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  1. #1
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    Xenon flash tube triggers from atmopheric radiation?

    If a xenon flash tube has say 300V across it, what is the chance that it can trigger by atmospheric radiation alone, (ie without needing a trigger pulse)?

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    Re: Xenon flash tube triggers from atmopheric radiation?

    I would say anything that causes the gas to ionize, will trigger it.
    Whether comic radiation is sufficiently energetic at ground level to cause ionisation, I don't know.


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    Re: Xenon flash tube triggers from atmopheric radiation?

    Possible, yes. Probable, no. Charged particles seldom
    reach ground level (although high-altitude server farms
    are a bit concerned due to huge number of exposed
    logic bits and the need to advertise an acceptably low
    bit error rate).

    So the question is, can you live with one flashover in
    a couple million years?

    You would like to know just how much energy would
    be needed to initiate an arc. Then you would figure
    the dE/dx (in low pressure Xenon) that turns out to
    be, what ion can drop that energy and then turn to
    atmospheric radiation transport codes and "space
    weather" flux charts to figure probabilities.

    The whole thing is basically a poor-boy Geiger
    counter, only lacking the actual counter, and not
    at all optimized for sensitivity. The low density of
    material makes it reasonably likely that even if an
    ionizing particle or energetic-enough photon did
    traverse the lamp volume, it would not interact.
    Energy dropped by an ion track is proportional to
    material density.

    One question is whether 300V is enough to sustain
    an arc in avalanche. There would be a holding current
    and a holding voltage, I'd expect. Can you quantify
    any of these aspects? Particularly, any kind of "kernel"
    initiation energy? Flash tubes tend to need a lot of
    high voltage energy to fire. You might be able to get
    at this by determining the minimum Joule energy
    behind a trigger pulse that does, and doesn't, fire
    the tube off (i.e. where does the arc bloom vs self-
    quench). Then compare this to what a worst case
    ion, if it ever made the trip, could deliver.

    In silicon you're talking picocoulombs of deposited
    charge. That would be like 1mA for 1nS. Gas tube will
    deliver way less from an ion strike. What is your critical
    trigger pulse current*time?


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    Re: Xenon flash tube triggers from atmopheric radiation?

    The whole thing is basically a poor-boy Geiger
    counter, only lacking the actual counter, and not
    at all optimized for sensitivity. The low density of
    material makes it reasonably likely that even if an
    ionizing particle or energetic-enough photon did
    traverse the lamp volume, it would not interact.
    Energy dropped by an ion track is proportional to
    material density.
    The one big difference between a flash tube and a geiger tube is the electrode geometry.

    A geiger tube has an outer cylinder with an axial wire down the middle. Most ionisig particles will enter the tube radially from the side, commonly creating an ionising trail roughly between the centre electrode and the outer sleeve. So its very prone to firing, if the electrode voltage is just below where the tube would self ionise. The geometry is deliberately arranged to make geiger "events" happen fairly regularly to give highest sensitivity.

    A xenon tube has end electrodes usually separated by a reasonable distance.
    It would take a fairly energetic particle to travel the full length of the tube between the electrodes, but it would certainly be possible. Ionising particles entering from the side are very unlikely to trigger the tube, because the geometry is just not favourable.

    But essentially both types of tube work in fairly similar ways by ionising a low pressure inert gas, but the electrode configuration is completely different.

    I guess the whole thing also hinges on background radiation level, or where you are.
    Cheers, Tony.


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    Re: Xenon flash tube triggers from atmopheric radiation?

    Xenon flash tube are high pressure discharge lamps resulting in very short ion free path, and the electrode shape doesn't promote avalanche multiplication as in a Geiger-Mueller tube.

    Only if a single particle could generate a complete ionized path along the tube, it might trigger a discharge at voltages considerably below the self-trigger threshold. I believe that's effectively impossible.


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    Re: Xenon flash tube triggers from atmopheric radiation?

    I worked on a xenon flash tube circuit that flashed when you walked across the room and touched the case. This was due to the high impedance trigger circuit detecting the electrostatic discharge as a command to fire the xenon tube.



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