Non uniform angular neutron output from fusors

[fusordoug]

We have noticed, while trying to get a good calibration on our big neutron detectors (moderated 3He and BF3 tubes in 6" dia HDPE) that we are not seeing uniform isotropic neutron output from our cylindrical fusor.
We placed two BTI detectors at the same distance from grid center-axis, but at different spots along the cylindrical grid's length, and got very different, but repeatable results in back to back runs from the two detector locations.

In all cases here, there is a hot spot near the open end of the cylinder grid, well away from the middle of its length where the visible rays appear to emit from (even with our cylinder grid, the rays appear circular in cross section and emit from the center of the length). We placed two BTI's side by side, vertical, at the same distance from grid axis (cylinder is horizontal), one about even with the end, and one a BTI diameter (about 3/4") closer to the middle of the grid length, and the one nearer the grid end consistently counts 3-5 times the millirems, and it doesn't matter if we switch which BTI is where -- we get the same output ratio from the same locations either way -- the indication stays in the same place re the fusor and doesn't move with the switched BTI detectors. Which convinces me at least that it's not a problem with the BTI's -- they are consistent.

This calls into question the widely held idea that neutron output is isotropic from a fusor, and the ratio we see here seems too high to be accounted for by square law effects alone (assuming even that most of the neutrons are made right at the tank wall, rather than elsewhere as seems more reasonable). It also makes the idea that you can calibrate a large, comparatively farther away moderated detector with a BTI reading alone seem very iffy.

In our case, the moderated detectors subtend a much larger angle than the BTI's do, even though they are closer in, and would tend to average or smear out any spatial non uniformities so they are reading some kind of average, while the BTI's seem to be telling us that the output isn't uniform over all angles, and that it is very variable over rather small angles.

I have constructed, but not quantified and rigorously tested a directional neutron camera to investigate this effect, and obviously it is now a high priority to get that into action and map out the angular distribution of the neutron output from my fusor. As it is large and heavy, and due to the way it works, not very sensitive, I will need to rig a better support for it than just sitting it on a sawhorse nearby -- perhaps a camera tripod or something like that will help the effort, as it needs to sit still for some tens of seconds to count enough neutrons to be reliable statistically.

I'll add some pictures of it on a later edit of this post, but it is a plastic scintillator/phototube arrangement behind a moderator with a hole in it to only past neutrons where the hole is, which were already going down the hole axis of the 1/2" by 6" long hole. Any other neutrons would encounter borated wax and cadmium washers and not make the tube count. Due to the narrow field of view, it's not very sensitive, and seems to also see some scattered gammas despite being encased fully in 1/8" thick lead (thicker in some spots), so there's a bit more work to do on that design yet.

Hopefully, Tyler, who sees something similar, will report his results here as well. This seems to be very real, we've seen the effect here every time we measure it.

Now to figure out why, and if we can use it to do interesting things, or harness the effect to advantage. We should learn important things along that path.

The plot below is from some beam-on-target work at higher energies that seems to show that the output from the reaction is not isotropic at all, even up to energies well above the Q of the reaction, so the effect might be even more pronounced at the lower eV inputs we are using (here it is usually about 50kv).

[Tyler Christensen]

I have indeed seen this, I have attached a picture of a recent run in which there is a hard cut-off line in a bubble detector where neutrons appear to no longer be. I had some instrumentation damage that I'm repairing so I haven't had a chance to investigate this further, but in the coming days I will do this more and see if it can be repeated in other locations, and more dramatically with more bubbles.

I also did an investigation on net neutron output at various points around the fusor a while back which I published to fusor.net, but will also put up here since it is somewhat relevant to this post (since max attachment size is 4MB: http://musicman500.tropical-forest.fera ... ormity.xls).