faq 1

Dear Photomultiplier Doctor,

Can you please explain why photomultipliers are offered with different pin configurations. I have searched manufacturer’s published data hoping to find guidance on the merits of : flying leads, hard pin and capped, but to no avail.

Yours sincerely,

Dr James Rautenbach

Dear Dr Rautenbach,

This is a very good question.

First let me explain about the four types of pin connections that are on offer: hard pin, capped, flying leads for direct soldering to a pcb and flying leads with temporary cap for the customer to solder in place.

The majority of photomultipliers are supplied with hard pins sealed into the glass base and a matching socket. The seated height of the photomultiplier is the same as the length of the glass nvelope and this may be a consideration where space is a concern.

The hard pin base photomultipliers are generally lower cost. Careful mounting on axis is important here and strain must not be exerted on the glass base of the photomultiplier, particularly when inserting and removing the tube from its socket.

Some, but not all photomultipliers, are available capped. Which means we attach a cap made of a special plastic material as an extension to the base of the tube. The pins of the cap are longer and broader than hard pins and so better electrical contact is assured. In addition the photomultiplier seats more securely in its socket.

1. The cap is opaque and makes life a lot easier when attempting to make a light-tight enclosure. Commercially available scintillation crystal assemblies are a good example of this. What you get is a photomultiplier and a NaI(Tl) crystal in a metal tube with the blue cap completing the enclosure - it keeps out the light and the atmosphere.
2. Capped photomultipliers are recommended for use in all housings because the cap removes mechanical pressure from the glass-to-metal pin seals at the base of the photomultiplier. Beware of do-it-yourself, spring-loaded housings used with hard pin photomultipliers.
3. They have lower capacitance, which is important if you have to deal with fast signals and they are usually lower cost.

Finally we turn to flying leads photomultipliers which don’t need a socket.

There are cost saving possibilities here and some reliability gains too because you can solder a voltage divider board straight on to the leads (don’t ever do this to a hard pin photomultiplier).

Photomultipliers used in satellite experiments, where the demands on reliability are paramount, invariably call for flying leads; the mechanical interface offered by the leads provides isolation against the severe shock and vibration created during launch.

We supply flying lead photomultipliers with loose fitting caps so that customer can position the cap in the assembly to achieve a length tolerance of a fraction of a mm. Note glass envelopes are hand-made and hence vary in length by a millimetre or two.

I hope this has cleared up the problem and that nothing has been overlooked.

Regards,

Photomultiplier Doctor.

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