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Thread: Help with x-rays from scotch tape?

  1. #1 Help with x-rays from scotch tape? 
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    Concerning the experiment in this story where X-rays are generated by peeling Scotch tape in a vacuum chamber:

    I wanted to try this, so I built a similar apparatus to fit my vacuum chamber. I'm not detecting anything beyond background radiation and I'm hoping someone can spot my error. Here is the setup:

    Vacuum pump, Robinair 5CFM, 50 microns absolute vacuum.

    Detection probes:
    Ludlum 44-9 Alpha-Beta-Gamma pancake probe (ludlums.com).
    Ludlum 44-2 NaI Gamma scintillator probe (ludlums.com).

    Feed rate: 1-2 inches per second

    Tape: Scotch photo-safe transparent (Cat. 600, UPC 21200-52396)

    Below are some photos of the setup. It uses a hacked servo to act as a gearmotor. In the photos, the tape is being peeled away so that the sticky side is facing up (smooth side rides against the bolt)

    Detection is being made through the 1/2" polycarbonate lid on the vacuum chamber directly over the point of tape separation (about 1/2" below the lid). The gauge shows 24.3 in Hg, which at my altitude (5900ft) is roughly equivalent to 29.9 in Hg at sea level. The gauge is not of high accuracy, but I let the pump run until I'm sure the chamber is fully evacuated to the extent the pump is capable (50 microns).

    Any thoughts? As far as I know, the probes are sensitive to x-rays, but have never confirmed that. I have no x-ray film or scintillator material laying around. Is a vacuum of 50 microns not enough to see some kind of output from this?

    Vacuum chamber:


    Peeling apparatus:


    Peeling close-up:


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  3. #2  
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    In the Nature paper where it was reported, they say they did the experiment at about 10^-3 torr, which I believe is equivalent to around one micron of pressure - so I would guess that your pressure is around 30x higher than theirs. As for whether or not that would actually matter, I don't know.

    Do you know how sensitive your probes are to xrays? How does your sensitivity compare to what was reported in the Nature paper?


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    Quote Originally Posted by Scifor Refugee
    In the Nature paper where it was reported, they say they did the experiment at about 10^-3 torr, which I believe is equivalent to around one micron of pressure - so I would guess that your pressure is around 30x higher than theirs. As for whether or not that would actually matter, I don't know.

    Do you know how sensitive your probes are to xrays? How does your sensitivity compare to what was reported in the Nature paper?
    It appears to me that the experiments are in a sensitive region of the Paschen curve. The vacuum might be an important factor.

    http://www.duniway.com/images/pdf/pg/Paschen-Curve.pdf

    http://www.google.com/imgres?imgurl=...num=7&ct=image
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    Do you know how sensitive your probes are to xrays? How does your sensitivity compare to what was reported in the Nature paper?
    That is a problem - I really don't know. From all of the reading I have done, NaI scintillators are good for detecting x-rays, but I haven't found any numbers to match against the sensitivity graphs provided in the links.

    I later tried to see if a fluorescent bulb would glow at all when placed against the lid (tested in total darkness) and did not see anything except the blue light coming off the tape at the separation point.
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    You might be able to figure out whether or not the pressure would really matter by looking up the extinction coefficient in air for the xray wavelengths you're expecting.
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  7. #6  
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    Quote Originally Posted by Scifor Refugee
    You might be able to figure out whether or not the pressure would really matter by looking up the extinction coefficient in air for the xray wavelengths you're expecting.
    Good point. But there is more involved.

    The wavlength will depend on the energy of the discharge, the voltage, which in turn will depend on the dieletric breakdown strength of the rarified air in the chamber, which is the subject of the Paschen curves.
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    Well, I personally hate it when someone posts silly information to a thread. Hope this doesn't seem silly.

    I simply cannot think of anything that would create X-rays from such actions. Is it even explained what the hypothetical mechanism is? Tribology doesn't explain it, and X-Ray film will expose to things other than X-Rays!
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  9. #8  
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    Quote Originally Posted by Wild Cobra
    Well, I personally hate it when someone posts silly information to a thread. Hope this doesn't seem silly.

    I simply cannot think of anything that would create X-rays from such actions. Is it even explained what the hypothetical mechanism is? Tribology doesn't explain it, and X-Ray film will expose to things other than X-Rays!
    It is not as silly as it sounds. Take a look at the video from Nature that shows the experiment.
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    Some additional information:

    Audio-only interview with one of the researchers (2nd posting from the top).

    A PDF of the experiment.

    DrRocket may be correct about the pressure based on the references provided. If so, I'm dead in the water since I have no access to a vacuum pump of the class needed to get down to 1 micron.
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    Regarding Paschen curve:

    According to the Nature paper, the voltages were in the 15-30 kV range, which should be way more than enough to achieve a discharge even in the 10-100 torr range. So I doubt that failure to achieve a discharge is your problem.

    Upon further consideration, I suspect that the real reason for the low pressure is to prevent the electrons from colliding with air molecules (and thus losing kinetic energy) while they travel from one surface to another.
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    Quote Originally Posted by Scifor Refugee
    Regarding Paschen curve:

    According to the Nature paper, the voltages were in the 15-30 kV range, which should be way more than enough to achieve a discharge even in the 10-100 torr range. So I doubt that failure to achieve a discharge is your problem.

    Upon further consideration, I suspect that the real reason for the low pressure is to prevent the electrons from colliding with air molecules (and thus losing kinetic energy) while they travel from one surface to another.
    The issue is not so much the ability to achieve a discharge, as the energy of the individual elecrons released and hence the frequency of associated x-rays. The higher the discharge voltage the higher the x-ray frequency. That may be significant with regard to the ability to detect the emitted photons.
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  13. #12  
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    Quote Originally Posted by DrRocket
    The issue is not so much the ability to achieve a discharge, as the energy of the individual elecrons released and hence the frequency of associated x-rays. The higher the discharge voltage the higher the x-ray frequency. That may be significant with regard to the ability to detect the emitted photons.
    Are you thinking that higher pressure will = higher voltage and hence more energetic electrons/x-rays?
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  14. #13  
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    Quote Originally Posted by Scifor Refugee
    Quote Originally Posted by DrRocket
    The issue is not so much the ability to achieve a discharge, as the energy of the individual elecrons released and hence the frequency of associated x-rays. The higher the discharge voltage the higher the x-ray frequency. That may be significant with regard to the ability to detect the emitted photons.
    Are you thinking that higher pressure will = higher voltage and hence more energetic electrons/x-rays?
    Actually you are on the part of the Paschen curve where lower pressure gives higher breakdown voltage, so what I am thinking is

    lower pressure will = higher voltage and hence more energetic electrons/x-rays
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  15. #14  
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    Quote Originally Posted by DrRocket
    Actually you are on the part of the Paschen curve where lower pressure gives higher breakdown voltage, so what I am thinking is

    lower pressure will = higher voltage and hence more energetic electrons/x-rays
    The thing is, the x-ray energies they detected seem to be waaay above what the Paschen curve would predict should be possible at the pressure they were working at. I don't claim to completely understand how the x-rays were being generated, but it appears that they were able to somehow create extremely short-lived regions of voltage far above what would be necessary to create a discharge, and so generate correspondingly more energetic x-rays when the discharge occurs.
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