Dear Photomultiplier Doctor,
I would like to know if it was possible to extend the shelf life of photomultipliers by storing them under a gas with a low He content.
Air has typically about 5.5 ppm He. I wondered if standard bottled N2 had significantly less He than this (say less than 1 ppm). I was thinking of storing tubes before use for about 5 years.
Do you know if anyone had tried these techniques or has details of the He content of standard industrial gasses.
Regards,
Paul Flower.
Rutherford Appleton Laboratories UK
Our standard glass photomultipliers can be stored in normal atmosphere even over the extended period
described without significant permeation of helium. To avoid oxidation of the pins over this extended
period it is however advisable to store the photomultipliers in dry air.
If the photomultipliers have a quartz window there will be sufficient helium permeation over this extended period to be measurable in the afterpulse spectrum. The photomultiplier will continue to function because helium does not react chemically but the electron currents flowing inside the photomultiplier will cause temporary ionisation of the helium during operation. In pulsed light applications the impact of these ions on the active surfaces produces late pulses around 0.3 µs after the initial pulse. In dc or slowly varying light applications the helium ionisation will cause a small percentage increase in the measured signal.
So for this extended period it is recommended that quartz photomultipliers are stored in dry nitrogen at normal atmospheric pressure in an air tight container (really helium tight). Any sealed container (usually metal, standard borosilicate glass or pyrex glass containers that can be sealed are suitable).
Regards,
Photomultiplier Doctor.
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