Another (Preliminary) Report on Anisotropy

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Liam David
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Another (Preliminary) Report on Anisotropy

Post by Liam David »

I put together another neutron detector based on a SNM-56 corona counter (3He 1.25"x5"). The intent was to make it mobile and more precise than my larger SNM-17 helium tube. I drive it with an old Ludlum model 12 and tapped into the output of a quad NAND chip to connect it to an Arduino interrupt pin and my DAQ system. I haven't embedded it in a moderator yet and am just using some leftover paraffin blocks.

All this means I can now measure the spatial neutron emission by normalizing the counts against the larger tube:

Plot.png

The red trace is the "TIER" as measured by the stationary tube, yellow is the small tube CPM, and blue is essentially yellow/red, all vertically scaled for convenience (and averaged with a moving mean). The left region is with the tube 90 degrees off-axis (I use a linear grid) and the right is on-axis. Both were equidistant ~6" from the cathode center. There's a lot of drift initially as the chamber and cathode heat up, but the on-axis trace is always higher even with reduced "TIER". Once steady-state is achieved, the ratio is about 2.2.

The statistics on the new tube aren't great. I had to turn up the discriminator since noise from the HV is leaking in somewhere, meaning it doesn't count nearly all true neutron events. The fluctuations in counts are thus rather large and the small moderator also doesn't help...but I verified that it only counts neutrons by removing all nearby moderators. Better measurements to come once I solve some of these problems.
Last edited by Liam David on Sat Feb 06, 2021 11:43 pm, edited 1 time in total.
John Futter
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Re: Another Report on Anisotropy

Post by John Futter »

Excellent thinking Liam!!

lots more data to collect
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Joe Gayo
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Re: Another Report on Anisotropy

Post by Joe Gayo »

6inches is probably too close to be sure... take a look at Jon R's post
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Liam David
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Re: Another Report on Anisotropy

Post by Liam David »

The 6" was only because that's the extent of my table past the lead shield, and I wanted to get some preliminary numbers. 20" is about the max distance for my table on-axis, and I'll have to cobble something together for other angles. Distance is but one of the things I'll change once I can get better statistics with the tube. In this run I had the tube parallel to the beam axis, so not ideal. I'll probably go for a front-facing design with a paraffin disk to up the resolution. If I manage to get the resolution and statistics good enough, I might try and fit the spherical harmonics, through that would take a lot of data/locations and runtime.
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Joe Gayo
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Re: Another Report on Anisotropy

Post by Joe Gayo »

I'm hoping my neutron camera will provide some more insight into my setup.
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Liam David
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Re: Another Report on Anisotropy

Post by Liam David »

Certainly looking forward to seeing those results. Looking again at Jon's data, he measured a ratio of 2.3 at 6.25", which is remarkably close to what I measured. Could be a coincidence, just need more data.
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Re: Another Report on Anisotropy

Post by Joe Gayo »

The similarity isn't important at the short distance, it's that the anisotropy holds up at greater distances. Additionally, the cross-section of the tube presented to the source is a major factor, especially so close.
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Liam David
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Re: Another Report on Anisotropy

Post by Liam David »

For sure, I know that far-field measurements with a point-like detector/source are needed to establish anisotropy (at least for low resolutions). This was just a very rough first pass and it was was mostly a "hmm, neat coincidence" comment. Lots to improve.
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Joe Gayo
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Re: Another (Preliminary) Report on Anisotropy

Post by Joe Gayo »

I'm glad you are looking into this ... I've long held that the data supports anisotropy in many fusor setups (but not all)
Robert Dwyer
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Re: Another (Preliminary) Report on Anisotropy

Post by Robert Dwyer »

I have always been interested in what the anisotropy in the fusor would look like. Glad to see people are are interested of taking measurements with their setups. The flux anisotropy as well as energy distributions at different angles with respect to the beams could give interesting insights into the where/why the neutrons are being produced. Of course this assumes that the beams follow the same trend as the visible light we see which may or may not be the case...

At any rate, great initial data! This is very interesting topic.
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Liam David
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Re: Another (Preliminary) Report on Anisotropy

Post by Liam David »

Progress report. I now have a measurement plan and have been working on the analysis tools for when the data exist. I've downloaded a spherical harmonics library for MATLAB that can use least-squares regression on a coarse (measurement) grid to estimate the spherical harmonic Y_l^m coefficients. For those unfamiliar, you can express any "suitably nice" function defined on the surface of a sphere as a linear combination of spherical harmonics. Finding these coefficients from a limited set of points allows you to reconstruct what the overall distribution may look like. To validate the method and code, I created some theoretical measurements for different scenarios (including some random error). The results are in the following images (higher res when you click on them). I'm assuming symmetry between all octants.

Dual beam-on-target, 2.5" distance, 20" measurement radius - flux
Dual beam-on-target, 2.5" distance, 20" measurement radius - flux

Isotropic 5" beam, 20" measurement radius - flux
Isotropic 5" beam, 20" measurement radius - flux

Anisotropic 5" beam, 20" measurement radius - flux
Anisotropic 5" beam, 20" measurement radius - flux

Same scenarios, equatorial flux
Same scenarios, equatorial flux


The reconstruction errors are of the same order as the measurement uncertainties. When removed, the reconstructions are accurate within ~1e-7, meaning the math works. Next step is to spend quite a few hours collecting data.
Last edited by Liam David on Fri Feb 12, 2021 1:49 am, edited 1 time in total.
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Richard Hull
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Re: Another (Preliminary) Report on Anisotropy

Post by Richard Hull »

Great work! I love the spiked porcupine balls related to hot emission areas!

Precision computed anisotropy reminds me of a one teraohm 10% resistor being a warranted open circuit to within +/- 100 gigohms of said warranted open. In general, D-D fusion wants isotropy, but in the end it can be steered and directed to the same order as herding cats. In the end, it is what it is actually measured to be, regardless of computational efforts. This is especially true within the amateur effort.

Still, it is fun to plug in values in the most complicated string of computations and then turn the crank, push the go button and to watch the fun begin based on one's musings amongst the supplied variables.

I often wondered, (I wonder a lot!),that should there be a God or omniscient presence, if we and the universe are just a running complex amusement, a wonderful and fascinating complex of self-assembling ever more complex, self-conscious, thinking organisms, scattered over trillions of worlds juxtaposed to a constantly running down clockwork of entropic death of all that is or ever will be in this one universe. What a gloriously wonderful universe! Such an entity could have multiple universes running at the same time! Just monkey with a few hundred key variables and spin her up! Never a dull day, right? Plus, absolutely no need to intervene lest the original startup be soiled and fouled to its original purpose. (Seeing where it goes if left undisturbed)

Sorry for the musings.....got wound up on our thinking we can figure it all out. I find it great that we try and will never stop trying. We have to think we are the "little engine that can" or there is nothing left to keep the train moving forward. Progress, science, engineering, art...Call it what you will....It moves and inspires those who think deeply.

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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Liam David
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Re: Another (Preliminary) Report on Anisotropy

Post by Liam David »

I now have some pretty good data on neutron anisotropy in the fusor "equatorial plane." Each run was ~5 minutes long at ~35kV, 8.56mA, 6-7e5n/s. The small 1.5" detector was 43cm from the center of the chamber and the output was monitored by my larger tube on the opposite side of the fusor. Count rates were ~50kcpm on the main tube and ~300cpm on the smaller detector. I measured at 8 angles between 0 and 90 degrees, with 0 corresponding to the beamline.


anisotropy.png


The ratio on/90 off axis is 1.34:1 and tapers off smoothly. The results are repeatable, and I conducted a few extra runs at various angles to check.

The endcaps are 6.35cm from the chamber center, so 6.35/43=0.15<<1 meaning it's in the far-field. Fusion in the beamline seems anisotropic.


Last two BOT curves have anisotropic source functions. Note that theta in the title is not the plot coordinate, but the relative angle between the measurement point and source. This distribution function is based on the Leslie Kicka paper on anisotropy, a rough estimate from the plots.
Last two BOT curves have anisotropic source functions. Note that theta in the title is not the plot coordinate, but the relative angle between the measurement point and source. This distribution function is based on the Leslie Kicka paper on anisotropy, a rough estimate from the plots.
Last edited by Liam David on Sat Feb 27, 2021 8:47 pm, edited 1 time in total.
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Richard Hull
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Re: Another (Preliminary) Report on Anisotropy

Post by Richard Hull »

I was in sympathy with Liam as I have thought a lot about this. Table space is an issue in quarters of limited size and fixed gear in this study at the amateur level. To serve the ideal, one would want an empty 20X20 room with the fusor device on a table or platform about 1 foot above any materials on that table. A roll-around cart with a good neutron detector system could then be positioned all about the fusion device at any number of ranges that would seem reasonable. This arrangement should rule out significant neutron scatter that might tend to confuse and confound results.

20X20 open spaces are rare outside of research and certain college labs. Out buildings on private property might not make a good lab, but if the right size, might be emptied and roll-around or portable fusors and measuring carts could be placed there in a temporary arrangements.

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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