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PMT operating voltage, who knew?

Discussion in 'Electronic Design' started by George Herold, Mar 28, 2013.

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  1. So someone was rewriting our manual for using a PMT.
    (Used in photon counting mode.)
    There was this confused procedure to set the discriminator. (The
    threshold level on the comparator before counter.) So I said let’s
    just take the data, and do it right. I took a PMT off the shelf that
    had a fair amount of dark current (3nA at 1000V) We looked at count
    rate with a light on versus the dark count rate at various
    discriminator setting and PMT voltages.
    Here’s a plot of the data,

    I must admit to being surprised. I’d always heard/ been told to
    operate the PMT near the voltage recommended by the manufacturer
    (1000 volts in this case.)

    I said to my boss that the best way to find the largest signal to
    noise is to set the discriminator to a tad above zero, then reduce the
    voltage till the count rate drops to a bit more than 1/2 it maximum
    value. I did this the voltage was 537 and the light on/ dark ratio
    was 455...more than 5 times better than at 1000V!

    George H.
  2. The power supply is a all in the socket thingie made by Hamamatsu.
    It's a Ccokroft Walton ladder so I can't really change the divider
    The maximum count rate with light one is ~10k/sec. The dark count is
    all over the map, 3k to 10's. If I pick the discriminator setting
    that gives the best ratio (~3-4k light counts per second.) Then dark
    counts are about

    1000V 60/sec
    800V 30/sec
    700V 20/sec
    600V ~14/sec
    537V 9/sec.

    (Like I said I picked a noisy pmt off the shelf.. the range of
    reported dark currents was from 5nA to 0.05nA I haven't looked at one
    with a low dark current.)

    The only thing I could find on the web that hinted at this is a graph
    of dark current vs voltage by hamamatsu... It showed a dark current
    'turning on' at ~400V.

    It's not really a problem or anything, I just never knew... and if
    someone was digging for a signal in the noise then it would be

    George H.
  3. rickman

    rickman Guest

    What that a metric "tad" or an imperial "tad"?
  4. tm

    tm Guest

    He maybe?

    Are the tubes old?
  5. Yeah maybe... looking at the 'scope and low rate dark counts
    there are a number of 'big' events. (maybe 1/3?)
    (I can see why you might want an upper window level on the
    I need some MCA...

    or maybe light emission being
    I'm not sure what you mean... light from the tube base/socket going up
    the side of the PMT? There was a lot of effort put into keeping light
    out of the PMT space. (work not done by me, I do know that some BNC
    sockets are better than others, and then we paint the ends with
    epoxy. And shouldn't stray light 'dark count' look like real light in
    terms pulse height?

    I'll have to look at a low noise pmt.
    (tomorrow should be a quite day.)
    George H.
  6. The discriminator is set with a ten turn pot and dial. I set it at
    ~0.01 turns, right at zero you start hitting the noise... Well it's
    not really all noise, there's some ~1 MHz interference getting in
    too... anyway right at zero the 'dark count' doubles.

    George H.
  7. Yeah I used 'good' pmt's with coolers built in to keep down the dark
    count... I never looked at how the rate changed with voltage.
    There are certainly speed advantages that come with higher voltage,
    but you don't always care about speed.

    George H.

    (Turning down the voltage is certainly easier than a cooler.)
  8. No. Dark count data on box had a 2012 date.

    George H.
  9. Bill Sloman

    Bill Sloman Guest

    Most of it. The first dynodes are reputed to produce a bit, particularly after they've been busy - not every incident electron produces only prompt secondaries.
    There's all sorts of interesting stuff that can go on. I used a zener to fix the voltage from photocathode to first dynode at close to it's maximum tomaximise the signal to noise ratio, and the RCa 8850 went further by usinga GaP semiconductor on the first dynode and some 800V between cathode and first dynode to get about 40 seconary electrons per photo-electron.

    It makes sense to use a zener diode between the last dynode and ground - that voltage drop doesn't add to the gain of the tube, and too much anode current can drop the voltage across this stage and increase the voltage acrossthe rest of the tube (which does affect the gain) to give you a perceptible positive non-linearity.

    If you are pulling large enough currents out of the anode, you can need to raise the voltage across the last few dynodes to avoid space charge effects, and Sauerbrey thought that the a single photo-electron in the photo-cathode to first dynode space could be shown to have created a significant spacecharge.

    Photomultipliers are fun devices.

    Sloman, A.W. "Comment on 'Computer aided simulation study of photomultiplier tubes'", IEEE Transactions on Electron Devices, ED-38 679-680 (1991).

    lists a few useful references.
  10. Robert Baer

    Robert Baer Guest

    Err...the 600V plot is almost two times better...
  11. Robert Baer

    Robert Baer Guest

    Excellent point; treat the PMT as a series of (noisy) amplifiers;
    that implies the first stage is the most critical,with the second stage
    contributing (to S/N) by about the inverse of its gain (i think).
  12. Bill Sloman

    Bill Sloman Guest

    Cambridge Instruments bought selected high gain photomultipliers so that they never ended up with more than 1000V across the glass face-plate - between the photocathode and the external metal work.

    Above 1000V you got electrical conduction through the glass, which was generating electro-luminescence, which showed up as extra dark current at high gain settings.

    Regular insulators between the glass and the metal-work didn't help - they weren't as resistive as the glass, so the voltage drop across the glass stayed at 1000V. We could have done better, but paying a bit extra for selected high-gain tubes was the cheapest option.
  13. Robert Baer

    Robert Baer Guest

    Use a plain socket and put in your own resistor / regulator chain,
    just like many did 30-40 years ago.
  14. Nice data sheet, too bad the dark current graph doesn't go to lower
    figure 9 here shows dark current 'turning on' at 600-700 volts.

    Did you ever look at the dark current at low voltages?

    George H.
  15. Sorry I cut the top off the graph and the single data point at 537
    volts was lost.
    The light /dark ratio was 455 compared to ~70 at 1000V.
    The pulses are only a few mV high basically lost in the noise... I
    can't get the 'scope to trigger on them, the comparator finds them
    just fine though. Looking at the 'scope display I'd never have
    guessed that the light/dark count ratio would be best there.

    George H.
  16. OK I got a low noise pmt off the shelf.
    Here's it's plot. Dark count rates are ~ 10 times lower.
    (There should be some big error bars on some of the data points... I'm
    getting rates of only 1/sec or so... (ten second counting window.)

    George H.
  17. tm

    tm Guest

    OK I got a low noise pmt off the shelf.
    Here's it's plot. Dark count rates are ~ 10 times lower.
    (There should be some big error bars on some of the data points... I'm
    getting rates of only 1/sec or so... (ten second counting window.)

    George H.

    Are you operating the PC at ground potential? I.e. positive HV?

    I have found that having all the shielding at the same potential as the PC
    helps lower the noise.

  18. Hmm the number of light counts is prety much constant except at the
    lowest voltage (537 V) With the discriminator set at zero (probably
    about 1mV) I see,

    voltage light counts dark counts
    (in 10 seconds)
    537 40.5k 89
    600 78.8k 1k
    700 92.1k 4.4k
    800 100k 9.7k
    1000V 135k 31k

    I get the best light to dark ratio when the discriminator is set such
    that the light count rate is down to 1/2 to 1/3 it's maximum

    NEA photocathode?
    Here's a data sheet...
    (well maybe not sure that will work)

    Don't spend any skull sweat on this on my account. It's pretty much a
    done deal.. just getting data for a manual re-write. (And scratching
    my head.)

    It doesn't seem that photoexcited electron's from the photocathode can
    have that much more energy than the thermal ones... all green photons
    so a maximum energy of a volt or two. Would that make such a big

    George H.
  19. No the anode is near ground... that's a lot easier for the pulse amp
    (It's a CFA run as a current to voltage converter.... I didn't quite
    understand how my circuit was working till I tried to get a bit more
    gain out of it, then I had to give myself a dope slap.)

    George H.
  20. Bill Sloman

    Bill Sloman Guest

    Note my second contribution to this thread yesterday. The PMT where we
    started getting extra dark current when the photocathode was more than
    1KV negative than the metalwork on the othere side of the glass was a
    EMI tube, not Hamamatsu - so the leakage current through the glass
    might not be generating electroluminescence or might start to generate
    it at different voltage difference, depending on the transition metal
    impurities in the glass used to make the tube envelope, the thickness
    of the glass and the phase of the moon.
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