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