Dear Photomultiplier Doctor,
Whilst cooling my extended red response photomultiplier I can decrease the dark count (aka background) but how do I keep the signal from vanishing?
Ganesan Ramanathan
National Physical Laboratory
You must cool as little as possible and use the photomultplier with optimal signal/background ratio.
The change in photocathode sensitivity with temperature depends on the photocathode type and the wavelength of incident light. Note how the variation is greatest at the long wavelength limits of sensitivity. The loss in red sensitivity should be noted whenever cooling is employed to reduce dark counts. The temperature coefficient of the photomultiplier is the combination of the change in photocathode sensitivity and electron multiplier gain, e.g. +0.4%/ ° C for the S20 @ 900 nm.
It is never a good thing to be operating a photomultplier where the QE is a fraction of a percent - this is where the temperature coefficient is very large. You are unfortunately up against fundamentals of physics. Maybe you need to consider some light chopping.
However, we do offer photomultipliers with prisms on the inside of the window to bend the incident light to enhance the QE for this purpose. If your light is parallel and of diameter less than 4 mm then we can also offer an external optical enhancer that would work. This gives from 2 to 4 times increase.
Regards,
Photomultiplier Doctor.
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