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New tests with neutron detector - EMI rejection

Posted: Fri Dec 01, 2017 7:03 pm
by Silviu Tamasdan
I refined my detector a bit. It's now enclosed in a Faraday cage to reduce electromagnetic interference (well it's been for a while but didn't have time to test). The cage is a 3/4" copper tube from a home improvement store plumbing section; ID of 3/4 copper is 19.5mm, OD of SNM-11 tube is 18mm. The SNM-11 was sanded carefully over its whole length and wrapped in several layers of aluminum foil, enough to make a tight fit (but not too tight to allow for disassembly). One end of the copper tube has a lid soldered, the other end is a tight-fitting lid with a hole for BNC connector, and is held over the end of the copper tube with a metal hose clamp.

The connections of the tube are: cathode/aluminum foil/copper tube/BNC jacket, and anode/copper fitted connector/soldered to central BNC connector. Everything is nice and tight with no mechanical play at all.

As a source of EMI I used a high voltage arc in air; it's produced between 2 copper electrodes about 1cm apart, fed by a NST nominally 3.2kV/30mA (but it doesn't get anywhere close to that). The NST is specced for 220V primary, so I also use a upconverting transformer 110V-to-220V/500W. Using a kill-a-watt type device the total power used by this chain is 52VA, so allowing for losses in the 2 transformers I will assume that no more than 25W is fed into the arc. That's why I said above "doesn't get anywhere close" - but good enough for this test.

Setup:
- tube + Faraday cage held in a fixed position above bench in a lab clamp, with the other end supported by a metal ruler which is exactly 15cm. (because the lab clamp isn't strong enough). It's connected to the MCA then to acquisition computer.
- arc is set 30cm away from the tube between electrodes as described above, supported by a flower pot for insulation.
- neutron source placed in relevant experiments at predetermined position, 15cm away from the tube. The source I have posted about before, built from a 5mCi 210-Po source + beryllium. Calculated current activity of the source is 3.68mCi.
- moderator built around the detector tube in certain parts of the experiment, consists of a 4x5x20inch block of HDPE and 12 quarts of mineral oil.

Counted neutrons 8 times, 10 minutes each; recorded all data including pulse and baseline waveforms.

Experiment truth table (and quick results for the impatient):

Code: Select all

test        EMI(HV arc)         source        moderator    |  counts in 10 minutes
N1               0                 0             0         |               3          
N2               1                 0             0         |               3             
N3               0                 1             0         |               5              
N4               1                 1             0         |               5            
N5               0                 0             1         |               4             
N6               1                 0             1         |               2                  
N7               0                 1             1         |              29                            
N8               1                 1             1         |              27                    
(where 0=not present/not turned on, 1=present/turned on)

I think the numbers speak for themselves. I did not notice any abnormality of the baseline when the electric arc was turned on. The SNR remained excellent even in the presence of interference.

I'm attaching the full datasets to this post.
The next post will contain screenshots of the final result for each of the 8 datapoints above. Post #3 will contain pictures of the experimental setup.

Re: New tests with neutron detector - EMI rejection

Posted: Fri Dec 01, 2017 7:05 pm
by Silviu Tamasdan
Screenshots

Re: New tests with neutron detector - EMI rejection

Posted: Fri Dec 01, 2017 7:10 pm
by Dennis P Brown
A good start on working with more realistic issues (except, maybe failure issues with a fusor ;) . I've had my fill of those today with my own system.) A bit limited data - could use more runs with the source and moderator (say, ten) and maybe five runs without a moderator. Also, 29 counts in 10 minutes is rather low. I thought the source was more active then that - or is that the result of the detector bias circuit applied to the detector tube? Flynn was asking about that and as I only far too recently discovered, not an insignificant effect on the output signal.

Aside: a fusor can also cause issues with ground loops and feed-back problems along power lines within the house resulting in complex interference issues with detector systems, besides the RF signature that you are partly simulating.

Re: New tests with neutron detector - EMI rejection

Posted: Fri Dec 01, 2017 7:15 pm
by Silviu Tamasdan
Pictures

Re: New tests with neutron detector - EMI rejection

Posted: Fri Dec 01, 2017 8:41 pm
by Silviu Tamasdan
Dennis P Brown wrote: Fri Dec 01, 2017 7:10 pm A good start on working with more realistic issues (except, maybe failure issues with a fusor ;) . I've had my fill of those today with my own system.) A bit limited data - could use more runs with the source and moderator (say, ten) and maybe five runs without a moderator. Also, 29 counts in 10 minutes is rather low. I thought the source was more active then that - or is that the result of the detector bias circuit applied to the detector tube? Flynn was asking about that and as I only far too recently discovered, not an insignificant effect on the output signal.
The detector is optimized already. With this system it's the best I can get. Poor sensitivity but can detect enough neutrons to see a difference. I am not trying to achieve high quality neutron flux measurement, but rather obtain reliable detection of thermalized neutrons above background. Precision will come later, with a completely different system that I've already started working on.
Aside: a fusor can also cause issues with ground loops and feed-back problems along power lines within the house resulting in complex interference issues with detector systems, besides the RF signature that you are partly simulating.
I can run the detection system disconnected from any power supply, just off the laptop battery. It runs for about 90 minutes this way. If I need more I can rig a car battery based charger. No ground loops, measuring system air-gapped.

Re: New tests with neutron detector - EMI rejection

Posted: Fri Dec 01, 2017 11:56 pm
by Richard Hull
Is your source a nucleospot? If so you should have many more counts in 10 minutes in moderator. If it is how is the Be placed on the source.

Richard Hull

Re: New tests with neutron detector - EMI rejection

Posted: Sat Dec 02, 2017 2:46 am
by Silviu Tamasdan

Re: New tests with neutron detector - EMI rejection

Posted: Sat Dec 02, 2017 3:31 am
by Silviu Tamasdan
Of course I get a lot more counts with the source placed inside the moderator block right next to the detector as opposed to 15cm away from it.


But again, that wasn't the point of this test.

Re: New tests with neutron detector - EMI rejection

Posted: Sat Dec 02, 2017 6:15 am
by Dennis P Brown
That is good work - just the statistics are low for the limited data sets. That is, if one is counting between 2 and 3 events a minute, then the data rate isn't terribly significant relative to counting over ten minutes )count rate is just 10% of the total data set.) However, your noise floor is very low and your source is a known, which certainly indicates your data is solid - just sparse. Of course, for most fusion proofs, where the source isn't a given, and noise can be significant, higher data rates are necessary.

Do look forward to you doing fusion when you either build or locate a fusor class x-former or power supply - then you will have far more counts and a very strong & (hopefully!) better signal. My mentioning of the ground loops & feed-back issues on power lines was more a FYI for readers since this isn't always considered by people and can cause problems. A battery operated neutron detector system has a number of advantages.

Re: New tests with neutron detector - EMI rejection

Posted: Tue Dec 05, 2017 8:50 am
by Silviu Tamasdan
I am working on 3 power supplies now. One is the switching PS that I discussed in the other section. Another is modifying a Spellman PS for higher output current. The third is based on a more conventional iron-core transformer. The trigger points for more work on those are in order: 1.ferrites being shipped from China and free time; 2. resistors for appropriate power load being shipped from Russia, and time; 3. huge capacitors being shipped from China, and time.
In the best case scenario if all 3 are successful they wouldn't be wasted; I have enough ideas for using all 3 and more.