A semi-DIY preamplifier for radiation detectors

This area is for discussions involving any fusion related radiation metrology issues. Neutrons are the key signature of fusion, but other radiations are of interest to the amateur fusioneer as well.
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Rich Feldman
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A semi-DIY preamplifier for radiation detectors

Post by Rich Feldman »

Yay! Got some own work to report. One more step toward counting neutrons at home. I scampered & cut corners last weekend. Then my birthday came and went, and project still needs work.

My BF3 tube wanted a Charge Sensitive Preamplifier (more about CSP's later).
Following in the footsteps of Dan Knapp, I started with a Cremat brand hybrid circuit module called CR-110.
Chose not to buy their almost-ready-to-fly circuit board (CR-150) or matching aluminum box. Partly because of wanting to make component substitutions. Mainly because of habit: the sacrifice of hours of "making" time, on the altar of low out-of-pocket cost.

You couldn't get the amplifier much closer to the detector tube.
prop.JPG
.
The circuit differences between my build and a CR-150 rev5 are highlighted in schematic diagram:
prop2.JPG
First: both HV caps are rated for 3 kV. Online pictures of rev4 and rev5 show 2 kV capacitors, but Fred from Cremat told me his latest batch was assembled with 3 kV parts.
Second: In email discussion about resistor voltage ratings in bias circuit, Fred warned me about a more important kind of fault that can destroy amplifiers, when detector is a HV gas tube. Not sparking bias resistors. A sparking detector (or detector connector?), can cause C1's HV charge to suddenly appear on amplifier input pin. I followed his suggested mitigation: Add series resistor (100 to 200 ohms) and reduce C1 to 1000 pF -- which is OK when detector node capacitance is very small.
Third: Expedient power source is a pair of 9 volt batteries. Originally they were going to be inside the box, but I decided to skip that in order to see pulses sooner.
Fourth: In other places I used components that were handy. Didn't need to use the HV version of 10M resistor. CR-150 is OK with regular (non-HV) resistors in bias circuit, though they see full voltage momentarily if bias is switched on suddenly, or when there's a spark discharge in detector circuit. The CR-150 design had chip resistors in rev 4, changed to regular axial lead resistors in rev 5.

Circuit was assembled on a scrap of perf board with copper pads on one side, copper ground plane & antipads on the other. The hybrid module is in back corner of picture, leaning like a shed roof, in a bent-over SIP socket.
DSCN1109.JPG
.
Got a setback last night. Connected bias supply for a proof test at 3000 volts, before plugging in the expensive CR-110.
I'd cleared lots of metal around each HV circuit node, but not enough. A couple times a minute there were little snappy sparks next to one or the other of the HV nodes. Good thing the bias supply is very weak on purpose -- equivalent resistance of a couple megohms, and output capacitor that stores only 0.045 joules. My response was to remove even more metal -- already done when the picture above was taken. But soldered components interfered with doing that properly around the detector node connections. I think the best course is to rebuild the circuit on a perfboard that has much less metal to begin with.

In the meantime, with the bias input cable removed: Plugged in CR-110. Powered up without loss of smoke. Exercised the Test function, a valuable feature for which I applaud Cremat or whoever they got the idea from. We can verify that the amplifier is in good working order, without needing HV bias or even a detector. It worked as expected on the first try!
DSCN1102.JPG
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Rich Feldman
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Re: A semi-DIY preamplifier for radiation detectors

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Progress report.
1. Sparking was solved by removing the R's and C's in bias circuit, so perf-board metal could be cleared in a wide margin on both sides.
With the R's back in place, circuit sustained 4500 V for 5 minutes with no spark or series-NE2-lamp visible glow in the dark.
Then with both high voltage C's back in place, and another NE2 in place of amplifier input, circuit sustained 3300 V for 5 minutes. Lamps glowed only when HV was ramping up or down. I didn't attempt a more quantitative meggering.

With 10 megohm DVM at detector connector, and HV cranked up briefly as far as 1500 V, voltage division ratio was roughly as expected. This test took the four 33M resistors well above their voltage limit, and hinted at some nonlinearity (more current than expected for the higher voltages).

2. All back together, including BF3 tube, the amplifier Test function worked as before. This time on my home oscilloscope.

3. Next problem became evident when HV bias was cranked up to 500 or 1000 volts. Got a couple volts of 60 Hz ripple at amplifier output. I had never modeled or measured the ripple voltage of DIY bias supply, except to know the RC product of output cap and kV meter resistance: 300 millseconds.

Went to measure bias ripple by cabling "detector" connector to scope channel (1 megohm input) and observing expected divided voltage (6 V when bias meter said 1000 V). Flat line at that scale, so I switched scope to AC-coupled mode. Unexpected behavior led to an uh-oh moment: AC coupling removes the 1 megohm DC load, and consequent attenuation of 1000 V to 6 V. Time to call it a day.

4. Simple remedies may be a combination of: Increase bias supply output C. Make a better lowpass filter (inductive choke even?). Change rectifier from half-wave to bridge. Reduce C1 in amplifier circuit. AC-couple the output of amplifier circuit, with time constant small enough to suppress 60 Hz or 120 Hz.

Suggestions are welcome.
[edit] It's now obvious where to start. Simulation confirmed quick analysis: go with the bridge rectifier to double the ripple frequency. The existing circuit has 3 stages of low-pass filtering, so ripple amplitude will come down roughly by a factor of 8 (well maybe 4 at amplifier output, because of differentiating effect of HV blocking cap).
ripple.JPG
Unsymmetrical 60 Hz waveshape on scope looked like the blue curve in this simulation.
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Re: A semi-DIY preamplifier for radiation detectors

Post by Jerry Biehler »

You could use a cheapish isolated DC/DC converter module. They have basically no ripple. You definitely wont see any 60hz!
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Re: A semi-DIY preamplifier for radiation detectors

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Well said, Jerry. I could probably find one or two, perhaps a laser power supply module, in one of many HV junk boxes in the garage. Or swap the transformer in a cheap low-voltage regulated DC power supply, with flyback topology, like we find in appliance controller boards these days.

The primitive bias supply design attracted me. In fact it's one 1X2A tube short of being semiconductor-free.
1x2a~~1.jpg
A tube like the one in picture was measured to have a lower forward voltage drop than the 30 kV silicon diode it would replace.
Of course vacuum technology would make the change from 1-to 4-diode rectification much less trivial.

Wonder what Nancy Wood used to bias her BF3 tubes in the Manhattan project?

Any other retro electronics fans out there? I forgot to bring, to HEAS, my red laser pointer (commercially manufactured) with a He-Ne laser tube and 9-volt batteries.
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Re: A semi-DIY preamplifier for radiation detectors

Post by Richard Hull »

I am about as retro as anyone in electronics. A smaller and cooler tube rectifier was used in all Textronics O'scopes for their anode supplies for years. It was a flying lead diode and could easily handle 10 KV with less filament current. I have seen them in a few early portable GM counters. The smallest in the image below is from a Sears and Roebuck "tower" brand scintillation counter of the early 50's.

As such GM HV supplies only needed mere microamps of current, portable counters quickly moved over to long tubular selenium pellet stack HV diodes.
Most were 2" long, slender 3/16" diameter stick phenolic bodies filled with tiny bonded, stacked Se pellets.

Richard Hull

How retro am I?..... go to my old post here

viewtopic.php?f=13&t=10989


Data on the tubes below...https://www.radiomuseum.org/tubes/tube_5642.html
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Re: A semi-DIY preamplifier for radiation detectors

Post by Harald_Consul »

Very interesting work Rich!

Just for the stupid guys. How much MHZ/ nanosec rise time is your from-scratch-preamplifier?
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Re: A semi-DIY preamplifier for radiation detectors

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Thank you, Harald.
I don't have meaningful risetime measurements yet. The CR-100 is supposed to have an impulse response like:
csp1.JPG
csp1.JPG (20.97 KiB) Viewed 14886 times
with risetime of about 7 ns and decay time constant of about 140 us (you could work it out from the test mode waveform picture).

I think it's common for CSP's to be followed by shaping amplifiers that get rid of the long tail. Here is a picture of two long-tailed events piled up, and a signal that can be recovered from that waveform using trivial analog electronics.
csp2.JPG
csp2.JPG (23.77 KiB) Viewed 14886 times
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Re: A semi-DIY preamplifier for radiation detectors

Post by Dan Knapp »

There are two companies that use the Cremat modules in their preamps for which they post the manuals online. These user manuals include circuits and other useful information:
https://www.fastcomtec.com/fileadmin/ft ... /csp1x.pdf
http://www.ptcusa.com/files/manual/CR10_UM_180312.pdf
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Re: A semi-DIY preamplifier for radiation detectors

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Thanks, Dan.

Here's a question for all, about turning HV bias on or off. The cited manual from Fastcomtec says "Increase or decrease the high voltage only at a very slow rate. Observe output during bias voltage change with an oscilloscope. Do not allow the output to saturate during change of high voltage."
Do people really do that, when dealing with three- and four-digit bias levels for gas tubes? In my experience it takes tens of seconds to get from 0 to 1000 volts.

The numbers make sense. Manual for the CR-110 hybrid module recommends AC coupling for detectors with bias current exceeding 10 nA, to avoid saturation. A steady 10 nA produces 1 volt at output of the first amplifier stage, and 2 volts out from buffer stage. That's within a factor of three of output clipping levels.
cr110.JPG
If external HV blocking capacitor were 10 nF, as found in all three products cited in this thread, a HV ramp rate of 1 volt per second would produce 10 nA at amplifier input. 3 volts per second would surely saturate the output. I'm using a 1 nF capacitor, so it can charge at around 30 volts per second without pegging the amp.

Why is saturating the amplifier bad? Reminds me of oscilloscopes in AC-coupled mode, when you put the probe on a signal with large DC offset, and trace disappears offscreen for a few seconds. If my HV were ramping from 0 to 1000 V in one second, a whole microampere would be flowing into the circuit module input. Rf and Cf can only pick up a small fraction of that, so the rest will find what I expect is a nondestructive alternate path to ground. Unless the module's electrical performance requirements call for a bare MOSFET gate and no protection diodes.
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Re: A semi-DIY preamplifier for radiation detectors

Post by John Futter »

Yes Dan
we increase voltage very slowly especially on solid state detectors (conditioning)it allows the depletion region to move into place ie extremely high impedance without puncturing (read expensive and now a low voltage zener not SSD)
we use ortec quad bias supplies
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Re: A semi-DIY preamplifier for radiation detectors

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Years and years ago. Carl Willis warned all of us about neutron preamp bias and I have taken his advice ever since. I always ramp up the bias slowly from zero to about 1600 volts and likewise I ramp it down to zero before I hit the bias supply on-off switch. This saves the input FET gate from possible destruction via capacitive pulsing when snapping on full bias or snapping it off. My advice is just do it!

As an old electronics engineer I knew how sensitive the ultra high impedance electrometer grade FET gates were, but in all my experience over the years, I never had 1600 volts DC in a gate circuit between the gate separated by only a capacitor. I only encountered this in my neutron detection scheme. and tend to obey the rules.

All the electrometers I ever used were paired 5886 electrometer vacuum tube front-ended. They would take a lightning strike! I would often, when measuring large static fields, actually have a spark leap to the input.....No problem. Do that with a modern FET front end electrometer, then reach for your wallet and a few weeks at kiethley.

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Re: A semi-DIY preamplifier for radiation detectors

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We agree that ramping HV bias up or down is good practice. Richard and John, how slow is your slow?

Designer at Cremat just agreed with me that ramping slow enough for preamp to stay linear is unnecessarily conservative.
I tried it while watching scope, not quite pegging CR-110 output.
Took about a minute to get from 0 to 1000 volts.

If my HV blocking capacitor were 10 nF, like most, it would take 10 minutes.

Thought of a feature that would be easy to add. Sort of like a transit lock, in the suspension of record turntables and other delicate instruments.
Put a switchable resistor or external clamp diodes between preamp input and ground. With that engaged, HV bias can change at a convenient rate w/o stress on preamp. Release the charging-current bypass switch before attempting to detect things.

Back to the primitive bias power supply:
Changing rectifer to bridge reduced voltage ripple to 780 mV (p-p, at amp output, with 1000 volt bias).
Augmenting the 0.01 uF output cap with a 0.95 uF microwave oven cap (w/ integral 10M bleeder) reduced ripple to 28 mV.
Reducing the HV blocking cap, and AC-coupling the preamp output, will help more.

Maybe not enough to make up for lack of regulation. When refrigerator starts, line voltage dips, so HV bias dips, and preamp output level moves substantially. Hmm -- just discovered that you can get 4500 volt transistors at Mouser.

I think detecting neutrons will be more rewarding than scratch-building a bias supply that's up to the task.
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Re: A semi-DIY preamplifier for radiation detectors

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I guess It takes me just under 45 seconds to manually ramp up and down my bias supply voltage. I often feel it is just the spike that kills. Ramping up and down are just a bit of warranty that feels comfortable. Certainly, I have not killed my first preamp in use since 1999 by just ramping up and down.

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Fusion is the energy of the future....and it always will be
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Re: A semi-DIY preamplifier for radiation detectors

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Broke down & got me an Ortec 459 Bias Supply ($75) and a powered NIM bin to put it in ($33).
Picked 'em up this morning, and in a quick test this evening both appear to work. Except +/- 6V power at the bin slot I checked, but the 459 doesn't use that.

The turns-counting dial on voltage setting pot has more friction than it should. I will need to open the box and switch HV polarity to positive. And buy or contrive a SHV connector, and clear plenty of bench space.
nim_stuff.jpg
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Re: A semi-DIY preamplifier for radiation detectors

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Good deal on the NIM bin, super cheap. The bias supply is about right on the price.

By the way, virtually no NIM bin has a +/- 6 volt supply in them. All NIM bins contain a +/-6v buss in them! A decent fraction of all bins have test probe, tip jack points on the power panel on the right side of the bin that include +/-24 v, +/-12v and +/-6v outputs so that the buss voltages can be easily read, verified and adjusted with ease.

Virtually 100% of all bins demand a +/- 6 volt plug in NIM module supply. This module then supplies the already extant buss to all plugs the +/- 6 volts.
In many cases, such modules only supply one 6 volts and might have a switch to change its polarity. A second 6 volt module will be required and its polarity selected accordingly to have both + and - busses powered. All such 6 volt plug-ins are heavy as the 6 volt requirement can be a good number of amps in other NIM modules demanding one or more of the 6 volt polarities. Such supplies often take up 2 or 3 front panel slots.

6 volts in NIM modules are effectively ancient history of long ago. 6 volt power supply plug-ins are very rare and are not common on the NIM surplus market. I lucked onto a total of two 6 volt supplies now. I purchased them 6 years apart and the last one I paid $135 for.

I only need one +6 volt module in my fusor NIM setup. It is one of the demanded voltages for an LRS custom pulse matching and shaping module.

Never expect to find or obtain an empty NIM bin that has a built-in +/- 6 volt capability.

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Fusion is the energy of the future....and it always will be
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Re: A semi-DIY preamplifier for radiation detectors

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Reminded me of an ebay purchase about a year ago.

Was looking for an adjustable supply to use for PMTs or neutron tubes. Happened on a cheap one listed.

A Canberra 3002, 500-3000V. Listed as for parts only. Seller said they measured no output when set in the 500V range.

It is a NIM module but from pics I could see it only used 115V input. Good 'cause I don't have a bin to plug it into.

I'm usually pretty good at fixing things and it was cheap so I bid. Got it for less than $30 with shipping.

When it got here I plugged it in and measured at 500 setting -- no problem. With a higher voltage probe I measured every step from 500 to 3kV and it worked fine. I switched the polarity and that worked fine too. So fully functional. From specs on a later version manual I found, I think it is good for up to 10 mA.

Looks like this.
front2.jpg
Would have preferred one with a meter and one multi-turn setting knob, but this will be fine, especially at the price.

A bit back onto the topic of this thread -- I noticed that when I change the setting it takes a few seconds to get to the new output value. I guess that's part of the design to help avoid blowing what might be attached with fast voltage transients.

Good thing it worked because I looked inside and everything is in a bunch of completely sealed plastic modules. I doubt if I could have fixed anything but a loose wire.

Sometimes you get lucky.
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Re: A semi-DIY preamplifier for radiation detectors

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This is an excellent example of the "Art of Bidding" that matters for getting cheap but good equipment.

Newbie's should take note!

Having the ability to do limited repairs allows one to 'gamble' on equipment and get extremely good prices; it is no accident that he 'lucked out' and it didn't need any repairs. Luck is often made just like this; if you have some knowledge, good things will often occur because you can afford to take limited risks and sometimes, they pay off completely like this. That does also require getting non-working stuff in bids, often not too difficult to fix if one gets (develops) the skills required.
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Re: A semi-DIY preamplifier for radiation detectors

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OK, I think I've seen a few neutron detections at last.

With bias from the Ortec, and preamp output connected to oscilloscope, there was a discouraging amount of low-frequency wandering. Much reduced with AC-coupling mode on the scope. It seemed to get better after I left the old bias power supply (by itself) on for a few days at 1900 volts, and worked the voltage-setting dial up and down a few times. AC coupling by user, with a much shorter time constant, will probably make it a non-issue. It was handy to use the scope's continuously-crawling display mode, with horizontal at 200 to 1000 milliseconds per division.

The BF3 tube is in a pile of 1/4-inch thick HDPE scraps from Tap Plastics, along with a sheet of beryllium and part of a small household safety appliance. Digital storage scope triggers about once every minute or two, reminiscent of Carl Willis's "counts per day" reports like this one from 2005: viewtopic.php?f=13&t=5566

Occasional trigger events resulted in weird waveform shapes, but most showed the expected 140-us-tailed positive pulses. Scope automatic measurement feature indicated peak millivolt values of 32.8, 34.4, 33.6, 36.0, 33.6, 38.4, 23.2, 91.2, and 47.2 in about 10 minutes.
Nothing smaller, even with trigger level at 6.4 mV (barely above the noise). To be re-visited after noise reduction steps. Most of the gamma flux is expected to be at 60 keV, which I suspect is strongly attenuated by the metal wall of BF3 tube. Got to find something radioactive that makes higher energy gammas. Maybe the test source on yellow CD geiger counter.

Am considering a couple ideas for gathering pulse-height statistics from an unattended setup, without buying a MCA or starting to deal with PC sound cards. Could get out a self-contained, battery-powered event logger.
elusb5.jpg
elusb5.jpg (2.96 KiB) Viewed 13583 times
It can record the time and sign of state changes with 0.1 second resolution. Preamp would be followed by a breadboarded pulse-stretcher circuit that might turn the 91 mV pulse into an start and stop event a few seconds apart. It might be easier to point a digital camera at oscilloscope screen, taking pictures at regular intervals or soon after each scope trigger event. Or a USB camera -- now how do we electrically activate the "capture image" key in camera monitor program? "Motion" sensitive capture mode? Other ideas are welcome!
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Re: A semi-DIY preamplifier for radiation detectors

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Here's a representative pulse from preamplifier output, into 1 MΩ scope channel with 20 MHz bandwidth setting, and bias at 1500 volts.
DSCN1190.JPG
.
After cutting up eight 7-inch-wide sheets of HDPE, and one smaller sheet,
the moderator pile was reassembled more scientifically. Still far short of Fermi's original Chicago piles.
DSCN1189.JPG
.
There's room in the middle for beryllium and alpha sources. Some might protest that there's no moderator between the predicted neutron source and the detector tube. And that the predicted neutron source is on the wrong side of a known "neutron reflector". Let the configuration experiments be done! Any requests? First need to automate the counting of pulses per hour or per day.

A forum FAQ thread about CAD has bubbled up lately. I found a simple 2-D drawing program, MS Visio, very helpful when figuring out how to cut up the plastic sheets. There's plenty of guesswork about which factors matter. Original lot of material is sufficient to fill a 4" diameter cylinder around the detector tube.
moderator2b.JPG
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Re: A semi-DIY preamplifier for radiation detectors

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Update:
Stacking the top half of pile differently resulted in, subjectively, a dramatic increase in count rate.
I guess by a factor of at least 3. Measured 9 pulses in about 5 minutes:
28.0, 2.4, 28.0, 23.2, 27.2, 20.8, 26.4, 28.0, 3.2 millivolts.

Now there's 1/2 inch of moderator between predicted neutron source and detector.
And the detector and alpha source are on the same side of 1/16" thick sheet of beryllium.
moderator3b.JPG
Is any reader in a position, with the inclination, to talk about neutron transport simulation?
For example, with the MCNP mentioned from time to time?
Or to present rules of thumb, seat-of-the-pants awareness, from practical experience?
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Re: A semi-DIY preamplifier for radiation detectors

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My first unattended data collection was inspired by Greg Courville's low-rate counting method, but might be more primitive.
No computer or MCA was occupied during a 115-minute run, then same configuration in a 94 minute run with accidental loss of 44 minutes in the middle. Electrical changes for noise reduction have not been made yet.

An old Wingscapes Birdcam was set in front of scope, as close as it can focus without reading glasses (which come later).
Time lapse mode with minimum delay setting, 30s, takes a picture about every 40s.
Each image shows the result of most recent scope trigger event. There could have been more than one trigger, or none, since the previous image.

Then pictures were individually categorized by eye, like Greg did IIRC.
Most are first image of a "good" pulse. The automatically measured height went into a spreadsheet.
About 28% of the images are redundant (no scope trigger since last event). Sometimes 3 or 4 in a row.
Then there are pictures of "bad" waveforms, where trigger event was some disturbance other than a charge impulse into CSP.
A few are of "deformed" pulses, and two show unmeasurably narrow "glitches" with no 140-us tail.

Here are the largest "good" pulse (36 mV), a small one (11.2 mV), four deformed ones, and one bad waveform.
cam_scope_0422mosaic.JPG
Now a chart of all good and deformed pulses, by height and when they happened. Run2 is appended right after Run1.
cam_scope_0422vt.JPG
cam_scope_0422vt.JPG (26.39 KiB) Viewed 13713 times
Finally a short numerical summary:
cam_scope_0422counts.JPG
I think most of these counts are from the intended AmBe source kluge, but don't claim to have proper evidence yet.
Have shown some evidence about the instrument's repeatability.
For later: try removing the alpha source, the beryllium, and both of them.
Then with those parts in the original positions, remove the moderator.
Scientists need to play Devil's advocate.
Don't infer success too quickly from "I saw just what I expected and wanted to happen".
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Re: A semi-DIY preamplifier for radiation detectors

Post by Richard Hull »

Decent Am-Be sources start to be highly useful at the 25uCi level in working up neutron detection schemes. Zero statistical work is needed at the 100uCi level. Much also depends on the detector tube type, active volume and pre-amp.

At best, one might expect a 10,000:1 ratio in well coupled sources. Thus, a 1uci isotropic source buried in powered Be would produce 3 neutrons/sec.... probably less and this is isotropic emission!
A 25 uCi source, well coupled, would do about 75 n/s iso. In a tight, pure water or plastic moderator a fabulous 3He detector might count 1 count every two seconds, reliably.


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Re: A semi-DIY preamplifier for radiation detectors

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Same experiment is now back in business on a different bench, with different oscilloscope.
20200207_191725.jpg
Surprised to see the forum accept such a needlessly large file. Haven't yet learned how to turn down the megapixel count in my first-ever smartphone.
Clockwise from lower right:
Blue box between cable and oscilloscope is a high-pass filter much faster than the AC coupling built into scope. Helps a lot with the microphonics.
Trail camera, with camouflage decoration, takes a picture every 10 seconds. To help it focus so close, an Optivisor lens in light blue frame is leaning against it.
Preamplifier power is two 9V batteries in a holder that used to be part of a smoke detector. HV bias is set to 1600 volts.
Beryllium sheet, near top of moderator stack, has rough edges on near end where pieces were cut off and sold.

In first 11 minutes I got 9 large (24.8 to 36.4 mV) and 7 small (6.0 to 11.2 mV) well formed pulses. Sort of matches middle image in my previous post.

[edit] Next timelapse ran for 50 minutes, with the beryllium removed after 35 minutes. A quick and casual inspection suggests that the rate of large and small pulses did not change. :-( Next week I'll try removing the alpha particle sources, then the moderator. Maybe the observed rates are background counts, unrelated to presence of Am or Be or HDPE. Need to repeat the configuration change that was reported to make a big difference, last April.
All models are wrong; some models are useful. -- George Box
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Rich Feldman
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Re: A semi-DIY preamplifier for radiation detectors

Post by Rich Feldman »

Looks like the 1.2 to 1.4 counts per minute, total of all pulse sizes, was a background level. Not easy to show any change when beryllium and/or alpha sources (totaling 3.2 to 3.6 uCi) were removed. On the bright side, the exercise validated a practical way to obtain and chart pulse height distributions at those rates.

On Monday, a quick test with much more active Am-241 based source brought obvious results:
about 5 CPM of large pulses
, before any effort to optimize the configuration.
Source was one of 2 or 3 separate and different emitters in a device, which all together amounted to nominal 80 uCi when new.

Details to follow after another session or two, with attention to "scientific" protocols. Before any claim to be in neutron-detecting club:
*Paper notebook. *Documented geometry. *Include measurements without the moderator, without the beryllium, and without the alpha source.
*Give pulse heights in charge units (pC and ion pairs) *Pulse height distribution with different bias voltages on BF3 tube.

I'd love to chat with someone familiar with PRA software (one program for detecting and measuring pulses in signal from PC microphone input).
All models are wrong; some models are useful. -- George Box
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Rich Feldman
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Joined: Mon Dec 21, 2009 6:59 pm
Real name: Rich Feldman
Location: Santa Clara County, CA, USA

Re: A semi-DIY preamplifier for radiation detectors

Post by Rich Feldman »

At last I think I can support a claim of counting neutrons (from kluge Am-Be source).
The alpha source has, I guess, between 50 and 70 uCi. There's plenty of room to improve its coupling to the beryllium sheet, and to improve the moderator configuration.
BF3 detector tube bias voltage is 1500 V.

Time lapse cam took 820 pictures in 137 minutes, including 281 that show a new pulse waveform.
Some pulses were missed, when scope triggered more than once between consecutive images.
neut1.png
neut1.png (269.17 KiB) Viewed 10941 times
I typed in the scope-measured heights of all pulses, and examined the distribution. Shown here sorted by height, which is easier and shows more detail than a histogram. 30% are below 20 mV and about 5% are above 28 mV.
neut2b.png
neut2b.png (9.49 KiB) Viewed 10941 times
.
A little after the middle of run, I removed the beryllium sheet and left all the moderator material in place.

Here is a chart of all 281 pulses, by height and when they happened.
Followed by cumulative count of pulses at least 15, 20, or 25 mV high.
neut3.png
Works out to 2.55, 2.31, or 1.87 CPM with beryllium present,
and 0.70, 0.63, 0.47 CPM with beryllium absent.
I still need to do a measurement with Be present and Am absent, to rule out radioactive material attached to the Be part.
But first, set up a less tedious way to discriminate pulses by height, and count them automatically.
All models are wrong; some models are useful. -- George Box
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