Posted by
Benjamin Smith on
URL: http://confocal-microscopy-list.275.s1.nabble.com/gated-PMTs-tp7585956p7585970.html
*****
To join, leave or search the confocal microscopy listserv, go to:
http://lists.umn.edu/cgi-bin/wa?A0=confocalmicroscopy
Post images on
http://www.imgur.com and include the link in your posting.
*****
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.
Hope this helps,
Ben Smith
Sent from my iPhone
*****
To join, leave or search the confocal microscopy listserv, go to:
http://lists.umn.edu/cgi-bin/wa?A0=confocalmicroscopy
Post images on
http://www.imgur.com and include the link in your posting.
*****
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
--
Simon Wiegert, PhD
Center for Molecular Neurobiology (ZMNH)
Institute for Synaptic Physiology
Falkenried 94
20251 Hamburg
Germany
Tel.: +49 40 7410 55085
simon-wiegert.com
--
Pflichtangaben gemäß Gesetz über elektronische Handelsregister und
Genossenschaftsregister sowie das Unternehmensregister (EHUG):
Universitätsklinikum Hamburg-Eppendorf
Körperschaft des öffentlichen Rechts
Gerichtsstand: Hamburg
Vorstandsmitglieder:
Prof. Dr. Burkhard Göke (Vorsitzender)
Prof. Dr. Dr. Uwe Koch-Gromus
Joachim Prölß
Rainer Schoppik