Question on which NIM amp for a 3He detector?

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Alex Aitken
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Re: Question on which NIM amp for a 3He detector?

Post by Alex Aitken »

Some test inputs are fine with a square wave. They have a capacitor, usually a very small one, in series. This is very common on preamp test inputs, though 1V sounds large for a preamp test pulse. The result is a charge injection/withdrawal on the falling and rising edges of the square wave. Each burst induces a waveform with a shape like a tail pulse. This ought to work just fine as the input to a shaping amplifier. If the output of the shaping amplifier takes a long time to decay, that would suggest something is wrong. I'm not clear on the setup, but the long decay makes me confused. I might be inclined to make sure the input to that is falling fast enough, and check things like impedance if it isn't.
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Richard Hull
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Re: Question on which NIM amp for a 3He detector?

Post by Richard Hull »

I speak just to testing the SCA input and discrimination here. Typically the preamp output is simply amplified first to give a very decent signal of a volt or more into the SCA which then tunes out the trash using the SCA. Such signals are typically very narrow 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
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Dennis P Brown
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Re: Question on which NIM amp for a 3He detector?

Post by Dennis P Brown »

My first results with the CSA chip/circuit- where the peak was barely above the curve which did not decrease very much - looks to have turned out to be the fault of the chip rather then due to the larger caps I installed in the board. By the next day the CSA chip's output performance had deteriorate till it was barely responding. Since I was applying all of 2 volts to the test input connector, I seriously doubt my setup harmed it (and I was using their circuit board). So I returned it for a replacement.

The replacement arrived today. Using the new chip in the identical setup the unit now gives the exact response they displayed in the diagram (strong & sharp peak, complete discharge to zero by the end of the square pulse.)

These chips make me nervous because they appear extremely touchy - too bad there is no work-around to avoid them or method that would better protect them. Every time I go to use this pre-amp I am convinced I'll do something wrong and destroy the chip - the last few days haven't made this worry any less.

Speaking of issues with my neutron detector system, one of the my HV coaxial cables had one of its center pins just fall out of its connector! So much for a good solder job by that company (no idea who made it) - this was supplied with my original 3He detector system, no less.

Starting to feel that Lt. Murphy would be interested in my work of late.
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Richard Hull
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Re: Question on which NIM amp for a 3He detector?

Post by Richard Hull »

Needless to say the input to the CR110 goes to the gate of an FET that has no issue responding to 100 Fa current and is easily destroyed by just clicking on the bias supply. Did we touch on bringing the bias voltage up very slowly and by the same token, lowering it very slowly. The larger the input capacitor value, the more instantaneous pulse current can pass with the instant a 1600 volt bias is just snapped on or off. I usually take about a full minute twirling the ten turn pot on my bias supply from zero to 1600 volts.

My input capacitor in the Princeton Gamma tech preamp is a .002ufd @ 3kv, but I have seen .01ufd caps on PMT preamps. My preamps are discrete component system boards with maybe 50-60 components on the board. (old school)

Richard Hester's fully functional system board used FET input op amp input, but also contained a discriminator circuit as well. He demo'd his board system at one of the early 2000's HEAS. I attach an image of Richard Hester from a 2004 HEAS event. He is a fabulous designer of modern circuitry.
Very missed now but he occasionally checks in here with a post or two.

Richard Hull
Attachments
L to R 2004 HEAS  Richard Hester, Richard Hull, Carl Willis
L to R 2004 HEAS Richard Hester, Richard Hull, Carl Willis
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
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