Aryeh wrote:

>BTW, all the comments on S/N ratio of photon counting assume that the
>photon counter only responds to true photon "hits", with some quantum
>efficiency. Is it really the case that the photon counting options on
>our systems, which dont use cooled PMTs (at least the MRC-1024
>doesnt), have negligible thermally generated counts?

As I understand it, by providing a threshhold against which pulses are
judged, the idea of photon counting is to reject the thermal counts
(low energy) and accept genuine photon events (high energy).  The
consequence of this is inevitably that some genuine photons are in
fact thrown away - total signal will be reduced - but even more noise
is thrown away so that S/N ratio is improved.

I must add that on my MRC 600 I have not found much use for photon counting
mode, though this may be a function of the projects that have come through
our lab.

                                                Guy Cox

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