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 »

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 »

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

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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

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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

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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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Re: Not Quite As Simple CSA

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I'm getting 50mV pulses out of the 3He tube and several volt pulses out of the 10B tube, although that one is a direct function of the bias voltage so it has little meaning.

This is with the 2pF charge storage capacitor.

This is well above the 2-5mV noise floor
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Re: Not Quite As Simple CSA

Post by Richard Hester »

This suggests that you're are getting about 100 nC/event from your He3 tube at your chosen bias voltage. This is a plausible value. It also suggests that a charge storage capacitor of 1-2 pF will be a good starting value for other experimenters. It also sounds like a post amp and shaper located inside the same housing as the preamp would be useful. 50 mV is a usable signal - 0.5 to1V would be better.
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Re: Not Quite As Simple CSA

Post by Richard Hester »

Here is my first PSpice tinkering run for a post-amp and shaping circuit to go along with the preamp. Some of the components are in place merely as probe points, and would not be included in an actual design. The first 2N5457 and the 2N3906 constitute the post amp, and the other two 2N5457s are the shaper. Since the shaper only has two poles, its output doesn't come near an ideal Gaussian pulse, but the peak is broadened sufficiently to make discrimination easier, at the cost of a good deal of amplitude, which will be seen in the next post. The blue probe monitors the input pulse, the red is the output of the postamp, and the green probe monitors the shaper output.
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Re: Not Quite As Simple CSA

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The picture here shows the input pulse (blue), the postamp output (red), and the shaper output (green) from the PSpice simulation. As can be seen, the shaper smooths and stretches the pulse, at the cost of a good deal of amplitude.This is very similar behavior.to the kind of circuits I've been using for my scintillator-based All-In-One detector setups.
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Re: Not Quite As Simple CSA

Post by Tyler Christensen »

My build of this circuit just stopped working, I'm now getting 0.6V P-P noise straight out of the pre-amp. It just suddenly started doing this the other day. At first it was just a few loud peaks then it went into chaos. I've replaced all three active transistors and it behaves exactly the same. Checked all the resistors, none are burned up. Any guess on what's causing this? This is with the HV off.

UPDATE: Solved it, it was the input diodes (just leaving the message incase anyone has the same problem after driving too much voltage into the input). I didn't initial replace them because they test fine on my diode test meter, interesting.
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Re: Not Quite As Simple CSA

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The diodes are there to catch a bullet (like switching on the HV to the detector with the preamp connected) that would normally take out the input FET first thing. Bad diodes also happen, and they will inject noise right into the input. The diodes might have been degraded enough to get noisy in the reverse direction. This may not necessarily show up on a simple diode tester. It would be interesting to put the original FET back into the circuit to see if it survived whatever took out the diodes.
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Re: Not Quite As Simple CSA

Post by Richard Hester »

I was looking at the postamp/shaper circuit I posted earlier on in this thread, and may have a solution for getting the signal amplitude higher while still keeping in the range needed to operate with a 9V battery. This is an offshoot of work I'm doing with jfet-based phono amps I'm documenting on a DIY audio site. More on that tomorrow when I can get to my simulator results. If I could get 0.5-1V output from a simple 2-pole pulse shaper things would be very happy-making.
A circuit for optically coupling the preamp output to a remote SCA would also be neat, but it may be more expedient to keep the SCA local to the detector housing and optically couple the resulting go/no-go pulse to a remote counter. This might require a pair of 9V batteries rather than a single, but that's still not a biggie.
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Re: Not Quite As Simple CSA

Post by Starfire »

Richard - I'm getting good results with a 555 using the trigger as input and set up for pulse o/p.
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Re: Not Quite As Simple CSA

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Attached is my second stab at a discrete postamp and shaper circuit to tie in with the charge sensitive amp. I relaxed my design constraints a bit and used a different fet for the input, which got me the gain I needed. As before, it runs off of a 9V battery.
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Re: Not Quite As Simple CSA

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Attached are the input and output waveforms. From a teensy ~50mV input, you get a solid 500mV pulse with a rounded top that will make it easier for a discriminator to work without needing a blindingly fast comparator.
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Re: Not Quite As Simple CSA

Post by Richard Hester »

Here's a simpler version of the postamp/shaper that works the same as the previous (same response).
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Re: Not Quite As Simple CSA

Post by Richard Hull »

Once again, nice amps and thanks for sharing. These with the usual input and bias circuitry added ought to do OK for a 3He counter tube.

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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Re: Not Quite As Simple CSA

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Keep in mind that the circuits I just posted are a postamp and shaper to take the output of a charge-sensitive preamp such as the one cited at the beginning of this tread, amplify the output, and shape the response to ease amplitude discrimination.They're not too useful without the preamp ahead of them. They are particularly germane to this thread, as the preamp that started the thread was aimed at He3 proportional tubes where the output pulse is fairly tiny, even with a small value for charge storage cap (~1pF) for the charge sensitive amplifier. The output of the charge sensitive amp is about ~50mV, so the postamp and shaper can make life much easier for a discriminator or SCA.
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Re: Not Quite As Simple CSA

Post by Richard Hull »

I am sorry, I was commenting on the entire thread. Sometimes I assume too much of the reader, being an electronics guy, myself. The original "CSA" linked to your last or next to the last "postamp" circuit is what I was referring to. This combo should replace most any older NIM based preamp setup and can be attached directly to the tube head, itself in a small box, utilizing batteries or the common 9 pin nim plug using the 12 volt line with a regulator.

I like attaching the preamp/postamp right to the head of the tube. It is a real noise reduction system if well executed.

I didn't look at the FET and transistor limits but I'll bet NIM 12volts could be used with only minor alterations.

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
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Re: Not Quite As Simple CSA

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If you really wanted to get nifty, the next step would be to gin up a low power discriminator circuit that you would adjust once to chop away all the gamma grass (assuming your HV to the tube is stable and repeatable). This would drive an LED coupled to an optical fiber, going to a remote counter. Having all the analog stuff in one box at the detector would go a long way toward making the detection system more noise immune.The optical fiber is perhaps gilding the lily, but it wouldn't be all that hard to do. Someone here (John Hendron, I think) mentioned using a 555 timer in the context of a discriminator. I'd vote for clever use of a CMOS 555 - I'll do some thinking.

I'll look at my simulations to see what 12V will do to the biasing in both circuits. It may not be much of an issue.
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Re: Not Quite As Simple CSA

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I figured once you had a 2 volt signal in hand, standard coax and good termination would obviate most of the noise in all but the harshest environments. However, it would, indeed be cool to run the signal thru fiber, especially if you are working a high pulsed energy fusion system.

I figured that 12 volts, NIM, might need some tweeking on the biasing networks. I imagine a linear regulator (LM317T) set for 9 volts would be a good, no bother, solution. Only a super "greenie-weenie" would worry about the 3V disappation losses with the few milliamp demand of the circuit.

Good work as always Richard....and thanks.

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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Re: Not Quite As Simple CSA

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Attached is a picture of a board incorporating variants of the charge sensitive amplifier and postamp/shaper circuits presented here, plus a simple discriminator/cable driver circuit constructed using an LM393 dual comparator. I've done the initial power-up and tweaking for DC operating conditions, and will try to mate this up with a detector system. Since I don't have any neutron tubes and sources configured at this time, my first step will be to feed this bad boy with the output of an NaI(Tl) scintillator with the PMT run at reduced voltage to bring the gain and pulse amplitude down.

My goal is to include the board inside of a detector assembly using one of the Russian corona tubes. I haven't figured out whether I want to use the B-10 or He3 corona tube to start with. If I interperet DC's results from his testing of a B10 tube correctly, I won't need to use a postamp with the B10 tube, so I would feed the output of the charge sensitive amp right into the shaper circuit. This may be eventually what happens with the NaI scintillator system two, as I will likely get a good, strong ~1V output from the charge sensitive preamp even with the PMT crippled by a low supply voltage. The post-amp will come in useful for people using He3 and B10 proportional tubes with their lower amplitude output.

Anyway, this one is still in the development phase, and I'll post schematics when I have a better idea of what's happening. The board shown is about 4" by 1 3/4", so it can very handily be mounted on the underside of a gallon paint can lid.
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Re: Not Quite As Simple CSA

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Regarding not using a post-amp, worked fine for me... my amp/shaper broke and I just fed right into my SCA directly from the CSA pre-amp box and it worked great for BF3, B10, He3, and Geiger... probably not "optimal" but I got the same count rates I'd expect on all detectors, so I didn't see any issues with it.
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Re: Not Quite As Simple CSA

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Maybe so in your case, but on this board, the discriminator is included. Without the post-amp, the threshold pot for the discriminator would have to be cranked all the way down to the low end, with out any real latitude for adjustment.

When all the homework is done, this should be a setup that woulld allow me to feed a detector head directly into a counter.
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Re: Not Quite As Simple CSA

Post by Richard Hull »

We have grown used to Richard's all-in-one solutions and the wait is worth while.

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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