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Advice Needed...PMT

Discussion in 'Electronic Design' started by jack, Aug 30, 2003.

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  1. jack

    jack Guest

    I'm working on a project now where I want to observe dark current
    pulses on a PMT. The output from a PMT can be considered a almost pure
    current source ( I've read) and I'm trying to figure out the best
    electronics to put between the anode and my scope. I'm configuring the
    tube as pulse mode to be used with a scintillator,so I have it as
    cathode grounded, and for starters I have a coupling capacitor just
    after the anode output. OK, several questions...
    Do I need a preamp or should I go straight into an op amp and if so
    what op amp is best? From my meager knowledge i have read a
    transimpedence op amp may be what I'm looking for as it takes a
    current and converts to voltage, and has the advantage of high slew
    rates which I like as these current pulses are in the 5 to 20 ns
    range. Before I go further with a scintillator,I would like to
    understand the nature of noise in these tubes and would like to build
    the electronics to do this. I have read the Hamamatsu handbook on this
    but in my opinion they did not give as much attention to pulse mode
    electronics as they did analogue operation. Does anyone have any
    experience with this or any suggestions? Any help would be
    appreciated. jack
  2. jack

    jack Guest

    Yes,sorry. I have a 10 stage linear focus tube with a rated gain of
    ..6x10^6 . It is a electron tube inc tube model9250B ,a 52 mm front
    end plate. So my initial question is the amplitude and width of the
    dark curent pulses. The rated dark current is 1.5 nA but I assume this
    is not the same as the dark current pulses. The pulse rise time is 4
    ns and the pulse fwhm is 6.5 ns. While not saying so directly I assume
    these are SER times. Just to observe the dark pulses,could I just run
    the anode line (after decoupling ) into my scope? Thanks for any help.
  3. jack

    jack Guest

    Thanks, now I'm going to show you how new I am too all this. What is
    AoE and where can I get this article? Thaks for the reply. jack
  4. AoE is "The Art of Electronics" by Horowitz and (Win)Hill. Check, used book dealers, or a good library.
  5. Eric Inazaki

    Eric Inazaki Guest

    Perhaps related to this discussion (or not).

    Some people I work with are setting up an ion
    counter using an electron multiplier. Typically,
    I've seen the EM connected to the preamp through a
    high voltage capacitor. Is there any reason not
    to directly couple the EM to the preamp, provided
    that end of the EM is at ground potential? Also
    what are the pros and cons of directly coupling vs.
    capacitively coupling the EM? This is a topic of
    some debate at work right now with each scheme
    having an advocate. The current plan is to have a
    shoot-off between the two designs but I'd like to
    see if there are any informed opinions out there.

    On an unrelated note, what ever happened to that rumor
    about an AoE 4th ed.?

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  7. Personally, I would prefer the isolation of the high voltage capacitor. For
    one thing, it is very easy, even with a ground reference potential in the
    system, to create extremely high voltages due to static charges.
    This will manifest itself as discharge events that spike the counter at
    random, indicating the presence of ions that are not there. Presently, as each
    ion passes your detector head, it triggers a cascade of electrons in a manner
    almost identical to that used in photomultipliers. The resulting signal is a
    varying voltage with spikes representing individual events, and if a large
    number of ions passes simultaneously, a barrage of events will produce a nearly
    continuous DC signal when the system saturates.
    The capacitor's size should be determined by the need to respond quickly to
    any single event, but also to allow enough "head room" so that it will not
    saturate. A DC coupled system would eliminate the saturation problem, but the
    real solution is to make a set of three or four "filters"- all AC coupled, all
    with varying sizes of capacitors, each with a Schottky rectifier directly after
    each capacitor. All the filters would be wired to the same input- that voltage
    created by the electron multiplier. Each of their outputs would be buffered.
    All the buffered outputs would represent some part of the even spectrum, and
    many would overlap- but that will be okay.
    Now, sum the events with a simple op amp circuit and you get fast response
    and head room at the same time. And, simultaneous signals step right on top of
    each other, "erasing" themselves. A simple second stage AC amplifier will clean
    the signal up and give you an accurate representation of the events, even if
    they are coming very rapidly.
    Just a thought.


    Chip Shults
    My robotics, space and CGI web page -
  8. Bill Sloman

    Bill Sloman Guest

    That would be the Thorn-EMI Electron Tubes 9250B tube - unless there
    has been a management buyout in the last few years. I worked at EMI
    from 1976 to 1979 - not long before they got taken over by Thorn - on
    medical ultrasound. I did have some contact with Electron Tubes on
    photomultiplier non-linearity, but I don't know if the German paper on
    the subject I sent over to them (with my translation) had any
    immediate effect. The section on photomultiplier non-linearity in the
    Thorn-EMI data book I've got is pretty sensible, but who knows where
    it came from.

    The simple-minded way of looking at dark current is to assume that it
    all comes from the photocathode as thermionic emission - with the red
    insensitive bialkali photocathode in the 9250B, cosmic rays and
    potassium-40 decay in the glass of the photocathode are probably
    nearly as important.

    The typical dark current of 0.1nA (your 1.5nA is worst case for four
    of the the other tubes of that family, but the 9250 is listed as 1.0nA
    worst case in my catalogue)at the anode would be 1.7x10^-16 amps at
    the photocathode, or about 1,000 photoelectrons per second, while the
    catalogue gives 300 counts per second as typical for all three
    bialkali photocathodes.

    IIRR some of the dark current comes from delayed secondary electron
    emission from the dynode surfaces, most of it from the later dynodes,
    whose surfaces see many more electrons than the early dynodes. If you
    do a pulse height analysis on the dark current, this proportion of the
    dark current shows up as long tail of low amplitude events. Because
    these events are small and numerous, the shot noise they contribute is

    If you want to look at the important - single photo-electron - dark
    current events, you are looking at 1.6x10^-19 coulombs of charge,
    multiplied by a gain of 6x10^5, 9.6x10^-14 coulombs, spread over
    6.5nsec, a current of about 15uA.

    You will only see that 6.5nsec pulse width if the tube is driving inot
    a 50R load, so that current becomes about 0.8mV, probably too small to
    see clearly with a fast scope (you'd want about 100MHz of bandwidth).

    There are integrated circuits around that can offer a gain of ten or
    twenty at around that sort of bandwidth. When I was working with these
    sorts of pulses we used Comlinear and Analog Devices current feedback
    op amps, which did the job nicely, but the parts I used then (late
    1980's) are now obsolete.

    The National Semiconductor LMH6624 voltage feedback amplifier is only
    stable when used as a gain-of-ten (or more) amplifier, but it looks as
    if it would do the job nicely, and offers rather fewer opportunities
    for disaster than Win's circuit in "The Art of Electronics" ISBN
    0-521-37095-7, which is in any event getting a bit dated these days -
    it would be fun to re-do it with surface mount wideband transistors
    (Farnell stocks PNP parts with bandwidths up to 5GHz and NPN parts up
    to 10GHz, and Siemens has datasheets for 50GHz parts).

    John Larkin will probably recommend a cheaper and faster amplifier -
    he seems to be designing for these sorts of bandwidths at the moment,
    and doing nicely out of it.
  9. jack

    jack Guest

    This has been extremely helpful, thank you.
    By assuming its mostly thermionic emission off the cathode,I guess
    that sets this contribution as the upper limit in terms of pulse
    amplitude .
    This is the part I was fuzzy on,i.e. how these numbers are
    calculated,.and it seems it was easier than I thought. It makes sense
    though. I'm not sure why the dark counts disagree so much ,however.
    Makes sense..
    So for a cathode grounded scheme, and a decoupling capacitor,a 100 Mhz
    scope should do the job.
    Analog make a current feedback op amp the 8011 with a very high slew
    rate. If I can figure out the proper way to wire it as a
    transimpedence amplifier....
    I'm still hunting down this book,should be interesting. Again thanks
    loads jack
  10. Eric Inazaki

    Eric Inazaki Guest

    LIke that's any excuse?

    Seriously, I should have said 3d ed.
  11. Bill Sloman

    Bill Sloman Guest

    You don't want to wire it as a transimpedance amplifier. The anode
    structure in fast linear-focussed photomultipliers seems to be
    designed to be terminaed with a 50R resistor, and you seem to get
    better results by amplifying the voltage developed across a 50R anode
    load than by making the anode a virtual ground.

    This was not what I expected when I started fooling around with fast
    tubes, and might not be true for someone with a really good feel for
    complex impedances, but it was certainly true for a couple of
    reasonably competent electronic engineers at Cambridge Instruments
    some 15 years ago.

    My rationalisation of this result was that the anode structure doesn't
    just have 8-10pF of capacitance to ground (some of it via capacitative
    coupling to the last dynodes) but also a significant inductance, and
    you need the 50R to get a well-damped RLC circuit, but we never played
    around enough to get a really good feel for what was going on.

    Bill Sloman, Nijmegen
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