PMT and resistor chain too high

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Chris IM
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PMT and resistor chain too high

Post by Chris IM »

Hello,
I'm making my own gamma spectrometer (I started a few months ago) and I still struggling with it as there many noise problem coming from my HV supply , comming from the USB port of the pc on wich I plugg my sound card... etc..

2 days ago I started to get some spectrum but there was a linearity problem and I can see that depending on the distance and activity of the specimen I'm trying to male the spectrum of , the spectrum is shifting : eg the low energies disapear and the high peaks rise...etc showing a voltage drop in the last dynodes.

My pmt is XP2020 Philips and a EMI THORN 9813KB.
crytal NaiTl 70mm almost new and clear as water.

The chain divider are mostly 10M resistors...and I calculated it for a 10uA current in the chain...because my HV supply wont be able to give much or more that let say 16uA..

My question is : what happens if the total resistance is too high? and what happens when too low ?

I read in different papers that the voltage divider current had to be 20-100 the current in the pmt or at the anode ....

As my pmt has 12 dynodes ... Ihave between 170M to 120M resistance...

thanks for your help..

To show you the shifting of the spectrum here's attached a rough gamma spectrum with radium at different distance from the pmt.

Background looks like a lot the "spectrum" of radium as there are noise problem that drowns part of the information of pulses , hence the difformed spectrum. I use Theremino and if i use the "compensation" the peaks become more conspicuous showing that the information is present but its not clen at all so far.
ThereminoMCA_2023_02_09_00_52_18 RADIUM DIFFERENTES DISTANCES.png


Chris.
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Richard Hull
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Re: PMT and resistor chain too high

Post by Richard Hull »

My 3" Bicron PMT drags 130 ua through the dynode string. I have found that 100k -to 220k to be the norm for each resistor on dynode resistor strings.

Attached is image of my radium spectrum taken on my Canberra gamma spec back in 2001. This is what you must see with a tolerably decent gamma spectrometer.

Richard Hull
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Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
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Jerry Biehler
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Re: PMT and resistor chain too high

Post by Jerry Biehler »

If you need a little HV power supply you might try one of these guys: https://www.digikey.com/en/products/det ... B/14543316

They make a negative and positive output version for however your setup is wired.

https://www.mornsun-power.com/html/pdf/ ... 1H-1B.html
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Jim Kovalchick
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Re: PMT and resistor chain too high

Post by Jim Kovalchick »

I use a Canberra 2007P base which has an integral preamp and a 7.2 Mohm dynode string resistance. I will put up my Goldilocks and the three bears to go along with Richard's.

Jim K
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John Futter
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Re: PMT and resistor chain too high

Post by John Futter »

Jerry
/richard
why encourage this guy who cannot read the joining rules?????!!!!!
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Richard Hull
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Re: PMT and resistor chain too high

Post by Richard Hull »

I have admonished him in his introduction posting, just now.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
Chris IM
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Re: PMT and resistor chain too high

Post by Chris IM »

thanks very much for your answers but still : could you tell me what happens (on the spectrum appearance) when the current isnt high enough when the resistor chain is too high ?

The HV module you proposed isnt powerfull enough for my PMT : 1200V is to low. I checked on my oscilloscope : the first pulses start to become visible when the voltage reach 1500 V minimum.

If i'm asking this its because I want to be able to recognize when the setup is correct.

I also have noise problems as there is a 33kHz frequency superimpose on the base line of the signal and I guess it comes from the oscillator of the HV module I use but with some RC filters it should be eliminated.
Chris IM
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Re: PMT and resistor chain too high

Post by Chris IM »

ALso I have to give more informations on my setup :
its all DIY ; i cant afford to buy expensive HV module or other already made preamp or PMT module so I have to make my own.

I'll post some schematics and photos of my crude setup of the moment. My HV is a HV module that I bought on amazon and that is the kind of module found inside a sort of flykiller racket. It works pretty well but it has to be filtered to eliminate the noise (wich is high : 500mV!!)

ps : i wanted to add that and mix the last 3 posts in the same ne but i dont know how to do it or even if its possible. sorry.

Chris
Chris IM
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Re: PMT and resistor chain too high

Post by Chris IM »

Hi again :
I've made some change tonight (Paris time) : I changed some of the 10M resistor for 5M and as a result there is more current available for the pulses to form "properly". The pulses observed on the oscilloscope and on Theremino software are more suited to be treated by the software and the sound card.

Also the PMT runs with more stability. And also one oddity I noticed on both my PMTs was that there was periodically a huge current spike (20v!) happening now and then and I couldnt understand how they buit up. Now that the current has less resistance these spikes have disappeared.

The spectrum looks much better (i'll post some photos). So now I know that these huge pulses could be a sign of too much resistance. Both Pmt have 12 dynodes and their total resistance were around 150 to 170M for some 16uA current max .

to be continued..

PS : so fat there is no preamp : only a simple passive shaper made of 2 RC integrator. I'm making a preamp I found the schematic of in a youtube video where a person made his own gamma spectrometer.

Chris
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Richard Hull
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Re: PMT and resistor chain too high

Post by Richard Hull »

Most all PMT scintillator combos tend to use only 8 dynodes....Rarely 10. The gain on 12 or more dynodes is in the million plus range. As such, they are sensitive to noise from many sources. 8 dynodes rely on less optical gain and demand a good preamp to give solid noiseless pulses.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
Jerry Biehler
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Re: PMT and resistor chain too high

Post by Jerry Biehler »

Looked at the data sheet for those and they are pretty high voltage, probably because of the dynode count.

I think you should follow the divider chain recommendation in the data sheet for the PMTs, The thorn also wants a 300v focus voltage and the other has special requirements too. Are you providing those voltages?

Check ebay for ultravolt high voltage power supplies, sometimes they go pretty cheap. But in the end you are just going to need a bigger power supply if you try to use these tubes. The Thorn at least starts working at about 900.
Chris IM
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Re: PMT and resistor chain too high

Post by Chris IM »

Yes I was following the recommendations and I tried many things but there is something very important that is very often overlooked : the sound card...

At the time Livio published THeremino MCA , he used a simple USB sound card and it had to be modified in order to make it suitable for gamma spectrometry .

I bought several of these cards and burned them off so easily as the slightest voltage pulses from my high gain pmt and high voltage source was too much for these cheap cards. Plus the more recent 2022 models arent as easy to modify as the old 2015 model were.

There is a capacitor that filtered out most of the pulses of my pmt . So as soon as I bought a proper sound card 10 days ago I was at last to record my first proper spectrum but the lack of linearity was an issue .

Also at the time I thought it was a problm of low current and I changed the rersistors for 5M resistors but as I didnt have the good sound card I couldnt se if there was actually an improvment and I thought it didnt work.

So , I changed most of the resistors for 5M except for the focusing and the 2 last dynodes. I also added a 10nF cap on the 2 last dynodes....and here is the result on radium :
ThereminoMCA_2023_02_13_00_31_37 RADIUM 5M++++++.png
compared to the previous results its nothing comparable....

There are still some improvment to do like making the preamp (there is no preamp yet) and adding 1 or 2 more staged of RC filter to get around 50 uV ripples on my HV module...

to be continued...
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Richard Hull
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Re: PMT and resistor chain too high

Post by Richard Hull »

Great work! Now you are getting results!

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
Jerry Biehler
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Re: PMT and resistor chain too high

Post by Jerry Biehler »

What card did you end up getting?
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