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Hi All,

There is some very good technical reference material on PMTs in the 
Hamamatsu PMT Handbook at their website: 
http://sales.hamamatsu.com/assets/pdf/catsandguides/PMT_handbook_v3aE.pdf
And a similar one from ElectronTubes (formerly Thorne_EMI): 
http://www.electrontubes.com/info/wedo.html
These are free downloads and have a wealth of info to dispel "folklore"....


Regarding:

2) It depends on the design of the HV circuit and if it has limiting. A 
stiff HV supply with no limiting will allow an enormous current to reach 
the anode if exposed to bright light. The dynode materials are various 
materials that, as I understand it, can be evaporated to deposit inside 
the PMT envelope, this reducing the sensitivity (as the dynode secondary 
emitter materials are depleted) and give more leakage currents. Having 
said this, no unit should ever be supplied with unrestricted voltage 
this way. The normal full scale signal output currents are very small 
and the circuit should have some limits that do not restrict linearity. 
The instrument with the PMT can also use a secondary light sensor to 
shut off the PMT HV if it is exposed to ambient light. In the SEms that 
I am familiar with the PMT HV is derived from the SEM HV generation, and 
so it is shut off if the chamber is vented.

3) Selection definitely makes sense for critical applications. These are 
individual devices with a spread in the characteristics and if some 
feature (low counts/sec, tight pulse shape, etc.) really matters to you, 
a "good" one can be selected from the production run.

4) The operating voltage. It is an easy way to change gain for general 
purpose detectors. Although the amplification is non-linear with 
Voltage, for a given voltage it is linear. The electron multiplier is 
extremely low noise amplification, at it's best in photon counting, but 
lower noise than most electronics of similar bandwidth. Read some of the 
Hamamatsu lit on photon counting for a specific case of finding the 
optimum operation voltage (for photon counting) of a particular tube.

5) Photon counting tubes are a different design in general and require 
very different preamps and discriminators/window discriminators. 
Although Biorad offered a photon counting setting, I am not convinced it 
was very good at all. The PMTs they used were NOT considered to be good 
counting tubes.

Dale Callaham


Michael Weber wrote:
> Search the CONFOCAL archive at
> http://listserv.acsu.buffalo.edu/cgi-bin/wa?S1=confocal
> 
> 1) This is afaik the way it's done in commercial confocals. PMTs need some
> time to stabilize and they also heat up over time, so I doubt that they
> can be triggered.
> 
> 3) I think selection at some point in the production chain makes sense;
> PMTs differ quite a lot (no numbers here, unfortunately), but I am sure
> also Hamamatsu selects them before sending them to vendors. A commercial
> response would be interesting here!
> 
> 4) Good question, I am also interested in that one. As already mentioned,
> voltage offers a wider range, but at a certain level S/N gets down, here
> one could think about the gain instead. But every time I used it in the
> past (Zeiss "Amplifier Gain", Olympus "Gain") I was quite disappointed...
> and Leica doesn't have "Gain" at all.
> 
> 5) Olympus and Zeiss have it for PMTs, and it's also available in FCS
> upgrades with APDs. Older PMT systems where maybe to noisy for this?!
> 
> Michael
> 
> 
>> Search the CONFOCAL archive at
>> http://listserv.acsu.buffalo.edu/cgi-bin/wa?S1=confocal
>>
>> 	Dear all,
>>
>> I have encountered a good deal of folklore regarding PMT operation in
>> scanning microscopy. Maybe someone in the list can confirm or debunk
>> these informations.
>>
>> 1) Some insist that PMTs should never be turned on and off during an
>> experiment, but instead that their voltage should be turned way down
>> when not in use. Is that true?
>>
>> 2) We have routinely exposed accidentally our PMTs to ambient light
>> (sometime for a few minutes at time). The dark noise should have
>> increased. Is this irreversible? Does sensitivity take a hit as well?
>> We use Hamamatsu PMTs with multialkali (R6357) or GaAs  photocathodes
>> (H7422-40).
>>
>> 3) Some microscope vendors claim that they hand-pick their PMTs from a
>> large lot. Does this make a difference?
>>
>> 4) What is the advantage of changing the PMT voltage except for
>> providing a crude gain control? Would it be better to always set the
>> voltage to an optimal value for best signal to noise ratio and amplify
>> the output current?
>>
>> 5) Why is photon counting not widely available on commercial scopes,
>> since computers would deal easily with digitized counts from the PMT?
>>
>> Thanks to all.
>>
>>
>> Quoc Thang NGUYEN, Ph.D.
>> Assistant Project Scientist
>> Physics Department, UCSD
>> e-mail: [log in to unmask]
>> Cell Phone: (949) 246-8143
>> Fax: (858) 534-7697