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 »

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

Post by Richard Hester »

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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Richard Hull
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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
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
Richard Hester
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Re: Not Quite As Simple CSA

Post by Richard Hester »

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
Richard Hester
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Re: Not Quite As Simple CSA

Post by Richard Hester »

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

Post by Tyler Christensen »

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

Post by Richard Hester »

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

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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
Richard Hester
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Re: Not Quite As Simple CSA

Post by Richard Hester »

I found my LED-Photodiode pulser in a box last night (I'd forgotten that I'd put in a nice case with a switch and output connector and all). At any rate, I hooked it up to the preamp/shaper/discriminator board today, and things worked first time around. I'll post some waveforms when I get a chance to take some pictures. When I find my little Power Designs HV supply, the next thing will be to hook it up to an NaI(Tl) scintillator with the PMT operating at reduced voltage.
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Re: Not Quite As Simple CSA

Post by Richard Hester »

For something to chew on, the next two posts feature the schematic for the preamp/shaper/discriminator board. The schematic here shows the charge sensitive amplifier, followed by a post amplifier (useful with He3 and B10-lined proportional tubes), and a shaper that takes the sharp-tipped exponential waveform from the preamp/postamp and rounds it off into a waveform more easily handled by a discriminator.
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Re: Not Quite As Simple CSA

Post by Richard Hester »

The next schematic shows the discriminator, constructed using an LM393 dual comparitor. One half of the comparator is used to compare the input waveform with a voltage reference generated using a TL431 shunt regulator. The pot is used to adjust the trip threshold of the input comparator. The second comparator of the 393 is configured as a monostable multivibrator, which puts out a fixed ~5usec pulse when the input comparator fires. A couple of transistors are used to buffer the output of the monostagle, so that one can drive a long cable if necessary. I plan to mount one of these boards right inside a neutron detector head. The output would directly drive a counter.
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Re: Not Quite As Simple CSA

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Attached is a digital oscilloscope picture of the waveforms from the charge senstive amp, postamp, and shaper portions of the circuit, driven by a led/photodiode pulser circuit. The top waveform is the output of the charge sensitve amplifier (probed at the junction of C8 and R17) at 50mV/div. The peak value of the signal is about 35mV. The center trace shows the signal at the output of the post-amplifier (probed at the junction of R10 and R14) at 500mV/div. It has an amplitude of ~1.7V. Finally, the shaper output (probed at C7) is shown on the bottom trace, at 500mV/div. The shaping process has reduced the signal amplitude to ~0.7V. This is the signal that gets fed to the discriminator circuit.
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Re: Not Quite As Simple CSA

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This next oscilloscope picture shows the input signal to the discriminator comparator (middle trace, pin 4 of U2A), the DC level that his circuit is compared to (top trace, probed at pin 3 of U2a), and the resulting output pulse (bottom trace, probed at OUT), The "OUT" signal is used to drive a counter.The perturbations on the pin 3 trace should not be there. These will get fixed, and a new schematic and waveform set shown. Having said that, the circuit is still functioning as intended.
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David Housley
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Re: Not Quite As Simple CSA

Post by David Housley »

I realize this is an old thread, but I only recently started my fora into the wonderful world of nuclear instrumentation, so I hope you'll indulge me a bit. While overall this looks like a great circuit topology, and should be very stable, the biasing on the fet stage looks a little strange. Specifically, there is no resistor to ground on the gate for the self bias mode, and there is no voltage divider bias on the gate, either--it is essentially left floating as far as bias considerations are concerned. Also, the source resistor seems to be an awfully high value for the required current level, and to have any voltage left across the drain to source. Finally, there is no drain resistor, meaning the output of this stage must be developed only across the rds of the fet itself. Please don't take any of this as a criticism of the circuit, only trying to understand the particular topology better. I welcome any comments along this line. Thanks!
Richard Hester wrote:
> This next oscilloscope picture shows the input signal to the discriminator comparator (middle trace, pin 4 of U2A), the DC level that his circuit is compared to (top trace, probed at pin 3 of U2a), and the resulting output pulse (bottom trace, probed at OUT), The "OUT" signal is used to drive a counter.The perturbations on the pin 3 trace should not be there. These will get fixed, and a new schematic and waveform set shown. Having said that, the circuit is still functioning as intended.
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Re: Not Quite As Simple CSA

Post by Richard Hester »

All comments refer to the first schematic presented in the thread, for reference purposes.

The gate of input jfet Q1 is biased from the emitter of Q3 via resistor R1.

The source resistor is as large as it is because the output at the emitter of Q3 is supplying 4-5V to the gate of Q1 through R1. The bias current through Q1 is a few tenths of a milliamp (appropriate for the 2N5457), depending on the individual characteristics of Q1 (jfets are generally specially selected for this role).

When a signal hits the gate of Q1, the output signal is developed across inductor L1, which, being an inductor, will resist a change in current by developing a voltage across it.

This is a variation on bog-standard circuitry used in these applications by several big-name companies. If you dig back into files. you'll see an analysis I did a few years ago of a couple of circuits from Ortec and Bicron that have some of the features used in this preamp.

BTW, this is not just a circuit just slung out there for the heck of it. It has been built several times (by myself and others), and works quite well. If you check this thread, you will also see waveforms recorded from a prototype of this circuit.
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Re: Not Quite As Simple CSA

Post by David Housley »

Thank you, Richard for the clarification. I wasn't attacking the veracity of you or the circuit, really! I know it has been built, and your reputation for good circuit design is well founded. I know this particular topology has great stability, due to the second stage being a common base configuration which provides great isolation between the input stage and the final buffer stage. I just had never really seen this particular biasing and output load configuration before and wanted some clarfication, which you have graciously provided. I just haven't seen a gate biased by the main feedback resistor before, but no reason it can't be--clever, really. Plus, the 2n5457 has an Idss of 1ma minimum, according to the data sheets, so that is why the source resistor value seemed high. Please don't take my wanting clarification of the topology as an attack on you or your circuit---it certainly isn't.
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Re: Not Quite As Simple CSA

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As I pointed out, a lot of the features of the charge sensitive amp described in this thread are fairly standard. Check out the paper I mentioned in "files" for documentation of two commercial charge sensitive amp circuits and an experimental circuit I designed a few years ago that attempted to address the variability of individual jfets in charge sensitive amplifier circuits.

This thread started out describing a preamp for He3 and BF3 proportional counter tubes, both rather thin on the ground for casual experimenters these days. The only neutron detector tubes readily available now seem to be the Russian corona counter tubes. Three flavors are available on Ebay for ~20 dollars apiece. These may not need a post amplifier for manageable signal level. A charge sensitive amplifier/shaper will still probably be useful for these tubes to tone down the low level "grass" that is the byproduct of the background corona discharge that makes them work.
I'm working off and on on a scintillator moderator combination that will detect both proton recoil (via PMT) and low energy moderated neutrons. The combination is so obvious that I'm really surprised no one's mentioned it before.
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Re: Not Quite As Simple CSA

Post by Richard Hull »

I'm looking forward to future Richard Hester circuits and innovations.

Richard is right. For those just viewing this and new to neutron detection, the proportional 3He tubes and BF3 tubes are in short supply and are currently outrageously priced, unlike a few years back when Richard developed the original CSA. Old surplus 3He tubes could be had for $200.00 - $400.00 in the 90's and early 2000's. Now, they are ~$1,000 - $2000.00, if you can find them. This is just for the tube!

Now, Richard is on a quest to go back to a PMT "beginning mode". Again, for those not familiar, all early neutron data was gathered here using PMT's and various scintillator arrangements. Going back deep into the posts, you will see this.

I applaud Richard's efforts to circumvent the current proportional tube shortage and help find a more inexpensive, electronic neutron detection scheme. Stay tuned.......

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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Joe Gayo
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Re: Not Quite As Simple CSA

Post by Joe Gayo »

Cascode JFET input
Moved the feedback cap after the input cap
2nd Order RC Bias Filter
Photodiode Pulser w/ adjustable current source (driven by an external microcontroller or signal generator)
9V Battery Holder

I recommend 1p and 47 Meg feedback.
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Joe Gayo
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Re: Not Quite As Simple CSA

Post by Joe Gayo »

I'll sell the bare boards for $10 each and tested assemblies for $50 (without pulser) and $75 (with pulser).

Bare boards without the pulser circuit are $5 each
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Richard Hull
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Re: Not Quite As Simple CSA

Post by Richard Hull »

Joe, I PM'd you with my order.

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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Joe Gayo
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Re: Not Quite As Simple CSA

Post by Joe Gayo »

Thanks.

I just acquired qty. 300 of original PN4416A parts. I'll be measuring and matching for the cascode stage.

Here is the final board -
CSA-02.jpg
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Re: Not Quite As Simple CSA

Post by Joe Gayo »

Built the amplifier and pulse section today. Everything tested great!

The pulser can generate 0.1 to 2pc pulses and the amplifier portion is approximately 300 mV/pc. I also feed the pulser randomized triggers (Another board I designed and built) at an average 76kHz and saw no roll-off in amplifier performance.
PXL_20201103_211151521.jpg
I've also decided to offer the board and parts as a kit that you assemble yourself.
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