Hello Tim,
Meridian instruments (Ultima and ACAS) are used a lot for Calcium work with
Indo, and we used side window PMTs, which give better quantum efficiency and
longer spectral response than end-window designs.  They are actually more
sensitive at 400 than at 600nm.  I am a little rusty on what our former
competitors are using, especially in their new scanners.  But last I knew,
Leica was using a small side-window PMT, and picking blue optimized versions
in their UV sustems.

Sending the light through the side of the tube onto an internal phosphor
screen is more sensitive because electrons are liberated from the same side
as the photons enter, and they can be steered more readily onto the
cascade-forming row of dynodes.  The down side is that the light has to hit a
very small sweet spot, only a few mm across, rather than the inch or so which
is more typical of end window designs.  This makes setting up the system much
more challenging, since every tube has to be rotated and displaced until it
gives the highest response.

I don't know if you have the option of getting a side window detector for
your Indo work, but there are so many designs of PMT out there, there must be
something which would work at 400.  For vendors, try Hamamatsu and Thomas.
If nothing else, you may be able to use the trick CCD cameras have of coating
the incident surface with a UV absorbing coating, which then emits at a
longer wavelength and hence extends the spectral response.  In cameras, this
ups QE from zero to 15%, when peak QE is about 70%.  Perhaps this, and
BioRad's photon counting expertese might be enough to get it to work, but be
aware also that Indo is tricky even on a custom designed UV system.  If you
can get it to work well with multiphoton, you would have something to brag
about.

Hope this helps.

Non-disclaimer:  I am an industrial scientist, working for a company that
makes laser scanning imagers.  Therefore, I cannot claim to be impartial, but
I do hope this input is useful.

David Carter
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