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External Quenching of GM Tubes

Discussion in 'General Electronics' started by Ed Rooke, Nov 12, 2003.

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  1. Ed Rooke

    Ed Rooke Guest

    I presently working on a GM detector. Unfortunately the source I`m
    using has a fairly high count rate and the period between events seen
    by the tube is much shorter than the dead time. I thought I would get
    around the problem with external quenching; switching the supply to
    the anode to below the starting voltage (750 V) of the tube for the
    dead time (200uS) this period is given from a monostable triggered by
    the event.

    The problem seemed to me to be trivial but after trying various shunt
    and series arrangements with FETs I have realised it is a bit more
    difficult than I originally thought.

    One problem is my power supply; it was originally designed for a PMT
    so it cant provide that much current. This has forced me to use a 16M
    potenial divider to switch down to 750V and the big resistances are
    giving me a large time constant even though the capacitances are quite
    small. This makes the rise and fall of the voltage too slow (I was
    aiming for 20 uS or so).

    I cant drop the supply right down to deck because some voltage (not
    sure how much - would be interested if anyone knows?) is required at
    the anode to let the tube recovery from an event.

    The other problem is milli capacitance accross the FETs and emfs being
    induced by the large voltages. The most successful circuits I have
    made respond to the first pulse but as the tube is switched back on
    again after the dead time a pulse appears on the cathode hence
    retriggering the monostable and causing the tube to keep switching

    I found one circuit using valves on the net but it didnt make any
    sense to me. Does any one/ has anyone done this with FETs ?

    Any advice would be much appreciated; I`m pretty stuck on this one.

  2. Robert Baer

    Robert Baer Guest

    Perhaps the use of the equivalent of a "programmable" or "gated" shunt
    regulator, where one can switch the regulating voltage from (say) 1050V
    to almost 0V.
    The possible problems with this scheme, is that the roughly 400pF
    drain capacitance of the regulating FET, coupled with the needed series
    resistor from supply to shunt regulator may give a too slow rise time.

    Be advised, that once something ionizes the gas in the GM tube, the
    time for those ions to dissipate to a low enough level will also cause
    "deadtime" problems.

    I could supply you with a gated 900V shunt regulator, which couuld be
    used in series with a 600V (max recommended) regulator - which would
    result in two shunt regulated voltages: 1500V and 600V.
    If you choose to use two regulators this way, you can specify these
    two voltages as you see fit, to a maximum of 900V each, in 25 volt
    steps. See:
    Another possible way is to use a FET to create a negative pulse that
    is capacitively coupled to the tube; obviously this can cause risetime
    problems related to the series capacitance (coupling in series with FET
    drain capacitance).
    Yet another way is to add a pulse transformer in series with the GM
    tube, and pulse it with a FET, so the secondary voltage cancels out the
    GM supply voltage. Make sure you terminate the transformer so the
    inductance does not ring too much.
    One can even use a 555 to drive a step-up transformer, giving a
    flat-topped pulse.
  3. Rich Grise

    Rich Grise Guest

    Why not just move the source farther away?

    Good Luck!
  4. Ed Rooke

    Ed Rooke Guest

    A good suggestion. Unfortunately, within the environment that the
    device must work there is a lot of background radiation. The source
    must be located by the detector and the high count rate recieved to
    get any kind of accuracy. Otherwise I`d just be measuring the
    background all the time.

    Thanks anyway
  5. Robert Baer

    Robert Baer Guest

    I do not remember if i mentioned that the voltage across the tube
    should be decreased to below 32 volts, as that is the typical ionization
    potential of most gasses once ionized.
    The trick using a pulse transformer may be the best method.
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