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How to figure out the HV Polarity of Hamamatsu Photo Multiplier Tube inside a Scintillator

Posted: Thu Mar 28, 2019 2:02 pm
by Harald_Consul

This issue is about my Rigaku scintillation tube probe I've bought second hand on Ebay.

With the great help of the German electronics forum I have figured out the pin assignements of the integrated preamplifer, respectively the of the 5 pins of the Hirose connector (which are connected to it.)

There's just one little point open, the HV polarity of the Hamamatsu photo muliplier tube, which is assembled within the scintillator. Its exact labeling is "Hamamatsu Photo Multiplier Tube 7K". (No further labeling available.) The scintillator has built between 2000 and 2010.

Basically, two electrical scintillator designs are pertinent:
  1. Negative supply HV, no preamplifer in the scintillator casing, "as is" negative signal to the multi channel analyzer (which contains the preamplifer)
  2. Positive supply HV, preamplifer within the scintillator casing, preamplified positive signal to the multi chanel analyzer (without preamplifer)
In general negative going signal design (no. 1) is more frequent (especially in experimental university research), however in commercial scintillators also positive going signal design (no. 2) is well known.

The current scintillator has been unmounted from a commercial scintillation system.

There are some indications, that the polarity of the supply HV is positive:
  • The scintillator contains a preamplifier, which hints to positive going signal design. (However, the supply voltage of the preamplifer is symmetric -12V/ +12 V. Thus, theoretically negative going signal is also possible.)
  • The HV cable is red.
  • The chromed outer ring of the HV cable has corroded. Usually metals corrode in positive voltage conditions (e.g. metal ions are charged positive).

However, your expertise in scintillators might meet the issue much more appropriate than my indications.

Here come the photos:

1. Corroded HV BNC Plug

2. Voltage divider of the photo multiplier tube
Explanation of the cables in the photo:
  • thick black cable, which connects to the stay bolt in the background == Ground of HV-BNC cable
  • thin black cable in the foreground == Ground from preamplifer
  • the view-shadowed end of the tantal-like light-blue capacitor == connect to the input of the preamplifer
  • And the thick red cable out of the HV BNC plug is the cable, of which the polarity is in question

For the worst case, you can't tell me the right polarity, I have got both, a positive 2 kV Spellman transformer and negative 2kV Spellman transformer. (Both Spellman transformers are limited to 2mA, thus safe.)

If I would have to try it out, the photo multiplier will not be damaged by wrong polarity HV, because the only thing that can damage a PMT is too high voltage in correct polarity, right? (In this case the current gets to high and burns the PMT). But in wrong polarity there won't be any current at all (thus no damage), right ?

Re: How to figure out the HV Polarity of Hamamatsu Photo Multiplier Tube inside a Scintillator

Posted: Thu Mar 28, 2019 6:07 pm
by Rich Feldman
Harald, you made a good presentation of the circuit configurations.
I bet you can get your answer from studying the PMT socket connections, which I think are pretty standard.
Here's a snip from a Hamamatsu catalog ... Z0003E.pdf
that shows two that appear to be about the right size. One might be called 7K series, and both have the same 12-pin base pinout

p.s. Tube pins are numbered clockwise when viewed from bottom. That convention was continued with IC's in TO-cans, DIPS, SOIC's, and quad flatpacks.

Re: How to figure out the HV Polarity of Hamamatsu Photo Multiplier Tube inside a Scintillator

Posted: Fri Mar 29, 2019 10:41 am
by Harald_Consul
Thanks Rich. You can be sure, I invested some time on learning the ropes of electrical scintillator designs.

I am going to study your clue, now.

Am I correct with "wrong HV polarity does not harm a photo multiplier tube" (as it would not generate any current at alll) ? Maybe I should just precautionaly disconnect the preamplifer, if testing so (just as backup possibility).