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Registered Member #1134
Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351
Sorry for the delay regarding more pics, and the vacuum testing, but I am getting there! It been a busy week
I have already prepared a tube for the vacuum rig (removed it from its base, and carefully cut open and polished the sealing stem), and noted that the getter on the inside of the glass pretty much vanishes once the tube is up to air (as opposed to tuning milky white). Any ideas why?
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
plazmatron wrote ...
Stella, how is your setup coming along?
Between baking bread and making great quantities of goulash, I think I'll be able to bring everything together this weekend coming.
I've given a little thought to the geometry of the setup. In the light of your recent demonstration that the heat shield was the major X-ray source, I've decided to place the longitudinal axis of the valve at the apex of an equilateral triangle such that the pancake ionisation chamber is irradiated by a 60° segment across its 75mm width at a depth 10mm below the top surface - i.e. at the half-height of the chamber.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
2X2A Inverted Mode Measurement Findings
The findings will go on in fits and starts throughout the day as edits to this post.
X-ray emission becomes statistically significant - 10% over background - at 13.5kV, with 100mm between source and detector. Radiation tools: ZP1430 mica end window GM tube, modified Mini Instruments Mini Monitor output pulses to Black Star Nova 2400 counter-timer. Voltage measurement: Meter Unit Type 100, 0- 18.5kV electrostatic.
Measurements using calibrated Fluke Victoreen Rad-Check Plus, Model 06-526, with ionisation chamber 30cc. Energy response is ± 5% across 15kV - 65kV. Electrometer drift is 5 - 10µGy/minute typical. Full specification is here:
Cathode axis of 2x2A 40mm above ionisation chamber half-height.
Voltages are measured directly across the tube via a 1000:1 divider. (1G/1.11M into 10M DVM) Current is measured by a protected mirror scale moving coil microammeter in the effective cathode line (i.e. the original anode)
The effective anode is held at Earth potential, and the effective cathode at HV negative.
Integration time for all readings is 10 mins.
18kV 11uA 0.00033 Gy 18kV 12uA 0.00034 Gy 18kV 14uA 0.00035 Gy Dose rate averaged over one hour at 18kV = 2.04 mGy/hr
19kV 21uA 0.00059 Gy 19kV 21uA 0.00060 Gy 19kV 22uA 0.00061 Gy Dose rate averaged over one hour at 19kV = 3.6 mGy/hr
20kV 44uA 0.00190 Gy 20kV 46uA 0.00200 Gy 20kV 47uA 0.00210 Gy Dose rate averaged over one hour at 20kV = 12 mGy/hr
21kV 61uA 0.00353 Gy 21kV 64uA 0.00269 Gy 21kV 60uA 0.00351 Gy Dose rate averaged over one hour at 21kV = 19 mGy/hr
22kV 105uA 0.01056 Gy 22kV 102uA 0.01047 Gy 22kV 106uA 0.01055 Gy Dose rate averaged over one hour at 22kV = 63 mGy/hr
For these next readings, I had to change to a cathode current meter of lower resolution (10uA divisions), and have made only one reading per kilovolt at this stage.
23kV 160uA 0.0152 Gy Dose rate averaged over one hour at 23kV = 91 mGy/hr
24kV 190uA 0.0194 Gy Dose rate averaged over one hour at 24kV = 116 mGy/hr
That's it for today, though I might put up some images and diagrams later if I can find the energy.
Note the remarkable jump in X-ray output between 21 kV and 22 kV - a phenomenon needing investigation
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
2X2A Inverted Mode Measurement Findings
Part Two - 25kV to 30kV
25kV 500μA 227 mGy/hr
I am not happy with this measurement because:
1. I could only measure the dose for 4 mins, so as not to exceed the dosimeter's maximum reading (after which it re-sets)
2. The current climbed rapidly over the 4 mins and reached full scale 500μA where it stuck when current was removed, a sign that it had wanted to go higher. (restored to use by a light tap)
3. Anode voltage had fallen to 21.6kV after the 4 mins, as a protest against too much current being drawn. I see what Les meant when he wrote of 2X2A taking all the current he could give it.
Anyway, I have to replace the 500μA moving coil meter with a 1mA type and increase the distance between the tube and the ionization chamber before I can carry on.
I'm having no difficulty at all in resisting the temptation to over-stretch my PSU. With instabilities setting in, and the tube temperature rising steeply, trying to push it further would just be an expensive waste of good parts, time and money. Failure of something can't be much further down the road.
I have to make some paprika dumplings now, which will give me time to decide in which direction to go next.
Errata
For some reason, I assumed that 1mA was where the meter needle came to a halt, but I now see that the current had exceeded 1mA, and was stopped by the end of the scale.
I'll have to sort out some nichrome wire to make a shunt, as I don't have a 5mA FSD moving coil meter, and wouldn't risk a DVM in such an unruly circuit.
Registered Member #1938
Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 701
Proud Mary wrote ...
I've given a little thought to the geometry of the setup. In the light of your recent demonstration that the heat shield was the major X-ray source, I've decided to place the longitudinal axis of the valve at the apex of an equilateral triangle such that the pancake ionisation chamber is irradiated by a 60° segment across its 75mm width at a depth 10mm below the top surface - i.e. at the half-height of the chamber.
Would be extremely helpful to add a few pictures/diagrams for all geometry related tests, but also for your measurements (V/A) (even some hand made sketches), and if I'm not asking too much a photo of the entire setup - Since the users here speak tens of languages, pictures are extremely valuable.
Keep the measurements coming! I'd like to see some readings over 50KV.
LE: for X-rays the Gray results numerically equal to the Sievert. Might be an useful observation when comparing the emissions above with various values found in other sources (dental x-ray dose, CT scan, etc).
* Living near a nuclear power station: 0.0001–0.01 mSv/year * Cosmic radiation (from sky) at sea level: 0.24 mSv/year * Terrestrial radiation (from ground): 0.28 mSv/year * Natural radiation in the human body: 0.40 mSv/year * Average individual background radiation dose: 2 mSv/year; 1.5 mSv/year for Australians, 3.0 mSv/year for Americans * New York-Tokyo flights for airline crew: 9 mSv/year * Atmospheric sources (mostly radon): 2 mSv/year * Total average radiation dose for Americans: 6.2 mSv/year * Current average limit for nuclear workers: 20 mSv/year * Lowest clearly carcinogenic level: 100 mSv/year * Elevated limit for workers during Fukushima emergency: 250 mSv/year
Acute radiation: * 0 – 0.25 Sv (0 - 250 mSv): None * 0.25 – 1 Sv (250 - 1000 mSv): Some people feel nausea and loss of appetite; bone marrow, lymph nodes, spleen damaged. ...
And some uncommon (sense) examples: * International Commission on Radiological Protection recommended limit for volunteers averting major nuclear escalation: 500 mSv * International Commission on Radiological Protection recommended limit for volunteers rescuing lives or preventing serious injuries: 1000 mSv
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Radu, I hope I shall be able to spare four or five hours tomorrow - Sunday - and hope to do the X-ray output and current measurements for 25 - 30kV, and take some pictures for you. With the last run it took me about an hour to do each kV. (Great quantities of tea are needed! )
It would not be correct to use Sievert units in our calculations. While the Weighting Factor ("W") for X-ray photons is 1, such that 1 Gy = 1 Sv for X-rays, the Sievert is a unit used in the radiobiology of living systems, with which our experiments are not concerned. (But if we were to make a risk assessment of the experiments, then we would use the Sievert.)
Have you tried out your new dental tube yet? Don't blow that filament!
For grounded cathode circuits, I have found the variable voltage regulator LM338K very good for providing 1.2V - 3.0V, at up to 5A, the usual range needed for XRF and XRD tube filaments.
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Vacuum Rectifiers X-rays report
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