Neutron Field Nonuniformity

[Tyler Christensen]

It recently came up in discussion in some emails whether the neutron field really is isotropic or not after I had some issues with calibration not matching in different geometrical orientations of detectors, and someone else had that same thing happen. Today I decided to test this in a controlled method by comparing results from a 3He tube in a fixed position 4 feet from the fusor to that of a bubble detector at a number of different places around the fusor, and found that the emissions at the fusor's surface are definitely not at all isotropic. The flux at the poles (the feedthrough and opposite the feedthrough) are half the flux as that at the equator, along the edge of the grid. Additionally, there is slightly more flux at openings between grid wires along the equator versus along grid wires. I tested a number of positions and had results of shocking precision, and the value found at the last 45* off-equator test was exactly as predicted (confirming a gradient between equator to pole).

All the tests were right around 40kV, 3.5mA, and 15.5 microns (on a thermocouple, so out of calibration for deuterium). The highest neutron flux (at the "best" place to put it for high numbers) according to the bubble detector was a little over 2e5 n/s during these runs. Nothing was changed including gas flow, the only control I used the whole test was the on-off switch on the variac.

Attached is an excel spreadsheet showing photographs of the test positions and the actual numerical results (looks like it's too big at 4.8MB, you can download it here: http://musicman500.tropical-forest.fera ... ormity.xls).

This means one of two things, all of the fusion is definitely not occurring at the center of the poissor, or it is and the neutrons do not come off of the reaction in an isotropic manner. The first option seems far more likely, but is by no means confirmed. Regardless of which it is, it brings up the question of where people are calibrating. I have always calibrated right above the feedthrough, so I guess that would have to bump my highest stable record count from 1e6n/s to 2e6n/s, but a fixed number without geographical justification has little meaning according to these results.

It would be interesting to see the results of someone else performing this same experiment.

[Andrew Seltzman]

The IEC group at UW Madison has shown that a significant(measurable) amount of fusion occurs in the star mode jets. This may contribute to non-uniform measurements when a detector is not positioned much farther away from the fusor then the spatial extent of the jets.

Andrew Seltzman

[Brett]

Or, it could mean a third thing: The fusor enclosure is modulating the neutron flux. It's not, after all, a uniform sphere, or perfectly transparent to neutrons.

[Andrew Seltzman]

The fusor enclosure should not deflect fast neutrons to any measurable extent.

see:
http://www.rtftechnologies.org/physics/ ... tector.htm

(near the bottom of the page)
About 1.5" of lead does not effect fast neutron detection rates to any measurable extent.

Andrew Seltzman

[DaveC]

I'm inclined to think your results are likely correct, Tyler.

The premise of Isotropic emission has always had the unspoken caveat that it assumed conditions were more or less uniform along circumferential paths of constant radius. In other words, while the density of ions must certainly vary from the center outward, it was assumed to be axially uniform. But clearly the center grid structure must cast "shadows" in ion flux, at least.

Simulations I've done with small numbers of particles show them looping all over the place, to the eye, apparently passing through some regions more often then others. I never tried to do simulations with thousands of particles, for obvious reasons of computational speed.

So it does seem plausible that there may be places where the ion density is higher and thus more fusion occurs there. Isotropic emission occurs at every zone, and it remains to be seen what the superpositions of all this might mean.

Probably works in favor of higher N/sec rates, for local regions, and quite possibly lower overall numbers.. But the higher numbers point the way.
.
Very interesting.

Dave Cooper

[honickmonster]

Great experiment, I remember you talking to me about this before.

One concern I have is the way you mounted the bubble detectors. I noticed you said that you taped them onto the fusor chamber for the run. If the chamber got hot during the run, then the parts closer the the main chamber (the equatorial flanges) would have reached a higher temperature than the "extremities." If the large flanges did indeed reach a higher temperature than the tubes protruding from the chamber, then the bubble detector may have been at a higher temperature and thus given a (falsely) higher reading.

If the water cooling I see in the pictures was active and was able to keep the chamber temperature from rising too much above ambient then this concern is moot.

Matthew

[Tyler Christensen]

Water cooling was on and the equatorial flanges were ever so slightly warm to touch, perhaps 2-3 degrees over ambient, that was not an issue in the measurements.

[Doug Coulter]

Good show Tyler!

I'll be duping those results with my own gear real soon. Just got my fusor back up running again today, didn't quite get to fire up the neutron camera.

We have seen this here too with BTI's, two brand new ones, one diameter apart, standing vertically at the side of a horizontal cylinder grid. Grid is 3.5" long, Bti's were 4" away, touching one another, and one got 3x the bubbles of the other. Switch them, make another run, same result -- but the more bubbles went with the position, not the particular BTI. So at least in our case, the variance was even larger than you are reporting over a very short distance -- one BTI diameter.

Interestingly, the one near the end of the grid was the one that got the most bubbles, not the one nearer where the "rays" hit. FWIW. More work needs to be done on this one.

All the data I can find from beam on target shows more or less perfect isotropy of neutron direction up until the beam energy gets way high (megavolts).

Our limited data (more soon!) shows so much anisotropy it can't just be that neutrons are coming from the beams, something more is needed to explain 3::1 I think, and the fact that near the end of the grid (where there are no visible rays) gets more is interesting too.

[Carl Willis]

This is a great experiment Tyler. Thanks for sharing it.

IF the differences are significant (in this experiment they aren't, or barely are; see below) Andrew's suggestion is a pretty plausible one. Other issues possibly in play are reflected neutrons due to the fusor's surroundings. Ideally there would be no table, no gas bottles, etc. This is a real complication unless the differences are huge. Matthew's suggestion was good, too, but you ruled that out I'd say.

If I had to add some new advice of my own, it would be to try and get more precise data to help steer the conclusions more forcefully. You now have two data points for the "B" position (and the two are consistent), but only one for the others. The biggest limitation in drawing decisive conclusions is the uncertainty in the bubble counts, sometimes as high as 30% (see graph attached with error bars shown). I can certainly understand the difficulty in keeping the system stable for long enough periods of time to get lots of bubbles though.

This is a fertile avenue for more experimentation and of considerable relevance to those who are striving for consistency between measurements on their fusor or want to know where to site an activation sample.

-Carl

[Frank Sanns]

Nice work Tyler.

The 1/r^2 relationship can answer your question: is fusion not occuring in the geometric center of the fusor or fusion producing neutrons that are not isotropically directed. By moving your detectors out by increments, you should be able to back calculate where r0 (point of production). This will give a crude map of where the fusion is occuring within your chamber.

To get to this point with any kind of sensitivity, you will need many more replicates. Any one number has an infinite uncertainty. As you add numbers the uncertainty gets smaller but even two does not give a good confidence interval. From the data you presented, even though your confirmation of B looks good, the confidence in that number without some other statistical basis to determine the uncertainty of your set up, it suspect at best.

One factor at a time experiments are the trickiest to do. It is better to vary a couple of factors like distance, position, orientation and the like and then sum up orthogonal columns. This gives values for noise, sensitivity, and cause and effect relationships all in one.

Frank Sanns

[John Futter]

Tyler
try it again and see if the results are similar
if they are, test Franks and Carls observations and again do them twice

to Test Carls lab environment effect some runs using a large block of wax or similar hydrogen containing substance in fusor- detector- wax and wax - fusor - detector type configurations would be in order with nothing else in the lab moving especially the fusor detector relationship

PS
Excellent work!!!!!!

[Tyler Christensen]

I will do some repeat testing this weekend and a bit more investigating. I'm particularly curious with what Frank said and seeing what happens when the detector is 2 feet off the end of the pole versus 2 feet off the equator, of course I'll need to do super long runs since my best stable rate right now is a quarter million per second (my record of a million plus was in a resonant state that I can't for the life of me repeat)

[Chris Bradley]

Tyler,

Frank has it right. A fusor is a radiator and all radiators have a near and far field.

If you are in the far field, then a radial displacement won't affect your distribution.
If you are in the near field, then a radial displacement will.

At the moment, you have evidence that suggests you are in the near field. To get far field results you will need to be several times the size of the device away from it.

If you do prove that there are anisotropic neutron emissions, then you have effectively proved the neutrons are not nuclear fusion in origin, unless you go on to demonstrate something absolutely so radical that it turn nuclear collision theory on its head. It would be an interesting discovery, but be careful what you wish for!! For now, the simple explanation is - near field radiation pattern.

[Carl Willis]

Chris, what are you talking about?!

Seems to me that Frank and Andrew previously were suggesting that the neutron source is distributed or is not concentric. Very straightforward and reasonable hypotheses, good suggestions.

If you have a theory invoking the concepts of "near field" and "far field" such as are encountered in optics / electromagnetism, then that's a radically different animal from anything else up here. It's also bizarre from a basic physics consideration. Maybe you have appropriated or re-invented the terminology itself to apply to the ideas of geometric attenuation with which Frank's and Andrew's posts are more lucidly directed.

You need to seriously take stock of your role as a sower of confusion, inaccuracy, and irrelevant nonsense on these boards, all delivered with self-assured and trollish obstinacy. Between this and the tail end of your metals-smelling thread, you have been throwing nothing but gutterballs lately.

[Steven Sesselmann]

Tyler,

I echo the others here and say good work, but I also think that the sensitivity of the bubble detectors may be giving you unreliable results. i know that too well from my own experiments. I always used two bubble detectors next to eachother, and sometimes one had twice as many bubbles.

If it is at all possible, I suggest you repeat the experiment with your 3He detector, to see if you get the same results.

Steven

[Tyler Christensen]

I don't have a small enough 3He detector to stand any chance of really isolating areas close to the fusor. It would work for the "Far field", but not near it (2 foot reuter stokes). I also have some 10B tubes but they are also far too large. I would need something like a G-10-2 to do that, unless there is some method I'm not thinking of?

[Chris Bradley]

If anyone wishes to agree with Dr.Willis and his florid description of his confusion over my choice of words, then please speak up so that I may provide a supplemental clarification for you.

[Carl Willis]

For my sake alone, I would appreciate a clarification.

[Dan Tibbets]

Would silver or other appropriate isotope activation provide the necessary spacial resolution and precision? IE: is it more precise/ reproducible than BTI bubble detectors?

Dan Tibbets

[Richard Hull]

Activation of silver here would offer little as any differrential would go statistical into a noise as the flux is low. Add to this the nature and volume of a required moderator and hot zones would be tough to see at a happy statistical level.

I wouldn't trust any bubble detector that was placed on, in contact with or even near a fusor that was in its air vent heat plume; I don't care if the thing was water cooled. The bubble detector must be at ambient or it should not be trusted. I have always placed my bub detectors on a lab ring stand, held in a lab clamp, below the horizontal belly band of the fusor and about 4-5 cm off the shell. In this manner, I am in no rising heat plume and well out of contact with the shell.

He3 detector data, close by, is also unreliable due to the size of the moderator and detector volume, unless you are using a tiny He3 tube. Tyler has already spoken to this issue, however.

While I will not leap into any argument related to near and far fields. I bet an He3 detector placed 1 meter out would show true isotropic emission of the system.

Even if you have hot zones of neutron production along more idealic ion zones, once the fusor looks nearer to a point source to any bulky detector the more unifromity is seen.

It is all good experiment though and if any truly significant hot spots are found just at the fusor, critical detection methods will be needed which at first blush seem beyond us. Time will tell, as always.

Finally, I would tend to believe a 30 count differential in a total He3 count of 500 at 1 meter than almost any measurement just at or near the fusor shell as an indicator of any anisotropic neutron production. (This assumes rigidly smooth and pefectly controlled fusor operation over several counts at several positions. - Another tough matter to warrant with a fusor). Variability of fusor operation could be nullified with a fixed He3 counter at one meter while moving an identical counter around at the same range.

Interesting work Tyler.

This would have been far better placed in the neutron radiation detection forum.

Richard Hull