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Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
According to Lederer,* cathode oxide coatings consist of mixtures of any of barium, strontium, calcium, and zinc, with mixtures of barium and strontium oxides being by far the most common.
A glance at the characteristic spectra of these four elements suggests that with Va = 40kV only a barium-containing target will emit X-rays energetic enough to escape through the anode bell and the glass envelope. Increasing the anode voltage from c. 38kV to c. 55kV will increase the output fluence of the barium target - the quantity of rays - but will not increase their penetrating power at all. Increasing Va to 65kV will produce little significant increase in penetration, as the graph makes clear. But the extra energy imparted to the electrons will be dissipated as heat in the fragile barium target, and very likely ruin it.
Strontium target: feeble bremmstrahlung and main peak below 15keV
Calcium target: assorted weak peaks over 20keV - a possible contender, but not very likely.
Zn target: feeble bremmstrahlung and peak below 10keV.
Now if you were to gradually reduce the tube voltage to 30kV, if there was a sudden fall in X-ray output once those Ba peaks are past, it would support the barium hypothesis.
The light source may be due to incandescence, as of calcium limelight.
Field Emission is a type of quantum tunneling which occurs at points of very high charge density - as in the needle and flat plate type of cold cathode X-ray source. The needle model goes right down to the nano-scale, with field emission cathodes made of carbon nanotubes now being used in sub-miniature X-ray sources the size of a grain of rice.
This macro image shows a number of sharp points on the anode spot weld:
Notice strong shadows of the weld spikes in upper right quadrant of photomicrograph below:
x40 new
Here are photomicrographs of the anode bell coating at x80 and x200.
x80. Notice volcanic crater with three gas vents in the upper left quadrant. Also diagonal fissure below it. [color]re-done
x200 Note gold-coloured inclusions of unknown significance new
The cathode tube, and its supports, are made of a magnetic metal. The damage to the oxide coat was done by Spencer-Wells forceps.
Cathode bare metal tip
x40.
The tungsten-molybdenum alloy heater is insulated with either aluminium oxide - aluminia - or beryllium oxide - beryllia - HAZARD! - but may contain small quantities of other refractory materials necessary to the coating process.* The 2X2As folded heater design was simpler and less expensive to produce than the double helix kind. The rust-coloured markings suggest the presence of iron.
x40
*Shaw GR, Shardlow LR, Heaters & Heater-Cathode Insulation,Vacuum Tube Design, RCA, Privately Printed, 1940, p. 24 et seq.
*Lederer, LE, Filaments and Cathodes Part II, Lecture 2, Vacuum Tube Design, p.11 et seq. Privately printed, RCA, 1940.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
radhoo wrote ...
do you have a microscope adapter for your SLR?
I do have a microscope adapter for my DSLR, but didn't use it for the photomicrographs above, which I took using a 3MP digital eyepiece adapter in a Vickers metallurgical microscope.
Looking at the images in the cold light of day, I can see all their faults, and will replace them one by one with examples of better quality, when I can find the time. I am marking re-done and new images as "redone" and "new" in red.
LE: My own knowledge of chemistry goes little further than mixing up powders to keep slugs out of the cabbages, so I wish you luck with this chemical analysis. But I completely agree that we should try to find a verifiable explanation for 2X2's superior performance compared with PD500, 6BK4B etc.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
A very interesting project! :)
The pinhole camera might be worth a try. The "camera" of course is simply a lead sheet with a small hole poked in it. The instructions I saw for making it were to bash an indent in the sheet with a hammer and punch, then cut/file off the little bump that appears on the other side.
From looking at your results, the tube seems to have two point sources inside it. It would be interesting to get a pinhole camera picture of them.
I think the dark ring discussed earlier can be explained by the variation in thickness of the anode bell normal to the direction of the X-rays.
Registered Member #2893
Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242
I'm very impressed with all this testing. You should put this info somewhere on the open interwebs, and not in the for the most part unsearchable depths of this forum.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Steve McConner wrote ...
The pinhole camera might be worth a try. The "camera" of course is simply a lead sheet with a small hole poked in it. The instructions I saw for making it were to bash an indent in the sheet with a hammer and punch, then cut/file off the little bump that appears on the other side.
From looking at your results, the tube seems to have two point sources inside it. It would be interesting to get a pinhole camera picture of them.
I think the dark ring discussed earlier can be explained by the variation in thickness of the anode bell normal to the direction of the X-rays.
It's one thing to use a pinhole to image the anode spot of, for example only, a dental tube 70kV/7mA with a dose-rate of 1.5E+004 Sv/hr across the entrance to a pinhole 3cm from the target, (of which, say, 0.1% transits the pinhole) and another to form an image with the tiny fluence of a 2X2A in cold cathode mode and a few tens of microamps of current @40kV. Moreover, a dental tube's roughly conical beam is highly directional, while Radu's experiments suggest that the 2X2A output tends towards the isotropic, with only a small part of its X-ray output available for image formation on a plane surface.
As to the dark ring, I first thought the same as you - that it was the shadow of the anode wall cast by a source coaxial with, and below, the anode bell, but Radu's subsequent experiments suggest a multiple or disseminated locus of emission.
I am now wondering whether a slit formed from two razor blades might be able to form an image from which further deductions could be made.
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Vacuum Rectifiers X-rays report
