faq 4

Dear Photomultiplier Doctor,

My problem is: I accidentally exposed the side window of a photomultiplier to ambient light during a routine maintenance. My question is:

1. Is it a guarantee that the photomultiplier will be destroyed whenever exposed to bright light?
2. What is the threshold intensity for destruction of the photomultiplier?
3. Is time of exposure a factor?
4. What is the physics behind the damage if it occurs?

yours sincerely

John Popoolade

Dear John,

High interdynode currents are generated within a photomultiplier when exposed to bright light. The electron bombardment of the dynode surfaces releases ions and if sufficient in number, breakdown will occur causing the photomultiplier to glow. Removing the bright light leaves the photomultiplier with an ion-damaged photocathode and filled with gas.

This is the worst case scenario but a photomultiplier will self-protect if the voltage divider is sufficiently resistive. If the divider current is less than 100 µA the dynode currents will be limited to below this value and the photomultiplier is likely to survive.

A discoloured photocathode is a sure sign of damage and spiky signal traces are also indicative of an ex-photomultiplier.

Regards,

Photomultiplier Doctor.

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