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GM Detector help

Discussion in 'Electronic Design' started by Syd Rumpo, Nov 15, 2003.

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  1. Syd Rumpo

    Syd Rumpo Guest

    Reposted here 'cos Ed didn't realise that sci.electronics isn't
    available on many news servers, although he's had a couple of helpful


    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
    potential 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 miller capacitance across 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 didn't 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. James Meyer

    James Meyer Guest

    Perhaps you could pass this along to him...
    You are using the wrong tool for the job. It's like asking for advise
    on modifying a steam roller so that it would be suitable for making biscuits.

    For large fluxes of energetic particles, a better detector would be an
    ionization chamber. They're very easy to make from scratch using components
    found in the average hobby electronics experimenter's junk box and also very
    easy to calibrate from first principles.

  3. Russell Shaw

    Russell Shaw Guest

    ask on cdv700 yahoo group
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