things up, when he receives his 2X2s.
Registered Member #1134
Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351
My 2X2's turned up today, thanks to Gintaras!
So first off I had to test this out! The radiation this little tube produces is totally amazing! It will actually conduct mA's if pushed hard enough, but of course this will quickly destroy it.
So I set up the pinhole camera, and produced a pair of images that settles where the radiation comes from.
Nearly all of the x-rays come from..................the heat shield, making the edge of the anode bell the most likely emitter of electrons. (I had had incorrectly surmised the entire surface was emitting electrons in the direction of the heater)
The photo below is a pinhole image looking down onto the heat shield.
The top surface of the shield is obviously a bright emitter of x-rays. The black structure in the middle is the heater and cathode support obscuring the x-rays.
Note underneath the heat shield some of the glass support can be seen, (note also the hole for evacuation!) There are slots cut in the heat shield, so electron beams make their way through, and impinge on the glass support, causing parts of it to emit x-rays.
Below is another view of the 2X2 this time looking up towards the underneath of the heat shield. Note the original heated cathode is a poor emitter of x-rays, and is actually emitting less x-rays than the glass support is! Note also that the underside of the heat shield is visible since the x-rays produced at the top surface make their way though.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
What super images, Les. Well done!
Didn't someone identify the heat shield as the probable X-ray emitter earlier in the thread?
The radiation on the underside of the shield must inevitably contain the characteristic spectra of the metallic elements used in the shield, including the pressed barium alloy 'pill', in addition to any target transmission shoot-through.
Anyway, that's all absolutely capital, and I'm looking forward to making my own experimental contribution this weekend, with dose rate measurements for the 2X2A.
I've potted the C&W module, so now only have the experimental jig to improvise out of lab scaffolding and stock lead laminates.
Registered Member #1134
Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351
Proud Mary wrote ...
What super images, Les. Well done!
Didn't someone identify the heat shield as the probable X-ray emitter earlier in the thread?
Thanks! I thought someone had mentioned it too, but I cant find the comment in the thread, I had thought it was you. Maybe it was Radhoo? All of this is coming together quite nicely.
Proud Mary wrote ...
The radiation on the underside of the shield must inevitably contain the characteristic spectra of the metallic elements used in the shield, including the pressed barium alloy 'pill', in addition to any target transmission shoot-through.
Absolutely. Do you know anyone with Sprectrometer?
Proud Mary wrote ...
I've potted the C&W module, so now only have the experimental jig to improvise out of lab scaffolding and
The tube I tried drew whatever current my HV supply would throw at it. Yours being somewhat beefier than mine, will need quite a bit of ballast resistance to stop the tube vapourising!
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
plazmatron wrote ...
Proud Mary wrote ...
What super images, Les. Well done!
Didn't someone identify the heat shield as the probable X-ray emitter earlier in the thread?
Thanks! I thought someone had mentioned it too, but I cant find the comment in the thread, I had thought it was you. Maybe it was Radhoo? All of this is coming together quite nicely.
I first proposed the shielding plate as the source on
Tue Jan 18 2011, 04:02PM
when I wrote:
Thinking about the concentric shadow surrounding the anode shadow in your radiograph, I wonder if it can be the longitudinal shadow of the anode bell from X-ray photons produced by field electron impacts on the circular shielding plate?
It certainly is an interesting feature worth investigating. If you have a good magnet, and place it near the shielding plate it might cause sufficient disturbance of the field emission electrons to show up on the radiograph
Which started me off on making a detailed investigation of the heat shield.
Now, Les, if you were to locate a strong magnet abreast of the heat shield, and then take another pin hole radiograph, we may see something of interest.
plazmatron wrote ...
Proud Mary wrote ...
The radiation on the underside of the shield must inevitably contain the characteristic spectra of the metallic elements used in the shield, including the pressed barium alloy 'pill', in addition to any target transmission shoot-through.
Absolutely. Do you know anyone with Sprectrometer?
I think I can feel an energy dispersal moment coming on:
Registered Member #1938
Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 699
The shielding plate seems to be a good answer for several observed aspects: - why do we get two concentric shadow rings, when the tube is placed with the anode perpendicular to the fluorescent screen (one would be the anode cap, the other could be from the anode bell) - the uniform distribution of the x-ray emission, all around the tube - the good performance at 45 degrees - the different performance between the Russian variant and the USA variant (that doesn't have a heat shield)
EDIT: for some reason I missed Les' post so here are a few comments:
plazmatron wrote ...
It will actually conduct mA's if pushed hard enough, but of course this will quickly destroy it.
My 100uAmp-meter went off-scale in normal polarization, and I was using a limiting resistor.
What surprised me event further, is that even if the glass envelope can get warm after a few radiographs (a few sets of 15sec usage), I've seen no metal part getting incandescent (in the Russian Variant).
plazmatron wrote ...
Nearly all of the x-rays come from..................the heat shield, making the edge of the anode bell the most likely emitter of electrons. (I had had incorrectly surmised the entire surface was emitting electrons in the direction of the heater)
Well, who would have had imagined such a trajectory for the electrons?
plazmatron wrote ...
Below is another view of the 2X2 this time looking up towards the underneath of the heat shield. Note the original heated cathode is a poor emitter of x-rays, and is actually emitting less x-rays than the glass support is! Note also that the underside of the heat shield is visible since the x-rays produced at the top surface make their way though.
Can you post a photo showing your setup for this experiment? What have you used to record those pictures with such an excellent quality?
ps: I got the tube on ebay. hope will perform better than the 2X2s :)
ps2: Les' images with some additional software enhacement. I would really like to know more on how these photos where made. Great job, so many details visible!
ps3: Still if the shield is a target for the electrons coming from above, how come we get so much radiation in the shield's plane?
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Vacuum Rectifiers X-rays report
