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Vacuum Rectifiers X-rays report

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plazmatron

Tue Feb 15 2011, 10:29PM

Registered Member #1134 Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351

OK the intensified pinhole camera works fine.

First off we have an image of the focal spot of a small dental x-ray tube (below).
Note, the resolution is good enough that is is obvious the spot is rectangular, corresponding to the spot etched to the anode face

So since I am waiting for KWTubes to send me the 2X2's I thought I would abuse a PD500. It is a poor x-ray emitter, and when driven in field emission mode, emits most of its radiation from near the base of the tube.
Due to the low flux this required a 15 sec exposure of the image intensifier screen.

Note the circular structure with two lines either side. This is the end of the heater coil, with metal grid supports either side (hidden visually behind mica end plates!). There are many other emission points from various metal parts near the base of the tube.

For future photos, I will take an AVI and stack the frames, and see if a longer integration period will yield a clearer image.

Above is the PD500 with the imaged area circled.

Les

Proud Mary

Tue Feb 15 2011, 10:34PM

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

What pd did you put across the PD500, Les?

The imaging of the element supports and leads were my expectation of 2X2As performance, before I saw Radu's results.

plazmatron

Tue Feb 15 2011, 10:40PM

Registered Member #1134 Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351

Proud Mary wrote ...

What pd did you put across the PD500, Les?

Approx 45kV. (no current measurement I'm afraid, but most assuredly very feeble). It is a really unstable tube driven like this, with occasional x-ray flashes as vacuum breaks down. It was really quite interesting to watch through the pinhole camera.

Proud Mary wrote ...

The imaging of the element supports and leads were my expectation of 2X2As performance, before I saw Radu's results.

I was really surprised at the detail, and the number of emission points. It will be interesting to see what happens with the 2X2.

Les

Proud Mary

Wed Feb 16 2011, 11:50AM

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

plazmatron wrote ...

I was really surprised at the detail, and the number of emission points. It will be interesting to see what happens with the 2X2.

While looking in a box of EHT shunt stabiliers to fish out a PD500 to compare with your image, I found another 2X2A, so I'll still be able to do dosimetry on two of them in my weekend session if the others don't show up, and then butcher one of them to do Radu's flame test.

Looking at the cathode and heater assembly of PD500, it's very clear why the emission is so low - in a word, it's all boxed in by screens and shields. I'll put some pictures up later.

radhoo

Wed Feb 16 2011, 12:01PM

Registered Member #1938 Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 701

Proud Mary wrote ...

Looking at the cathode and heater assembly of PD500, it's very clear why the emission is so low - in a word, it's all boxed in by screens and shields. I'll put some pictures up later.

On a smaller scale, it's the same problem with the PY88 .

Proud Mary

Wed Feb 16 2011, 01:00PM

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

PD500 base internal view

The equalising ring on the left seems to correspond to a nebulous incomplete circle at left in Les's radiograph.

What is the origin of the impacting electrons? Comparing the strong verticals in the radiograph with the vertical supports as we see them here, it seems reasonable to suppose that the electrons must have originated in the conductive metallic getter coat on the inside of the glass.

plazmatron

Wed Feb 16 2011, 01:57PM

Registered Member #1134 Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351

Proud Mary wrote ...

Looking at the cathode and heater assembly of PD500, it's very clear why the emission is so low - in a word, it's all boxed in by screens and shields..................

....................Wha t is the origin of the impacting electrons? Comparing the strong verticals in the radiograph with the vertical supports as we see them here, it seems reasonable to suppose that the electrons must have originated in the conductive metallic getter coat on the inside of the glass.

This is what appeared most confusing to me too. There appears to be no line of sight between the "cathode" and the little bundle of "anodes" at the base of the tube.

The problem with tubes like this is unpredictability of electron paths. This is evident to anyone that has seen the fluorescence on the inside of the glass caused by electron impacts. Not only do they appear quite random, but often these move around , as charges build up on the glass, electrostatically deflecting or reshaping the beams.

Hopefully the emission characteristics of the 2X2 will be a little more sane and easier to understand!

Les

Proud Mary

Wed Feb 16 2011, 02:27PM

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

On the basis that electrons won't travel any further than they have to, the only explanation for what seems distributed X-ray emission along the entire length of a vertical would be an electron source parallel to it - and hence from the getter on the glass walls.

Using PD500 as an improvised X-ray source, I think it a waste to use it in field emission mode when it is designed to operate at up to 27.5kV @ 1.6mA, and will hold off 45kV in extremis, (according to Note 2 in the Philips datasheet of 1969.) An X-ray shielding warning is printed on the glass. One can raise Va well above 27.5kV by operating the valve in starvation conditions to increase impedance and reduce Ia and so keep Wa within its 40W 'absolute max' dissipation rating. A combination of reduced heater current, and increased grid bias negative makes it possible to operate the valve at 40kV so long as flashover precautions are taken. The much greater anode current (compared with field emission) of the heated valve results in greater X-ray fluence.

Nah

Thu Feb 17 2011, 11:13PM

Registered Member #3567 Joined: Mon Jan 03 2011, 10:49PM
Location: USA, 1960s
Posts: 260

I'm going to ask a stupid question, why can't you use a burned out antique light bulb instead? Would the smaller surface area make a big difference in terms of xray output??

Proud Mary

Fri Feb 18 2011, 11:49AM

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

Nah wrote ...

I'm going to ask a stupid question, why can't you use a burned out antique light bulb instead? Would the smaller surface area make a big difference in terms of xray output??

Hi Nah,

thank you for your interest in this thread.

The main interest here is to try and found out why this obsolete HV thermionic diode, 2X2A, seems to produce a higher X-ray output than we would expect when compared with other common HV valve rectifiers of similar vintage. The consensus so far is that it may be due to the presence of barium, but more experiments need to be done to confirm this.

I'm not sure just what you mean by an "antique light bulb," so I'll note that X-rays are even produced by spark discharges in air at atmospheric pressure, but that relatively complex equipment is needed to detect them.

Stella

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