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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Re: A semi-DIY preamplifier for radiation detectors

Post by Richard Hull »

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.

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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Rich Feldman
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Re: A semi-DIY preamplifier for radiation detectors

Post by Rich Feldman »

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

Post by Richard Hull »

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.

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: A semi-DIY preamplifier for radiation detectors

Post by Rich Feldman »

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.

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: A semi-DIY preamplifier for radiation detectors

Post by Rex Allers »

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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Dennis P Brown
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Re: A semi-DIY preamplifier for radiation detectors

Post by Dennis P Brown »

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 13576 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 13706 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

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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.


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: 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.
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Rich Feldman
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Re: A semi-DIY preamplifier for radiation detectors

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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).
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Rich Feldman
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Re: A semi-DIY preamplifier for radiation detectors

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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 10934 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 10934 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.
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Re: A semi-DIY preamplifier for radiation detectors

Post by Rex Allers »

Good stuff, Rich.

What is your alpha source? Is it from one of those big old smoke detectors?
If so, I've heard bad stuff about them shedding radioactive material. I'm sure you are careful but it'd be good to monitor your work area.
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Re: A semi-DIY preamplifier for radiation detectors

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Yup, and you are not the first to relay that warning. In fact the larger of two sources in F5B model is directly visible from outside, through the window screen. Topic for a different thread.

As for heeding warnings,
1. Tool-free disassembly, to the point where we can handle one large part that contains inconveniently-disposed alpha sources, is as far as I intend to go.
2. I wash my hands frequently. Have not yet started wearing disposable gloves.
3. Just received a nice old alpha-particle detector, including electronics, the better to monitor stuff. Anybody here got experience with DIAD II - 900?
4. My first name is not Carl. Middle name is, but spelled with a K, so probably safe enough. :-)
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Re: A semi-DIY preamplifier for radiation detectors

Post by Peter Schmelcher »

Rich you might have a waveform template or limits test feature in your scope. It's a feature in my tds3054 circa 2000. Back in the day the shape of a digital one or zero was a telecom standard. My scope has 8 straight test violation lines (voltage vs time, at any angle). If the analog channel waveform crosses a test line a violation is counted and incremented to the total for that individual line. Overshoot or undershoot the waveform fails and if the duty cycle was not 50% it would also fail, however, the way I perceive the testing feature is 8 bins and you pick the width and height of the bins – sounds like an 8 bin multi-channel analyzer. Ever want to count 2 violation events over a week and know how the waveform failed so that it can be fix, I got the T-shirt.
Food for thought
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Re: A semi-DIY preamplifier for radiation detectors

Post by Richard Hull »

Background here is between 6 to 10 detections/min. 1-inch by 20-inch Reuter & Stokes 3He P4 (4 atm) tube 1700 volts bias, Princeton Gamma Tech preamp. Ortec spectro NIM amp and Ortec digital NIM counter. Tube in 6-inch diameter PVC tube water tank moderator. My system has been fixed rigidly from 2002- until 2020. (no alterations in position or settings).
High CME Solar storm time monitored in 2000-2005 and in 2012 time frame saw counts as high as time averaged 24 detections/minute. There could have been moments of higher count rates as I did not continuously monitor. I consider 8 CPM my norm here over vast periods of time. No diurnal differences noted worth mention.

I have wondered about neutron detects versus cosmic rays. As the bulk of neutrons at ground level are from cosmic starring at about 11 miles up in the atmosphere, what few counts I get might be from the rare fast or thermal neuts that make it to the ground here. (water in the lower atmosphere really impacts any fast neuts formed at the 11 mile high region.) For me, 100% of all neuts I detect are of cosmic origin with any increase being from severe CME events of solar proton storms.

Calibration done in 2002 with one month old calibration at Factory of my first PNC-1 Eberline while running fusor III. Rechecked between 2005-2007 with 3 BTI bubble detectors while running my Fusor IV.

I have checked water evaporation in the tank and winter versus summer backgrounds with about a 10% thermal variation (~ 1 cpm) and almost no detectable change of any significance with a 20% reduction in water level. Before major runs, I check and add water to a fill line in the tank to warrant no significant change in function.

Once working and calibrated, I am paranoid about keeping all things fixed in my neutron counter setup. Note: I have two 3He Reuter and Stokes tubes, one large 2-inch by 18-inch BF3 tube and three functional, portable, electronic neutron detection instruments on hand with two rem balls (All BF3 tubed). I had a big sell off of my many 3He tubes back at HEAS 2018. None left that I wish to sell.

I have noted that absolute neutron counting is virtually impossible at the amateur level and a +/- 20% level for excellent professional systems with good statisitics at the low end, the best the amateur can hope for with a perfectly adjusted neutron detection system is a +/- 5% differential accuracy at the lower end. I dare say a +/-1% differential at 200cpm count over a normal background. When running fusor IV and getting 5,000 or more CPM, background is of no consequence in the differential area. At this level, the differential for improvement recognition is up a bit. A run with 5167 cpm versus 5310cpm might not be an improvement based on the variability of the gas pressure, voltage and current in a fusor that is the norm. As always, knowing your source, your counting system and the environment it is in is a skill learned over time.

Rich is doing a great job in trying to discriminate his detection tube with the electronics he has. He will make it, but the hotter the neut source the better. If you can get that N source, then discrimination is a snap as you can see in the fabulous and without peer, Carl Willis videos on setting up you neutron tube and electronics for perfect setting of dials for warranted neutron detection.

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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