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 » Wed Jul 13, 2011 10:42 pm

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.

Richard Hester
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Re: Not Quite As Simple CSA

Post by Richard Hester » Thu Jul 14, 2011 12:33 am

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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CSA_POST_SHAP.gif

Richard Hester
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Re: Not Quite As Simple CSA

Post by Richard Hester » Thu Jul 14, 2011 12:40 am

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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DISC_DRVR2.gif

Richard Hester
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Re: Not Quite As Simple CSA

Post by Richard Hester » Thu Jul 14, 2011 8:43 pm

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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Disca.gif
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Richard Hester
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Re: Not Quite As Simple CSA

Post by Richard Hester » Thu Jul 14, 2011 8:57 pm

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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Discb.gif
Discb.gif (7.51 KiB) Viewed 672 times

David Housley
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Re: Not Quite As Simple CSA

Post by David Housley » Sun Mar 25, 2012 1:04 am

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.

Richard Hester
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Re: Not Quite As Simple CSA

Post by Richard Hester » Tue Mar 27, 2012 3:42 am

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.

David Housley
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Re: Not Quite As Simple CSA

Post by David Housley » Tue Mar 27, 2012 7:54 am

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.
Thanks,
David Housley

Richard Hester
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Re: Not Quite As Simple CSA

Post by Richard Hester » Tue Mar 27, 2012 8:21 am

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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Richard Hull
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Re: Not Quite As Simple CSA

Post by Richard Hull » Tue Mar 27, 2012 3:44 pm

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

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