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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Re: A semi-DIY preamplifier for radiation detectors

Post by Rich Feldman » Wed Apr 24, 2019 3:00 am

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
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".
All models are wrong; some models are useful. -- George Box

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Richard Hull
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Re: A semi-DIY preamplifier for radiation detectors

Post by Richard Hull » Wed Apr 24, 2019 5:59 am

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

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

Post by Rich Feldman » Sat Feb 08, 2020 3:41 am

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 » Tue Feb 11, 2020 8:29 pm

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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