Rebuilding my fusor.

[William_Estlick]

I will try usins these cathodes again tomarrow to see if any cracks formed.

I'm not sue why the plasma was turning from blue to red, maybe there was silicone on the cathodes when they got hot. Or maybe the plasma where at a higher or lower temperature causing the spectrum to shift.

And I still need to try and line up the cathodes better.

[William_Estlick]

Parifin wax makes a pretty good nutron moderator, even though distilled water apparently is better.

I cast 2 blocks, wrapped them in cellophane, and taped the detector to one of them.

I am not sure how thick the wax needs to be, but I can make more and stack them if needed.

[Liam David]

For maximum detection efficiency you should surround the tube with the moderator. The ideal configuration is something like (5 cm ???, have to check) of moderator between source and tube and then an infinite thickness of moderator acting as a reflector. A thicker moderator between source and tube is not always better.

[William_Estlick]

My thinking was that if the neutrons are coming from a single direction, the moderator would only need to be between the source and the detector. (Allowing for scatter.)

If I place the detector on top of the chamber, they will also have to pass through approximately 1" of glass.

[Dennis P Brown]

Backscatter of neutrons by a hydrogen dense substance isn't insignificant so some moderator behind the tube is still useful. Certainly, more towards the source.

[Liam David]

Back-scattering can be very significant. If I recall, I put some mineral oil jugs near my detector just to see what would happen and the count rate increased maybe 10-20%. There is a trade-off between moderator thickness and the inverse square law for a point-like source. The moderator limits how close to the source the tube can get, but there is a minimum moderator thickness for thermalization. The thicker the moderator, the less the fast flux. There are quite a few variables to consider. Carl Willis posted about this a long time ago and I posted a bit of work in that domain as well.

[William_Estlick]

Thank you, I did not know moderated neutrons could scatter back towards its source.

I would like to make a water filled moderator in the future and will try to make that one cylindrical. Then I will be looking to make more accurate neutron counts.

But for now, I am just looking conformation of neutrons. At the very least, I am sure I will need more than 15kv and will be building a voltage multiplyer.

[Jim Kovalchick]

The more common term for the phenomenon is reflection, and the material that does it is a reflector. Note that reflector material is often the same as the moderator.

Surrounding a detector with a moderator and reflector does not make the reading more accurate in determining true neutron production. In my opinion, it makes it more inaccurate. This is because neutron flux is very sensitive to geometry of the source and surrounding materials. Unless the detection array is carefully calibrated with a known neutron flux of the same energy and geometry, all one really knows of a system is a ball park and relative numbers from one measurement to the next. 1/r² is a gross misapplication often done by folks here including me. This is why I take claimed fusion rate numbers with a grain of salt. Some may argue with me, but I have come to this conclusion through academic study and coaching from my son who has a expertise is MCNP modeling.

All this being said, surrounding a detector designed to sense thermal neutrons is essential for a looking at a fast neutron field. Having the right material on all sides of the detector will make sure it sees as many thermal neutrons as possible.

Jim Kovalchick

[Richard Hull]

Jim is correct, as always. And the way we determine if anyone has really done fusion regardless of numbers is real easy.

You should have a removable neutron counter tube from the moderator. This is why most folks use a moderator cylinder with a hole in the center of it. They can also create block assembled pile of moderator material all around a central hole for the detector tube.

True proof of fusion is if you have a neutron count with the tube in the center of the moderator of, let us sasy 200 cpm and with the fusor still running just remove the tube and get nearly zero counts one or two feet away from the moderator.

A prime example is found in this video produced by someone with the handle of Clagwell. I assume this was someone from our forums.
His fusor is running and he is counting neutrons. Several times he removes the counter tube from the cylindrical moderator and the counts cease. He returns the tube to the moderator and counts resume. This proves fusion is taking place!

https://www.youtube.com/watch?v=MFOrEsPDrZw

In all cases the counts either cease completely or are 100 to 1000 times lower. If you count 200 cpm and pull the tube out and then count 180 cpm you are NOT doing fusion. If you count 200cpm and you pull the tube out and get 1 or 2 cpm then you are doing fusion.

Jim points out something I have posted for years. Accurate neutron counting and flux measurement is a nightmare and using a point source method is always going to be wrong and imprecise. However if you leave your counter system forever in one place and never move it relative to your fusor, you can make changes to your fusor and if the numbers go up or down, you will have a hyper accurate relative count as to whether you improved or messed up in your changes.

Richard Hull

[Dennis P Brown]

Glass is useless as a moderator so that isn't something to use. Just use your hydrogen rich wax blocks. Adding some such material behind into your detector when trying to prove you are doing fusion. These must be slowed neutrons since most detectors do not respond to fast neutrons created by the fusor.

Determining the exact amount of moderator to use is not something anyone can easily tell you for the reasons given. That could be done as an experiment in the future when you want to do such a experiment for your system. But as Richard and others point out - the gold standard is reading a count with the moderator in place and then essentially getting no count w/o the moderator. Just watch numerous examples of this that most people include with their request for admittance to the neutron club. You'll also see their geometry for their detector/moderator system.

[William_Estlick]

I havn't heard the term reflection and that only shows how much there is to learn.

I am very interested in ion paths, electrons and fields in a plasma.

I recently came across a youtube video that talks about electrons and electromagnetic fields in a conductor that may be of intrest.

https://m.youtube.com/results?sp=mAEA&s ... ough+wires

So, if electrons are barley moving in a conductor; could it also be possible that they are barley moving between the anode and the cathode in a plasma???

[Liam David]

No, in typical plasmas the electrons are moving very fast due to the low collisionality as compared to solids. This is due to both the much lower densities of typical plasmas as compared to solids, as well as the transition from the hard sphere to the 1/v^3 scattering regime.

Even at just 1eV, electrons are moving at about 6e5 m/s. Your glow discharge system has mean electron energies on the order of 1-10eV.

Think about it: if the electrons were moving at just, say, 0.1mm/s in your plasma as they do in a metal, the glow in your chamber would also move that fast when you first switch on the power. I'll bet you see that the glow establishes instantaneously.

There are some parallels between plasmas and metals... in grad school we sometimes joke that solids are just really, really cold plasmas. One can use a form of Ohm's law for collisional plasmas, most often under the resistive magnetohydrodynamic (MHD) model, but I would not suggest pursuing the analogy of plasmas as metals.

[Ryan Ginter]

Neutron reflection can be easily understood with a very generalized analogy. Think of a speeding bullet being fired at a target. If the target is made of a very large and dense material like a steel plate the bullet is likely to ricochet due to the large difference in mass. The bullet shoots of in another direction, but it's speed changes very little.

This is essentially what happens with some types of neutron reflectors, particularly materials like lead, tungsten, and uranium. This type of reflection is not useful for common forms of neutron detection because they remain fast neutrons.

Now imagine the bullet is fired at a wood plank. The wood has a much lower density than the bullet, and the effect we observe is that the bullet passes through the plank with only a small change in direction, but also a loss of some speed. This is like the moderators typically used with fusors, such as paraffin wax.

As the neutrons pass through the first layer of paraffin they slow down more and more, but they continue moving in a relatively straight line. This would continue until the neutron comes to a stop, except there is another factor at play. The moderator has a temperature, that is to say the atoms it's made out of are vibrating. The neutron cannot stop moving completely because it enters equilibrium with the vibrating atoms. These are known as thermal neutrons, and are what common detector tubes can read.

Once they reach this thermal state, they no longer move in a mostly straight line. The constant state of bouncing off of moderator atoms causes their directions to become completely randomized. This is another form of neutron reflection. If you imagine you have a wall of moderator with thermal neutrons passing through it, the randomization of their directions will result in roughly half of them returning from the direction they came (assuming an infinitely tall wall with finite thickness).

Thus the goal of the detector moderator is to be just thick enough to thermalize neutrons as they approach the detector, then be thick enough on all other sides to aid in reflecting as many back towards the tube as possible.

Please don't mistake this as the actual physics of what's occuring though, as I've stated it's just a rough analogy. The real physics and the guides for designing moderators can be found in the radiation FAQs.

[Richard Hull]

Ryan makes useful analogies. Unfortunately, the FAQs do too. It is not possible to teach neutron physics in FAQs. That is what college courses in the subject does and what can be found and self taught to a degree in college level texts on the subject. People come to this site to do fusion and hopefully also apply skills they bring here and learn new ones in doing so. FAQs are "gap fillers" in technology they have never been exposed to but will have to understand to not only be successful, but to expand their horizons in the FAQs devoted to that particular discipline.

No one does fusion here and leaves as dumb or ignorant of fusion and the many processes that are involved in making it happen as when they arrived.
As Bacon said in his essay on studies, paraphrasing again, 'reading and studies applied make for a ready man'.

FAQs are designed to head off the constant re-answering of common questions with the assumptions they will be read and therefore forestall a future barrage of similar queries. I doubt if 50% of those arriving here read the FAQs made especially for them. Still, that is 50% we don't have clogging the forums with asked and answered questions.

FAQs are corrected and added to as time goes on. I often find a faux pas that I have made and correct the FAQ or add to it in some way which makes it more understandable.

Analogies work if they create understanding, but are often not the whole of it. Only those who hunger beyond the analogy will seek the core understanding in a book on the topic.

Richard Hull

[Ryan Ginter]

I see, that was my mistake. I definitely remember reading about determining moderator thickness from this site, but now that I think about it, I believe it was a link to a paper. Possibly one from Carl Willis.

In either case, reading through the FAQs gives people the understanding to know what they don't know. Once they realize just how much more complicated the topic of nuclear fusion is, they can head off on their own self-directed study.

[Dennis P Brown]

Here is my use of a He-3 detector showing the effect of a moderator:

[William_Estlick]

My apologies Richard, I guess I am still trying to figuring out how to use a bulleten board.

Could you please tell me, is there is a place on Fuser.net to talk about electron drift and wells in electric fields? I am sure many of us would love to talk about the physics that are involved in fusors and fusion.

As far as colledge, my life just did not go in that direction; and with a family, mortgage and a job, I will probably never get my phd in physics and work in a gient lab with every resource at my fingertips. But that does not mean that I and a myriad of other amiture scientists do not have something to contribute to science and humanity.

Fuser.net has been an incredible resource and I have enjoyed sharing the progress of my fusor very much.

"I would just like to finally talk to someone who knows what I am talking about..."

[William_Estlick]

Hello Dennis,
Can you tell me, is that is a neutron source or your fusor on the other side of the moderator?
Also, is there a link to your fusor build?
I would like to see it.

[Liam David]

The "Fusor and/or General Fusion Theory" section would be the appropriate place for discussions about plasma physics and general fusion topics. I'd caution posting about something like electron drift in metals. A general physics forum would be better for that since the topic falls within the "standard college physics material" category.

Amateurs have much to contribute to physics and science/engineering in general. I'm all for trying new things and experimenting at home whether a degree is in hand or not.

[Dennis P Brown]

Yes, that short vid is for my fusor while it is producing neutrons (from deuterium; pressure 12 microns*, potential: 32 kV, and 28 ma.) Notice the moderators - about 10 inches (which I do think is overkill and likely reducing my count rate.) Moderators are essential to slow/thermalize the neutrons for the detector. I removed the x-ray shielding for the picture.

And in the spirit of the title of this thread, I'll mention that since that picture was taken I've recently rebuilt my fusor system into a far smaller footprint; however, I kept the chamber.

* Unit not calibrated

[William_Estlick]

Distilling the heavy water for the second time before I put it through my fuel cell.

[Dennis P Brown]

Why are you distilling the Heavy water?