Voltage divider for Scionix 38B57

Scintillation crystals, PMTs, voltage dividers etc...
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kotarak
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Voltage divider for Scionix 38B57

Post by kotarak » 07 May 2020, 10:06

Hey guys,
I am a complete newbie to Gamma Spectroscopy - just purchased a Gamma Spectacular MCA and will be building a probe based on the Scionix 38B57 /1.5M-E1 scintillator for starters.
I am about the remove the factory board off the back of the PMT and would like to build a voltage divider for use with the GS MCA.
Can someone point me to a schematic (with components values appropriate for use with GS) and how to wire the voltage divider to the PMT so I can use it with a single coax cable?
While I've been in the electronics industry and hobby for almost 40 years, I have not dealt with PMTs before so please don't assume any practical knowledge in this area.
Any help would be greatly appreciated!

Best regards,
Andrey Stoev

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Svilen
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Re: Voltage divider for Scionix 38B57

Post by Svilen » 07 May 2020, 19:01

Zdravey Andrey :)

Welcome to the forum, I'm glad that the bulgarian presence here grows :)
Please, add your real name and location in your signature (or in the proper place from the user control panel) and introduce yourself shortly in the "Introductions" section, since the forum rules require it.

About the voltage divider, please check the correct pinout of the PMT used in your detector, but a standard situation would be:

2R resistor between the cathode and the first dynode and R between the rest of the dynodes and between the last dynode and the anode. Then 2 or 3 HV (above 2KV) 10nF capacitors between the last 2 or 3 dynodes and ground. The ground and the cathode are connected together.
You may also try 10nF capacitors only between the last 3 dynodes (D7-D8; D8-D9; D9-D10) - 500V capacitors are enough in this case, with or without a 4th (this one High Voltage!) capacitor between the last dynode and the cathode.
For the resistors you can use 2MOhm-1MOhm-1-1-1... resistors with a total resistance of 12MOhm, and that's a good selection for the GS.
You would also like to buy a diheptal socket in case you don't have one - check eBay for it. If the PMT is without a socket and just wires coming out from the glass, then definitely find out the correct pinout, since the pins will most likely not be in the order from the picture below.

Here is one nice picture from Steven that I can attach to illustrate my words:
VD.png
Greetings,
Svilen

kotarak
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Re: Voltage divider for Scionix 38B57

Post by kotarak » 08 May 2020, 00:10

Zdravey Svilen I blgodaria!

I introduced myself as suggested.

This information is really helpful! Ill order the parts and will let you know how it turns out.
Tom from Irad inc. suggested that for this PMT, R should be doubled (R=2MOhm and corresponding 4MOhm for 2R) - does It mean that voltage range will be set differently as well?
Where do I supply HV if I want to separate the HV line and the signal line - do I just include a capacitor in series on the output and get the signal out this way clean of DC component? what would be the best value for such cap?
I *think* the PMT is Hamamatsu R580 or R980 but not sure how to find out which one.
Andrey E. Stoev
Brookfield, Connecticut, USA

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Svilen
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Re: Voltage divider for Scionix 38B57

Post by Svilen » 08 May 2020, 03:44

Tom is surely competent and have lot's of experience with these detectors, so you may try this too. I personally don't know what is the benefit from increasing the resistance, it just increases the noise too. This will reduce the current drawn and may be good for battery powered portable setups, but what else? Protective?
Yes, you can use just an 1nF 2-3Kv capacitor for the anode signal output. There is also no difference from using separate HV.
Svilen

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iRad
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Re: Voltage divider for Scionix 38B57

Post by iRad » 08 May 2020, 07:48

Hi Andrey - I sent the schematics and recommendations to your hotmail account yesterday. Did you not get them? Let me know and I can send them again if needed.
Cheers, Tom Hall / IRAD INC / Stuart, FL USA
Please check out my eBay Store: http://stores.ebay.com/The-Rad-Lab

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iRad
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Re: Voltage divider for Scionix 38B57

Post by iRad » 08 May 2020, 08:03

Andrey - I just sent you additional diagram for separate (decoupled) HV and signal.
Cheers, Tom Hall / IRAD INC / Stuart, FL USA
Please check out my eBay Store: http://stores.ebay.com/The-Rad-Lab

kotarak
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Re: Voltage divider for Scionix 38B57

Post by kotarak » 08 May 2020, 09:43

Hi Tom,
Thanks so much for sending those! I didn't get them because Hotmail decided to relocate them in my Junk Mail folder! I just dug them out! This is perfect - what you sent! Thanks again!
Andrey E. Stoev
Brookfield, Connecticut, USA

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