Photo multiplier tube question.

[Monochrome]

Hi folks,

I wonder if anyone could clarify some details regarding the use of photo multiplier tubes please?

After reading a few back issues of the NBTVA newsletter I note a number of warnings which state that a photo multiplier tube can be instantly destroyed if exposed to daylight while switched-on.

While I can see how excess current might flow in the final anode due to the huge electron multiplication would it not be possible to simply include some form of current limiter to ensure that the device will simply saturate if exposed to daylight and limit any pottentialy damaging currents?

It has also been said that exposure to daylight (even when not switched-on) can degrade the performance of the photo-multiplier for days at a time until it settles down again.

Trawling the net has uncovered conflicting accounts with some writing that exposure to daylight (when switched off) does no harm at all and that photo multipliers are electricaly very "rugged" while other texts seem to imply that an old tube will almost certainly be U/S or below full spec due to mis-handling.

Are there any group members with genuine experience of using/handling photo multiplier tubes and if so could they confirm/deny the scare stories. I am thinking of making use of a 931A photo-multiplier tube but only if its likely to be of any good. While the tube is brand new (old stock) I have no way of knowing if its been out-of-the-box in daylight though it does appear to be un-opened.

Many thanks,

Des (M0AYF)

[Steve Anderson]

Hi Des,

...erm, there's conflicting opinions on this. I did a fair bit of playing around with PMTs a couple of years ago, one thread you may find of use is this...

http://www.taswegian.com/NBTV/forum/vie ... se&start=0

One thing I was able to confirm was the dark current issue, that if a PMT has been unused for quite a while it takes some time for the dark current to settle down to a current within specification. You'll notice that in the tread above I was also using 931As.

Exposure to normal daylight whilst un-powered doesn't seem to affect the tubes in any major way, at least those similar to 931As. Just give them some time to 'dark-adapt' as the manufacturer Burle says. 24 hours should be ample.

Exposure to 'normal' light-levels while powered will destroy a PMT...well, that's what all the manufactures say and they should know. Within that thread above you'll find some links to Burle and Photonics handbooks and datasheets which are invaluable.

To incorporate some form of current limit wouldn't be easy whilst still providing the required voltages on the dynodes. The best bet is simply to ensure that the tubes do not get exposed to daylight while powered. This requires some discipline on your part and perhaps some interlocks on equipment covers.

If there's any other way I might be able to help, just ask. Another member here (amongst others) is Steve Ostler who has built a colour mechanical camera using PMTs.

Steve A.

[Monochrome]

Hi Steve and the group,

Many thanks for the response and also for the excellent references to the previous thread on this subject. I found the Burle and Hamamatsu handbooks particularly imformative. I have only had time to read a few chapters of each so far but "wow" such detail, so much I did not know about PMT's.

Following your comments and the chapters read it is much more clear to me now that your advice is probably the most practical solution to addressing the "scare stories" i.e. "ensure that the tubes do not get exposed to daylight while powered" and "some interlocks on equipment covers" both of which are much more straight forward than any current limiting which I had proposed.

Thanks again and best wishes to all.

Des (M0AYF)

[Steve Anderson]

Des, you're welcome,

Yes, I doubt there's much missing from those references regarding PMTs, certainly at the level we use them. When you do plug the soldering iron in would you keep us up-to-date as to what you're building? Or you might submit it to Jeremy for inclusion in the newsletter, he's always looking for content. Writing even a short piece is appreciated.

Steve A.

[Monochrome]

Hi again Steve,

When you do plug the soldering iron in would you keep us up-to-date as to what you're building? Or you might submit it to Jeremy for inclusion in the newsletter...

Understood, I will do both.

Thanks again for the references.

Regards,

Des.