Re: gated PMTs

Posted by Benjamin Smith on
URL: http://confocal-microscopy-list.275.s1.nabble.com/gated-PMTs-tp7585956p7585972.html

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From the Hamamatsu handbook (p. 97): http://psec.uchicago.edu/links/pmt_handbook_complete.pdf

   If the photomultiplier tube output is OFF at a gate input of 0V, a reverse bias of about 10 volts with respect
   to the focusing electrode and first dynode is supplied to the cathode. This prevents photoelectrons, if emitted
   by the cathode, from reaching the dynode section. Here, if a pulse signal of +3 to +4 volts is applied to the gate
   input terminal, the driver circuit gives a forward bias to the cathode via capacitance coupling, and sets the
   photomultiplier tube output to ON during the period determined by the gate pulse width and the time constant
   of the capacitance-coupled circuit. This gating circuit provides a switching ratio (or extinction ratio) of 10^4 or
   more. The capacitors are connected from the first through the center dynode to absorb the switching noises
   often encountered with this type of gating circuit.

I made a quick mock-up of the circuit schematic: http://tinyurl.com/jkzg5uf

  I know the capacitors and resistors aren't the exact values, but the point is more to show the potential switching behavior at the photocathode.  As you can see, if the TTL signal is high (switch closed) then there is a negative potential  from the cathode to the focusing anode/dynodes, which turns the PMT "ON" (electrons head toward the dynodes).  If the TTL signal is low (switch open), then the polarity of the potential between the cathode and focusing anode/dynodes gets flipped, without effecting any of the potential of the dynodes themselves, which turns the PMT "OFF" (electrons stay at the photocathode).  The MOSFETs are there simply as a crude surrogate for the PMT current, where negative dynode potentials drive the PMT current 

Cheers,

   Ben Smith

On Wed, Nov 2, 2016 at 3:37 PM, Mark Cannell <[hidden email]> wrote:

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Benjamin,

I don’t think he’s asking about hybrid detectors but Gaasp PMTs. By the way, the cathode to d1 voltage is typically 100V and this voltage does not focus the photoelectrons, it accelerates them into the first dynode to cause secondary emission.

Regards Mark

On 2/11/2016, at 7:52 pm, Benjamin E Smith <[hidden email]> wrote:

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Unfortunately, as far as hybrid PMTs go, this is a case of manufacturers using the same term to describe two things.

Because of capacitance and inductance alone, it is highly prohibitive to gate a high-voltage potential in microsecond time frames, and to do so would require large currents, which you are trying to avoid.

In gated PMTs, the gating is done by switching the focusing potential between the photocathode and the dynodes.  Since this potential is only about 10V, it can be rapidly switched.  With the potential flipped, the photoelectrons are scattered away from the dynodes, protecting the dynodes from what would otherwise be a large current.  By switching the focusing polarity, you can protect the dynodes while keeping them at a stable high voltage.

In a hybrid detector, gating refers to synchronizing the photon counting to a pulsed laser.  In this case, the detector is always on detecting photoelectrons, and what you are gating is the detection clock, basically a start and stop time when you want to actually count the photoelectrons vs ignore them.  Since autofluorescense has a relatively short fluorescence lifetime, while fluorophores have longer lifetimes, by waiting a short time after each laser pulse, you can enrich for detecting just the fluorophores.  

In short, in a gated PMT, the photocathode always sees the photons, but the gate protects the dynodes from the photoelectrons, protecting the rest o the PMT.  in a hybrid detector, the entire detector circuit sees all of the photoelectrons, and the gating simply tells the counting circuit which counts to ignore.

According to Hamamatsu, it sounds like the GaAsP photocathode is pretty resilient, because the guide says to avoid direct intensity light such as direct laser light, which is pretty extreme when you thing about normal light levels a PMT encounters.

For more reading on the topic, this is a good guide: http://psec.uchicago.edu/links/pmt_handbook_complete.pdf

Hope this helps,

  Ben Smith
Sent from my iPhone

On Nov 1, 2016, at 1:28 PM, Simon Wiegert <[hidden email]> wrote:

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Dear Listers,

I guess this question was discussed here previously, but I could not find any recent discussion on this topic. I am planning to set up a two-photon imaging microscope with forward detection through the condenser. Since there will be a lot of photostimulation happening from various light sources that more or less point directly into the condenser, the PMTs need to be protected. Until now we did it with Uniblitz shutters. However, there are some issues with these shutters (speed, noise, etc.) and I would rather like to use gated PMTs.

Does anyone have long-term experience with gated PMTs and (strong) light exposure? I would like to use GaAsPs and I am afraid that a lot of light might still kill the PMT since there is no physical light block in front of it. In my understanding, excessive light can still destroy the photocathode, i.e. the light-sensitive surface, even if the anode current is strongly suppressed by the gating circuit.

Does anyone have experience or even a systematic comparison with gated GaAsP PMTs that had been exposed to light vs. some non-exposed?

Any feedback is welcome.

Thanks, Simon

 

 

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Center for Molecular Neurobiology (ZMNH)

Institute for Synaptic Physiology

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