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Re: A semi-DIY preamplifier for radiation detectors

Posted: Wed Apr 24, 2019 3:00 am
by Rich Feldman
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".

Re: A semi-DIY preamplifier for radiation detectors

Posted: Wed Apr 24, 2019 5:59 am
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.


Richard Hull

Re: A semi-DIY preamplifier for radiation detectors

Posted: Sat Feb 08, 2020 3:41 am
by Rich Feldman
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.

Re: A semi-DIY preamplifier for radiation detectors

Posted: Tue Feb 11, 2020 8:29 pm
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).

Re: A semi-DIY preamplifier for radiation detectors

Posted: Sun Mar 01, 2020 9:07 am
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 1955 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 1955 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
neut3.png (47.92 KiB) Viewed 1955 times
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.

Re: A semi-DIY preamplifier for radiation detectors

Posted: Sun Mar 01, 2020 9:47 am
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.

Re: A semi-DIY preamplifier for radiation detectors

Posted: Sun Mar 01, 2020 10:14 pm
by Rich Feldman
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. :-)

Re: A semi-DIY preamplifier for radiation detectors

Posted: Mon Mar 02, 2020 5:22 am
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
-Peter

Re: A semi-DIY preamplifier for radiation detectors

Posted: Mon Mar 02, 2020 3:31 pm
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