Weve been following the discussion about Bio-Rads enhanced pmts with some
interest, as we have been investigating this subject in connection with
low-level light detection from photoproteins, and we feel that a positive
email from us might be helpful to anyone else who is interested in this
topic (with the possible exception of Bio-Rad [:-) but were not out to
knock them either).  The general idea has been around for a long time, and
anyone who is interested in it could well do worse than to consult the
paper Optical Devices to Increase Photocathode Quantum Efficiency  by W.D.
Gunter, G.R. Grant and S.A. Shaw, in Applied Optics Vol 9, pp251-257,
published way back in the mists of 1970.

This paper describes the technique of increasing sensitivity by multiple
reflections within the photocathode, and suggests a variety of external
optical attachments, based on prisms or prism-mirror combinations, that
allows it to be applied to any standard end-window photomultiplier.  The
simplest implementations have the photocathode at a 45 degree angle to the
incoming light, but other possible arrangements allowing the photocathode
to be at its conventional orientation of 90 degrees are also described
there.  Those are more complicated, but are likely to be more convenient in
practice.

We dont know what Bio-Rad are doing, but it looks as if anyone else could
achieve equivalent results to theirs by implementing one of those
arrangements or an optically equivalent one, and putting it onto a
conventional photomultiplier.  The data in the paper shows that the
improvement is greater at longer wavelengths, and also appears greater for
an S20 than for a bialkali photocathode.  With a bialkali tube, and using
what they called a two-bounce device, they got a 25% improvement at 500nm,
and 50% at 600nm, increasing to100% at 800nm (but youre talking about an
actual quantum efficiency of twice almost nothing at this wavelength with
bialkali!).  The improvement with an S20 tube was greater, being about 100%
(i.e. twofold) at 500nm, 140% at 600nm and 200% at 700nm.  The quantum
efficiency of bialkali exceeds that of S20 at wavelengths shorter than
about 550nm (data from Electron Tubes Ltd, formerly Thorn EMI), so the
greater enhancement with S20 would tend to suggest under these conditions
that bialkali would not overtake S20 until a somewhat shorter wavelength
(about 500nm?) was reached.

In case this sounds like a free lunch (although it can be sometimes), the
downside is that both the permissible area and the angle of the incoming
light beam are considerably reduced by these arrangements - unlikely to be
a problem for confocal, but it may be for other applications.  We expect to
be playing around with the method ourselves during the next few weeks, and
well be happy to share all results and technical details on request.

                Martin

Dr. Martin Thomas
Cairn Research Ltd
Unit 3G, Brents Shipyard Industrial Estate,
Faverahsm, Kent, ME13 7DZ
Tel: 01795 590140  Fax: 01795 590150
E-mail: [log in to unmask]
http://ourworld.compuserve.com/homepages/cairnresearch