Not Quite As Simple CSA

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Richard Hester
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Re: Not Quite As Simple CSA

Post by Richard Hester » Tue Mar 09, 2010 3:09 am

Attached is the output waveform for the circuit in question built dead bug style on a small piece of copper clad board. The excitation source is an LED/Photodiode pulser similar to the circuit described in "files". The charge storage cap used was a 1 pF +/- 0.25pF NPO disk. The coupling capacitor (4.7 nF) was moved to the input rather than inside the feedback loop to allow greater freedom of choice for the charge storage cap. The voltage rating for the charge storage I chose is unknown (though I suspect it is either 500 V or 1kV rating). The input, output, and summing nodes were built off the board on small standoffs for isolation and/or ease of connection. The picture shows a 1/e time of about 2.9 usec, which is somewhat longer than expected, but not too far off considering expected stray capacitance and the small value of charge storage cap.

Will post more info and pictures tomorrow as time permits. This represents ~15min of effort (the data taking , that is - the circuit took a couple of hours of effort, including digging up the fet for the input). The circuit came right up and required no extra effort. I spent a lot more time trying to find my pulser....
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Richard Hester
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Re: Not Quite As Simple CSA

Post by Richard Hester » Tue Mar 09, 2010 7:32 pm

Here is the actual build of the CSA circuit using dead bug construction. The circuit was constructed on single-sided copper clad board. An additional piece of board was tacked to the main board (superglue works ok) to serve as a Vcc bus. The circuit was laid out on the board to flow from left to right pretty much as presented in the schematic. It's not pretty, but it works. Small standoffs were used for input, output, and summing node connections. The summing node could very well have been an "air connection" (and it would have worked better that way), but I wanted a little more support for the components. The nailhead terminals on the input and output connectors make connection to the board a little easier. The large disc cap at the left hand side of the device is the input coupling cap . It's about as big around as a nickel, which gives you an idea of the overalll size.
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Carl Willis
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Re: Not Quite As Simple CSA

Post by Carl Willis » Tue Mar 09, 2010 8:40 pm

That's a nice little project, Richard.

I'm thinking that on account of its diminutive size and simplicity it might work well as a front-end for CDV-700s, Ludlum 3s, etc. to enable their function with proportional detectors.

Thanks for the update.

-Carl
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Richard Hester
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Re: Not Quite As Simple CSA

Post by Richard Hester » Tue Mar 09, 2010 8:58 pm

Test results are in a post a little farther up the chain - maybe I'll ask Mr. Hull or the Professor to move it down the line so it is in chronological order. I based the charge storage cap size on the numbers given in the Reuter-Stokes data sheet for their He3 detector. If i remember correctly, their charge/event was specified at 600V. As you stated earlier on in the boron tube thread (I think), this number will be higher for higher bias voltages. Getting the storage cap value up to near 10pF would help repeatability, and also make the cap a little easier to obtain. Having said that, a teflon wire "gimmick" with some heat shrink around the wires to preserve the twist would make for a high quality charge storage cap - make up a couple if inches of twist, and cut it to the desired capacitance...

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Re: Not Quite As Simple CSA

Post by Tyler Christensen » Tue Mar 23, 2010 4:12 am

I built this amplifier today and it is working great, another successful build. It is a huge improvement over my previous "pathetically simple pre-amp". I really like the battery power source since it simplifies on getting power to it noise-free.

Worked great on my 3He tube, I'll test it on a 10B tube at some point but I'm sure it will work just as well.
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Richard Hester
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Re: Not Quite As Simple CSA

Post by Richard Hester » Tue Mar 23, 2010 7:02 am

What kind of pulse heights are you seeing out of the preamp/tube combo and at what bias voltage? Are you using the 1pF charge storage cap or a different value? Some fine tuning may be possible. I threw some values together based on what I saw from a Reuter-Stokes data sheet.

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Re: Not Quite As Simple CSA

Post by Tyler Christensen » Tue Mar 23, 2010 1:54 pm

I'm using a 2pF charge storage cap. If I recall correctly, the output pulses are right around 100mV, but I could be mistaken about that, I won't have time to test it for a few days. The 3He bias was at ~1600V.

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Re: Not Quite As Simple CSA

Post by Richard Hester » Tue Mar 23, 2010 8:00 pm

Let me know when you get around to testing it. The optimum value of charge storage cap would be nice to know.

I have a charge sensitive preamp that I use with my homemade fast neutron scintillators, and it didn't get proper C values until I had a chance to pair it up with a neutron source at Mr. Hull's lab a few years ago. It was giving squirreley results there because valid neutron events were producing signals large enough to saturate the amp, puttting them outside the range of the SCA I had looking at the output (it was only doing its job...). Doubling the value for the charge storage cap did the trick. An added benefit of that was to push down the amplitude of low-level garbage signals and make them easier to discriminate out. For that reason, we want as high a cap value as we can get away with for the He3 tube preamp. 100mV of valid signal would be just peachy.

I may gin up a simple post-amp and shaper that could be included in the same box, making the output signal higher amplitude and easier to deal with.

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Re: Not Quite As Simple CSA

Post by Tyler Christensen » Tue Mar 23, 2010 8:47 pm

I'll work on optimizing that when I get a chance to run the fusor again. So the charge storage cap will determine the amplitude of the signal output? I don't have a storage scope to get traces of the exponential decay wave that others have posted if that is required to optimize it, I just see "blips" on my scope at the voltage when neutrons are going through it. Just optimize those "blips" to be 100mV by changing the storage cap?

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Re: Not Quite As Simple CSA

Post by Richard Hester » Tue Mar 23, 2010 10:24 pm

I'd set up the scope with the preamp and set the trigger level on the scope so as not to respond to "grass" from a gamma source, then look at the pulse amplitude with a neutron source. Maybe it's best to set the scope to"norm" trigger mode when looking at the neutron pulses. Set the trigger level to just above the value that rejects the grass, and inch it up as you look at neutron generated signals until you reach a point where the scope doesn't trigger any more. That will give you an amplitude range for the solid signals. You can get an idea of the real average signal level by the number of events you get for a given trigger setting. Too bad you don't have a digital scope, as it makes the job easier. See what you have first with the 2pF cap before messing with it. What we're looking for is a good solid output voltage above the noise with a reasonable value of capacitor - there's no one perfect number. Somewhere around100-200 mV looks like an attainable value.

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