Questions about Voltage Divider PMT bases

Scintillation crystals, PMTs, voltage dividers etc...
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Mike S
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Questions about Voltage Divider PMT bases

Post by Mike S » 26 Jun 2020, 17:31

I have not found a good source that explains the basic questions I have about voltage divider bases for PMTs. I'll ask my questions, but if someone just wants to say "go read this and it will explain it", that would also be great. I could not find my answers in Practical Gamma-ray Spectrometry by Gilmore.

1. How much does the total resistance matter? Is there a reason to choose a lower resistance string vs. higher resistance?
2. Does it make a notable difference when the signal is separated from the HV at the detector base (two cables vs. one)?
3. Is there any reason for a single detector setup to need gain adjustment? It seems like you'd want it all from the detector and then adjust the voltage and downstream electronics as needed.
4. What does focus do, really? I bought a used detector that has a built-in focus pot and I can't see any obvious difference in the spectra.

Thanks in advance for any advice.

Mike S.
Mike Sullivan
Central Coast of California, USA

luuk
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Re: Questions about Voltage Divider PMT bases

Post by luuk » 26 Jun 2020, 19:57

Hi Mike,
1: A low resistance vd will give less or non-peak shift at (very) high counter rates, so not very important for hobby use because hobby sources are (hopefully) not strong, if they are better you find a weak source!
High resistance vd will use less power from your Hv unit , therefore better for hobby use and perfect portable(battery powered) applications!
2: separating signal in or outside the detector makes no difference in performance of the detector, most counters and hand held detectors use single connection because one wire is more practical than two wires. For desktop detectors two cables will work perfect. So it depends on the use you have for your detector.
3: this question I don’t understand!
4: A grid or focus takes care that most/ all generated photons from the cathode are getting as fast and accurate as possible to the fist dynode with minimum loss.
This is important in fast timing application and mostly only used in high energy physics with fast scintillators e.g. BaF2 for hobby use it is not important and does not give any advantage
You can simply connect it as a dynode no. 0 in your vd chain.
Luuk

Mike S
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Re: Questions about Voltage Divider PMT bases

Post by Mike S » 27 Jun 2020, 04:40

luuk wrote:
26 Jun 2020, 19:57

3: this question I don’t understand!

Luuk
Luuk,

Thank you for your answers.

Question 3 was because I have a detector that has a "G" adjustment potentiometer. I'm assuming it is a gain trimmer, and it does seem to perform that function. My question is why would we want to adjust the gain at the detector and not just leave it at maximum? It seems we could just turn down the voltage if we wanted less gain. I assume it could be useful if you had multiple PMT's (for some reason) connected to the same HV source and you wanted to adjust them to match, or if you had a system with a fixed HV.

Ultimately, my question really is: If I'm shopping for a voltage divider PMT base, is a gain trimmer on the voltage divider base a useful feature for hobby gamma spectroscopy? My guess is that it is not useful, but I figured I'd ask.

Mike S.
Mike Sullivan
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luuk
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Re: Questions about Voltage Divider PMT bases

Post by luuk » 27 Jun 2020, 06:41

Hi Mike,
The main purpose for any gain adjustment on a detector is normally used to match one detector with other detectors, so if you do not couple different detectors together
and you have just one here is not a real use for it.
Set the gain to it maximum is always good because in that case you may need less high voltage to get the for you appropriated signal out of it.
The lower the high voltage the longer you will extend the life of the pmt.
Luuk

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Steven Sesselmann
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Re: Questions about Voltage Divider PMT bases

Post by Steven Sesselmann » 27 Jun 2020, 21:32

Mike,

Just one additional note on resistors.

Use a quality metal film resistor and not just any low grade carbon type, the reason is resistor thermal noise or "Johnson noise".

Long version here: https://en.wikipedia.org/wiki/Johnson–Nyquist_noise

In a laboratory where you have a mains powered NIM power supply one can use very low value resistors because you are able to supply plenty of current, but if you are using a battery operated meter or a USB powered GS, the amount of current is limited.

But you need not get too carried away with this idea because the amount of thermal noise from good resistors is relatively small compared to the typical gamma pulse, if anything it will affect the low energy signals <25 KeV. Remember the weakest link in the chain is usually the NaI crystal.

Steven

kotarak
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Re: Questions about Voltage Divider PMT bases

Post by kotarak » 28 Jun 2020, 12:12

Hi Steven,
is there a specific Temperature Coefficient threshold we should be aiming at? I built my divider with resistors listed as 100ppm/K - not sure if this is good enough?

Best,
Andrey E. Stoev
Brookfield, Connecticut, USA

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Steven Sesselmann
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Re: Questions about Voltage Divider PMT bases

Post by Steven Sesselmann » 28 Jun 2020, 14:42

Andrey,

I am not an expert on this subject, but there is plenty of information about resistor noise on the web.

http://www.resistorguide.com

I also found this calculator for Johnson noise here: https://www.daycounter.com/Calculators/ ... ator.phtml

According to that calculator a 12 M Ohm resistor at 20C˚ and 1MHz the noise is around 440 µV., this is still quite insignificant as your typical spectrum with 1000 channels has around 1 mV per channel, but in principal I agree you should optimise your design by using good quality resistors with appropriate values.

Steven

Mike S
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Re: Questions about Voltage Divider PMT bases

Post by Mike S » 14 Jul 2020, 04:38

luuk wrote:
26 Jun 2020, 19:57

4: A grid or focus takes care that most/ all generated photons from the cathode are getting as fast and accurate as possible to the fist dynode with minimum loss.
This is important in fast timing application and mostly only used in high energy physics with fast scintillators e.g. BaF2 for hobby use it is not important and does not give any advantage
You can simply connect it as a dynode no. 0 in your vd chain.
Luuk
Luuk,

I have another thread where I'm trying to figure out an apparent detector issue. One question is if the voltage divider is correct and your comment here is particularly relevant.

I'm not sure I'm understanding correctly. If I don't have an adjustment pot for the grid/focus, should the grid pin of the PMT be connected to:

1. Ground/Cathode?
2. Dynode 1 ?
3. In between the ground/cathode and Dynode 1 as a sort of additional dynode with a resistor divider so that its voltage is between ground and dynode 1?

It seems like you are saying #3, which matches what I'm being told in the other thread, but I wanted to be sure that's what you meant.

If the grid was connected straight to dynode 1 or straight to ground, what performance impact would you expect me to see? In the schematics I've seen with an adjustment pot, it seems like the adjustment range could set it to those extremes.

Thanks,

Mike
Mike Sullivan
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luuk
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Re: Questions about Voltage Divider PMT bases

Post by luuk » 14 Jul 2020, 04:52

Hi Mike,
The grid/focus can simply be seen as a dynode (name it dynode 0) so Cathode connected to ground then a resistor to the grid/focus=Dynode 0 and then a resistor between grid/focus=Dynode 0 to Dynode 1 then the rest of the vd as always. The resistor value can be all the same for every resistor in the vd chain.
Hope that answers your question.
Kind regards,
Luuk

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