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Registered Member #1938
Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 701
About Gintaras: Now I understand how does he manage to carry his bike all around the globe. Nice hobby, a bit risky.
I believe I finally have some answers explaining the 2X2 case: 1- I also believe the entire bell surface is an emitter. If this wasn't true, I wouldn't have obtained such an uniform illumination on the fluorescent screen. The emission is stronger at 360degrees around the tube, were the space between the bell and the central filament is shorter, and dims gradually towards the top of the bell. However the illumination remains uniform both on horizontal and vertical plans, as I have shown with all the pictures and measurements above. 2- some information from a Romanian forum, on a thread focused on vacuum tubes: the oxide covering the filament is either barium oxide or strontium oxide. I would assume this is barium, but Proud Mary could help me with a simple chemistry test: put a little piece of the oxide in an colorless flame: barium makes the flame green, while the strontium makes it red. Barium oxide would be an excellent target for electron bombardment and I believe it can hold more damage than a simple tungsten filament. 3- the metal grill with crystals pictured by Proud Mary under microscope, is used to absorb the remaining gases after the tube has been vacuumed. So the crystals could be the resulting compounds of highly reactive chemicals placed on that plate. I don't think the plate has any influence on the x-ray output. 4- All the above is nothing new for other tubes as well. So what makes the difference? Take the following picture as a reference: I invite the readers to take a guess on which of the tubes above would make the best X-ray emitter, from left to the right: 2X2 , V1-0.1/30 , VI3-70/30 , DY86, PY88, 3BW2 , 3A3C , 3CZ3 , 6LJ6A I believe I know the key criteria that causes the abundant emission for the 2X2 (at least as compared to the tubes I currently have). I still need to run some tests to confirm it, so I will get back on this.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
radhoo wrote ...
About Gintaras: Now I understand how does he manage to carry his bike all around the globe. Nice hobby, a bit risky.
I believe I finally have some answers explaining the 2X2 case: 1- I also believe the entire bell surface is an emitter. If this wasn't true, I wouldn't have obtained such an uniform illumination on the fluorescent screen. The emission is stronger at 360degrees around the tube, were the space between the bell and the central filament is shorter, and dims gradually towards the top of the bell. However the illumination remains uniform both on horizontal and vertical plans, as I have shown with all the pictures and measurements above. 2- some information from a Romanian forum, on a thread focused on vacuum tubes: the oxide covering the filament is either barium oxide or strontium oxide. I would assume this is barium, but Proud Mary could help me with a simple chemistry test: put a little piece of the oxide in an colorless flame: barium makes the flame green, while the strontium makes it red. Barium oxide would be an excellent target for electron bombardment and I believe it can hold more damage than a simple tungsten filament. 3- the metal grill with crystals pictured by Proud Mary under microscope, is used to absorb the remaining gases after the tube has been vacuumed. So the crystals could be the resulting compounds of highly reactive chemicals placed on that plate. I don't think the plate has any influence on the x-ray output. 4- All the above is nothing new for other tubes as well. So what makes the difference? Take the following picture as a reference: I invite the readers to take a guess on which of the tubes above would make the best X-ray emitter, from left to the right: 2X2 , V1-0.1/30 , VI3-70/30 , DY86, PY88, 3BW2 , 3A3C , 3CZ3 , 6LJ6A I believe I know the key criteria that causes the abundant emission for the 2X2 (at least as compared to the tubes I currently have). I still need to run some tests to confirm it, so I will get back on this.
One of the Russian kenotrons - 2nd and 3rd from the left, perhaps?
Do keep in mind in your theorising that there a number of distinctly different modes of X-ray production, which for simplicity I'll divide first into 'hard vacuum' and 'soft vacuum' classes.
The high vacuum Coolidge tube with thermionic cathode needs no explanation here, but the earlier unheated Röntgen tubes deserve closer attention:
Introduction to Gas Discharge Tubes and Cold Cathode X-ray Tubes Oak Ridge University Associates
Regulating Gas Pressure
For proper operation, the gas pressure inside the tube needed to be on the order of 0.2 to 0.5 mm Hg. A tube with higher pressures was too "soft" while a tube with a lower pressure had too "hard" a vacuum. Initially, outgassing from some of the tubes metal components (especially the aluminum) would soften the tube, but over time it was common for the gas pressure to decrease as the residual gas in the tube was adsorbed on the tube’s walls and other components. This hardening of the tube reduced the intensity of the x-rays. At the same time, the x-rays became more penetrating (higher energy).
While the long term trend was towards a hardening of the tube, the pressure in the tubes could vary in an unpredictable fashion over the short term. For example, during use the gas pressure could increase temporarily as the glass wall of the tube heated up and released some of the adsorbed gas.
In general, two methods were employed to correct for the hardening of the tube:
1. The most common method was to add a regulator (side arm) to the tube which contained a material that released a gas when heated, e.g., asbestos impregnated with some chemical such as sodium hydrate or potassium hydrate. Potash and charcoal were also used.
In the simplest designs, the regulator had to be heated directly in a flame. However, the more common self-regulating systems didn't require the operator to do anything. These self-regulating tubes employed a lever (usually a wire) attached at one end to the regulator. The free end was positioned at a specific distance from the connection for the cathode. As the tube hardened, the flow of current from the cathode to the anode decreased. Eventually, the current would jump the spark gap between the cathode connection and the tip of the lever. This caused the material in the regulator to heat up and increase the pressure in the x-ray tube. The sparking ceased when the pressure in the tube was reduced to an acceptable level. If a harder tube (more penetrating x-rays) tube was desired, a large gap was used. For a softer tube (more contrast), a small gap was employed. 2. Another approach, developed by Villard, employed the principle of osmosis. The glass wall was penetrated by a very fine capillary of platinum or palladium. Upon heating the latter to a red glow, hydrogen diffused through the platinum/palladium into the tube and increased the gas pressure.
Now perhaps you will understand my interest in the getter material, and the possible effects it might have on the gas pressure inside the tube. Of course, it may have nothing to do with the 2X2A X-ray production at all - but we should not exclude it as a possible factor without good evidence.
Registered Member #1134
Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351
Proud Mary wrote ...
Now perhaps you will understand my interest in the getter material, and the possible effects it might have on the gas pressure inside the tube. Of course, it may have nothing to do with the 2X2A X-ray production at all - but we should not exclude it as a possible factor without good evidence.
Yes, it is also completely possible that this particular make of 2X2's just so happen to have the correct level of vacuum to function as a good old 1900's cold cathode tube. The internal pressure may well be governed by the getter type, as well as the speed at which residual gas atoms are cleaned up, during use. However the envelope really looks to small to be able to pull off such a balancing act with ease.
Registered Member #1938
Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 701
I understand your interest in the getter, but the medium inside the tube doesn't change much on the short term. So given a particular tube, it will either emit "plentiful" x-rays, or it will not.
Regarding the choice you made, I have to admit that I was eagerly waiting for my new big Russian tubes to arrive. I got them today, and I didn't have to power them to guess something that after a few simple tests, proved to be true. Funny but I picked the small DY86 as a potential candidate, among the other tubes in the picture above. Why? Here are some facts:
11.Other vacuum tubes Note: Again, I will be using several vacuum rectifier tubes or shunts, in inverse polarization at 50KV.
1) The Russian V1 0.1/30 . I have 4 of these. Here are some results: TEST A: Tube in vertical position, fluorescent screen in close proximity (2-3cm): The first pictures shows the setup, The second and forth pictures show the fluorescent screen while the tube is applied 50KV in inverse polarization (no fluorescence), The third picture shows a closeup on the tube, while running. Notice the glass blue fluorescence, seen on other X-ray tubes, is missing! ; instead the bottom of the tube is strongly illuminated. One of my 4 tubes, has a small metal blocker right under the anode bell . Not this one. Wondering why, I turned the tube in horizontal position: TEST B: Tube in horizontal position, bottom oriented towards fluorescent screen: Results? Nothing on the fluorescent screen, after 15seconds of exposure (the same parameters as for the 2X2). The Radex 1706 was picking up small amounts of radiation. Placed at 3 meters : 0.42uSv/h
2. To make sure that my setup is functioning correctly, I replaced the v1-0.1/30 for the 2X2: Much better! The Radex 1706 was now indicating 54uSv/h placed in the same place at 3meters . Not reliable but a lot different (good text for a quote :) ).
3. So far so good. I had to try the PY88 . For reasons not related to X-rays, vacuum or tube size, this was unsuitable to test my idea. So going for the next test:
4. The DY86 . I did some tests with this kind of tube some time ago. But now I have another one that seems different. Applying 50KV created a lot of flashovers, a nice show of sparks, putting the tube in real danger. Some tape fixed the problem. Here are some pictures: First picture: The tube connected to the supply is covered in sparks Second picture: Using some tape to better insulate the tube's terminals Third picture: Some nice fluorescence on the screen, comparable in intensity (energy level) with that produced by the 2X2. Of course, the field is distributed in a narrower shape (among other disadvantages like small tube size: thermal and insulation problems, small vacuum volume inside possible cause for parameters changes in time, etc).
My conclusion Lately I got used to check the medium inside a sealed tube, for photoelectric effect, using a 405nm laser 5mW laser pointer. By doing so I quickly was able to distinguish between vacuum tubes and inert gas tubes (like spark gaps or light bulbs). Not only that, but using a multimeter I had the chance to notice some minimum and maximum values on the voltage outputted by a tube, while illuminated with the UV laser.
This was the first test I did today, to make sure the vacuum inside the new tubes is well preserved. Here are some maximum voltage values, obtained by continuously changing the laser target (I was usually aiming either the anode or the cathode). No matter what I did, I couldn't get more voltage out of a given tube, using the laser pointer: 2X2 --> 200mV 6LJ6A --> 0.1mV First V1-0.1/30 --> 0.1mV Second V1-0.1/30 --> 0.5mV Third V1-0.1/30 --> 0.1mV Fourth V1-0.1/30 --> 1mV PY88 --> 50mV DY86 --> 300mV!!! The previously used DY86 --> 0.3mV Another DY86 with the seal cracked and the metallic surface inside turned white: 0V
So the last property that I believe makes the 2X2 so good at emitting X-Rays, besides the first I discussed in the previous post, is the hard vacuum inside, a feature highlighted by the photoelectric experiment described above.
As can be seen, the big Russian tubes only have a so-called "soft" vacuum inside. Proud Mary I believe you brought the discussion to the right place, with the article you've quoted.
As for the 2X2, it is presented as a "High-vacuum Rectifier ", a feature that I believe it is responsible for the nice X-ray emission:
I will try to bring more evidence that a simple laser pointer test can indicate whether a vacuum rectifier tube is suitable for X-ray emission, given some preliminary requirements are met like: high-Z target, electrode geometry, etc . For the time I had for this evening's test, this is all I could come with.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Just a short reaction for now, as I am busy:
The appeal of small EHT rectifiers like DY86, and even smaller types like EY51 is that you get a correspondingly smaller anode spot than with, for example, the large EHT shunt stabiliser triodes. I found DY86 to emit X-rays when excited with 40kV, but didn't explore it further because of the flashover problems you've described, and having far better sources to hand.
Your photoelectric experiments are interesting. How much of your measured potential difference could be due to the Seebeck effect? Did you take into account the likely Galvani potential, or 'contact potential' values - the flow of electrons from a low work function metal to a high work function metal - which can be as much as one or two volts in some cases, and is always a factor in thermionic valve design. (You only need think of a permanent bias of 2V on the grid of an audio triode, for example, to see why, but in thermionic diode power rectifiers, compensating for Galvani potential will not have been a design consideration, as it is with multigrid valves) Even the smallest contamination of a metal surface can change its work function by a great amount. As you will know, work function directly affects field emission and so the production of X-rays in our experiment.
Moreover, according to Lederer, (1940 ibid) barium metal evaporates from barium oxide cathodes, and condenses on cooler electrodes in the valve envelope, so wherever electrons are impacting they are probably impacting on a barium nanolayer. I will try to show that this is so with a Gy:kV graph of 2X2A from 15kV - 40kV which will be ready at the weekend.
Registered Member #1938
Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 701
Perhaps I omitted some of the steps of my simple experiment: the bottom pins of the vacuum tube to be tested are connected together, and then to one of the two multimeter probes. The top anode connector goes to the second multimeter probe.
Given an approximately constant room normal temperature no Peltier effect has been observed based on the multimeter readings.
I am not sure about a possible Galvani potential, but in case it exists, it is not to a significant degree: When the multimeter is connected, small charges of 0.1mV can be recorded, but the experiment was not performed in the absence of light so the cause of these small readings is debatable.
Once the multimeter connected, we can ignore the negligible small values as the potential difference is close to 0.
When applying the UV Laser light, a relatively strong electrical field is detected instantaneously. The current persists as long as the target is illuminated and disappears as the laser light is turned off or moved from the conveniently selected target inside the tube. Using a light source with a bigger wavelength results in smaller voltages . Eg. a green laser (~500nm) will only produce a charge of aprox. 30mV in a 2X2 as compared to the 405nm laser that goes up to 200mV. In my opinion, these indications go in close correlation with the photoelectric effect, and we do not need to search for other, more complicated approaches to explain the voltage detected. It also appears that this property is related to the vacuum inside: probably the photons from my laser, with only a few eV energy, are causing the target to emit electrons, that cannot form a current between the two electrodes supervised by the multimeter, unless there is no barrier in the medium inside the tube (gases). Comparing various tubes with extremely convenient electrode shapes, there seems to be a relation between their "x-ray emitting performance" and the photoelectric effect observed. More tests are needed to support this empirically.
Regarding the barium nano layer, it is very likely to form in these conditions, still there is a problem considering this layer as a source of emission: this layer can't possibly be deposited perfectly uniform, while my previous tests showed the x-rays are emitted uniformly all around the tube. This is why I believe the main source of emission is in the center of the 2X2 tube - more precisely I believe it is the oxide deposited on the central filament
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Radu, the frequency of the incident light being the same in all cases, are you saying that the magnitude of your photoelectric voltage is related to the work function of the metal surfaces - as in Einstein's photoelectric equations - and that this property can predict the X-ray performance of the tube?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Well, the only problem I see is that the photoelectric effect is a kind of electron emission. It tests how well the material works as a photocathode.
I don't see any necessary connection between that, and its performance as an anode for X-ray generation, where it's not emitting electrons, but being bombarded by them.
Except in so far as atoms of heavy metals (barium) have characteristic rays of higher energy, so when bombarded with electrons of high enough voltage, they can make lots of hard X-rays that will penetrate the tube glass efficiently. And don't heavy atoms also have a lower work function, because the outer electrons are further from the nucleus and easier to knock off? The old "electric eye" tubes had photocathodes coated in cesium for that reason.
So maybe Radu's method just detects whether there is any barium in the cathode/anode.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Steve McConner wrote ...
Except in so far as atoms of heavy metals (barium) have characteristic rays of higher energy, so when bombarded with electrons of high enough voltage, they can make lots of hard X-rays that will penetrate the tube glass efficiently.
This was my conjecture in my post above (Wed Jan 26 2011, 05:07PM) where I compared and contrasted the characteristic X-ray spectrum of barium, with the spectra of other common cathode coatings, (i.e. of our effective anode) and concluded that - setting aside the continuous radiation - only the strong barium peaks above 30keV could expect to exit the glass envelope in any quantity.
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Vacuum Rectifiers X-rays report
