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What is best wiring method of Photomultiplier Tube dynodes?


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Showing 1-8 of 8 posts in this discussion
Initial post: Aug 6, 2012 8:45:36 AM PDT
This is money and time-saving question regarding the optimum method of hooking PMT to power supply.

The commonly used technique is using high voltage (1000-1500 volt) and resistive dividers along the 9 to 11 dynodes. Between each consecutive dynodes a resistor of 20 kΩ to 10 MΩ distribute the HV equally among the dynodes. Here alone, I have to build two socket assemblies to test the advantages and disadvantages of each resistive mode (the low 20 KΩ and the high 10 KΩ).

The HV method is expensive and bulky. Any regulated HV power supply costs over $200, used, and constitutes a technical challenge in insulation.

The seemingly best choice, I ponder, would be a regulated 100 VDC power supply applying potential in parallel to the 9 to 10 dynodes. I have not seen anyone doing that except an article from 1967 using three 300 VDC sources to power 10-pin PMT, each source supply 3 dynodes. Here also, the regulated DC power supply is not very cheap or small, though smaller and cheaper than the regulated HV one.

The main application in this question is photoscintillation detection, very sensitive to signal to noise ratio. Without high regulated power supply, the signal output from the PMT cannot be used reliably.

Any comment or recommendation should replete the web literature on such technical matter.

Mohamed F. El-Hewie

In reply to an earlier post on Aug 6, 2012 8:55:36 AM PDT
Last edited by the author on Aug 6, 2012 8:56:19 AM PDT
do you know how a PMT works ?

each dynode needs 100v more than the one before it
so ten of them would require a 1KV supply for the last one

no way you can run them in parallel and have it work

and if you could you would still need a KV supply for the last ones

read some amateur radio books
they will tell you how to make high voltage supplies
perhaps even buy a used/surplus one cheaper

you need the specs on your specific PMT to see its impedance and leakage hence how much current it uses

and allow for that voltage drop to pick the right resistors to divide the high voltage supply down for each dynode

look for manufacturer spec sheets and white papers for guidance too

In reply to an earlier post on Aug 6, 2012 9:51:57 AM PDT
andthehorseirodeinontoo? wrote:
"no way you can run them in parallel and have it work".
========================================
I really laughed when you posted your reply while I was messing with the parallel hook up, which does not make sense. Thank you for the quick comment and contribution.

I have the high voltage power supply made of TV flyback transformer triggered by regulated 12 VDC interrupted by signal from 555 timer. The primary of the flyback is fed by MOSFT which takes a signal from hex inverter placed between the timer and the MOSFT. The output of the flyback is rectified and regulated by a row of Zener diodes giving very steady 1200 VDC output. A sort of modification over the following circuit.

http://skemarangkaian.com/0-1000-volt-regulated-high-voltage-power-supply-ic-7805/

Yet, the choice of using 4 separate lines of 300 VDC, in lieu of a single line of 1200 VDC is appealing to me. It only takes 4 transformers to replace the flyback one. A option that solves the problem of fitting a massive transformer in power supply box.

While doing the construction, I warmly appreciate any interrupting thoughts, which I often make to manage the space of components.

Mohamed F. El-Hewie

In reply to an earlier post on Aug 6, 2012 10:07:19 AM PDT
you dont need a massive transformer

there is no current worth mentioning
so no power to need a big transformer

a low wattage small high voltage transformer should work

In reply to an earlier post on Aug 6, 2012 11:44:01 AM PDT
andthehorseirodeinontoo? wrote:
"you dont need a massive transformer
there is no current worth mentioning
so no power to need a big transformer
a low wattage small high voltage transformer should work "
==========================================
Now, you are talking business and power.

I will take you suggestion into consideration as I finagle with the various parts of the settings.

Three concerns relate to the choice of the hv source.

(1) In the setting I have, I need no sparking or strong electric field in the vicinity of the PMT that might deflect the incident electrons onto the wrong electrode. That requires some design consideration when dealing with high voltage.

(2) The transformer response to 2kHz to 10kHz oscillating input should not cause too much fluctuation on the hv output. I would have to play with a modular high voltage supply like EMCO +/-3000 Volts - 3.3mA versus the flyback transformer to see which one provides best regulated VDC.

(3) The flyback measures about 6 cm in height and 4 cm in diameter, but have the reputation of lowest peak-to-noise. Some of the flyback transformers performed decently over 20 years life-time trouble free. On the other hand, modular compact transformer are short lived, from my own experience.

Mohamed F. El-Hewie

In reply to an earlier post on Aug 6, 2012 2:00:56 PM PDT
look at the radio amateurs handbook and other arrl pubs
they will have info on circuits to use and how to do it

Posted on Aug 7, 2012 11:57:15 PM PDT
Last edited by the author on Aug 7, 2012 11:57:51 PM PDT
Deviating from the point high voltage transformer, in order to avoid arching and insulation problem, I found myself favoring the Cockroft-Walton Voltage Multiplier in the form of capacitors and rectifiers connected to the 10 dynodes of the PMT. That would reduce the size of the hv power supply to at least 50% had I used the flyback transformer.

http://www.inl.gov/technicalpublications/Documents/2808460.pdf
http://www.google.com/patents/US5523556?printsec=drawing#v=onepage&q&f=false

In fact the use of Cockroft-Walton Voltage Multiplier should greatly reduce the cost of biasing PMT due to the very low cost of diodes and capacitors. The IC 555 that oscillates the input voltage to the Cockroft-Walton voltage multiplier runs at 50 cents a piece.

A practical description of the this simple biasing approach is found here:

http://www.canit.se/~dlwebb/photometers/dominic_hvps/cw_circuit/cw_circuit.html

Mohamed F. El-Hewie

Posted on Aug 8, 2012 4:04:23 AM PDT
[Deleted by Amazon on Aug 8, 2012 3:45:29 PM PDT]
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Discussion in:  Science forum
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Initial post:  Aug 6, 2012
Latest post:  Aug 8, 2012

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