Preamplifier and P/Z troubles

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ChristofferBraestrup
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Preamplifier and P/Z troubles

Post by ChristofferBraestrup » Fri May 08, 2020 9:40 pm

Hey all, I'm stumped in my efforts to build a scintillation preamp. I'm hoping you might have a hint at what's going on.

Here's the issue: tail pulses overshoot, and I get bipolar peaks out of my shaping amplifier. This sounds like a classic pole-zero adjustment thing, but the p/z potentiometer on my shaping amplifier (ortec 572) does absolutely nothing, better or worse.

I don't think there's anything wrong with my shaping amplifier, I've tried two identical Ortec 572 amps and a 452, they behave identical.

I've also tried all combinations of termination, AC/DC coupling etc. I can think of. it doesn't change the issue much.

The preamplifier is a parasitic capacitance amplifier, in the style of the Ortec 113: http://courses.washington.edu/phys433/e ... ec_113.pdf

Furthermore this problem persists between two different homemade preamps.

Thanks in advance!
20200508-012214.jpg

John Myers
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Re: Preamplifier and P/Z troubles

Post by John Myers » Sat May 09, 2020 1:22 am

The Gain bandwidth product of the pre-amp may be too much. Have you tried a low pass filter with the pre-amp?

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Re: Preamplifier and P/Z troubles

Post by John Futter » Sat May 09, 2020 8:07 am

Christoffer
This is a common problem
you need to look up DC restoration circuits for
Video
video signals suffered from this without restoration due to the large negative excursions of the line and frame sync pulses. then the peak black was reduced by the time taken for the video signal dc level to return to normal
PZ adjustment has to to be the inverse basically of your shaping time constant

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Richard Hull
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Re: Preamplifier and P/Z troubles

Post by Richard Hull » Sat May 09, 2020 4:51 pm

What is the scoped value of this negative excursion??? If over .6 volts, throw a 1N914 or 1N4148 fast diode in the proper manner to clip it to where it will not matter. Like has been said, this is normal and typically can be ignored in follow on circuitry.

Richard Hull
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Fusion is the energy of the future....and it always will be
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ChristofferBraestrup
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Re: Preamplifier and P/Z troubles

Post by ChristofferBraestrup » Sun May 10, 2020 6:54 pm

I see, something like this restoration circuit, with inverted diode direction perhaps? : Image

I will try that. The negative component's size is dependent on pulse height, but around 1/3 of the positive pulse amplitude, usually more than the voltage drop of a diode but this will still leave a small negative spike before it's clipped to zero. I guess this isn't a huge problem though. I believe most pulse processing electronics expecting a positive pulse will just ignore the negative component.

It still feels like bad practice to just leave it be, though. I'll try the diode thing! Thanks!

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Richard Hull
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Re: Preamplifier and P/Z troubles

Post by Richard Hull » Sun May 10, 2020 7:22 pm

Many things in electronics and science seem wrongful to leave them less than ideal or perfect. As you note, while not leaving you feeling real good as a purist, If it doesn't enter into the mix as a negative thing, one can save effort and move on to the next issue in the chain of getting an end result.

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
Retired now...Doing only what I want and not what I should...every day is a saturday.

ChristofferBraestrup
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Re: Preamplifier and P/Z troubles

Post by ChristofferBraestrup » Mon May 11, 2020 12:04 am

Yeah that is true. I think I'll leave good enough alone and look for a preamp cheap on ebay.

I added in the diode and that did reduce the negative peak area, and it looks like whatever is left doesn't affect energy resolution, I get pretty decent spectra out of my new MCA!

I guess it's only really an issue at high count rates, then. Thanks for the tip!
k40 mca 1.jpg

ChristofferBraestrup
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Re: Preamplifier and P/Z troubles

Post by ChristofferBraestrup » Mon Jun 01, 2020 12:17 am

I just want to round off this topic saying I was fixing the wrong problem.

It turns out the scintillation detector resistor has ENORMOUS influence on resolution.

Here is an almost 3 hr spectrum of about 100g potassium carbonate: The bias resistor is 100K.
k40lowres.jpg
Here is the same sample again, bias resistor at 2.5M:
k40hires.jpg
That's what I call a difference!


And here's a spectrum of an antique uranium glass bowl: Not great but definitely useable.
uglasshires.jpg

Note the strange phenomenon that there are virtually no counts beyond the 40K peak - my scintillation crystal is only 6mm thick, it is meant for low energy x-rays I believe.

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Re: Preamplifier and P/Z troubles

Post by John Futter » Mon Jun 01, 2020 3:42 am

You are bang on with the limit of your crystal being low energy only
This is because of stopping power of NaI (i presume this is what it is), it is not very dense and therefore you need crystals of this to be 3" thick to provide energy info up to a few 100,000 keV. I have seen old Harshaw units that have NaI crystals 6" in diameter and 10" long, heavy as hell.

ChristofferBraestrup
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Re: Preamplifier and P/Z troubles

Post by ChristofferBraestrup » Sun Jun 14, 2020 2:45 pm

I've just built a brand-new low-cost praemplifier for NaI(Tl) detectors, one that eliminates the issue almost entirely.

See attached. It's an inverting amplifier with a P/Z network, and works much better than anticipated, even with the fairly low-performance TL072 FET op-amp.

The scope screenshot is the output of my shaping amplifier with a shaping time of 2 us.

I'm quite pleased with this outcome, I've tried to annotate the schematic such that it's possible to recreate the circuit for others.

thanks for all your help!

--Chris
PMT CSA.jpg
csa1.jpg

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