MODERATORS

[hartman3704]

Any one tried using berylium as a moderator, or even H2O? What happens in a S.S. chamber, is there contamination? In the Tokomak the problem is iron contamintaion (usualy) which leads to hard x-rays and lost power. Also what about using EMF to keep neuts from reaching the vessel walls? Any one wishing to discuss fusion meet on 1.865MHz, after 20:00 EST call KB8PDU. ~Robert

[henryhallam]

As far as I'm aware nobody has tried Beryllium as a moderator, although it's a light nucleus and would probably work (having a small cross section for thermal neutron absorption) it's generally pretty toxic stuff. I believe it's used as a neutron reflector in nuclear bombs.
Water has been used by a couple of people, notably Carl Willis who did some activation experiments using solution of a manganese compound. I think Craig Wallace used a stack of AOL CDs as moderator!
Good old paraffin works pretty well though and the rem-balls supplied with most neutron counters are made of that.

Could you elaborate a bit on what you mean by contamination? Are you talking about iron from the shell getting into the plasma itself?

I am a bit out of my depth talking about the neutron magnetic moment but I don't think there's a realistic way to stop neutrons exiting the chamber that way, besides you don't want to as the main point of the fusor is to produce neutrons...

Henry

[badflash]

Lots of people are using REM Ball type neutron detectors. These used a big ball of poly to moderate the neutrons so they can be detected by themal reactions. Beyond detection, we've not been very interested in doing anything with the neutrons that I know of.

The fusor doesn't run long enough or put out a high enough flux at the power most of us can run it at to cause much activation.

Berylium was used on some of the old test reactors, but it isn't a very effecient moderator. It can be used as a neutron source by bombarding it with high energy alpha or electrons.

[Richard Hester]

If you look at the archives here you will find a large body of activation work done by a couple of our members about a year or so ago (Carl and Jon, if I remember correctly). I don't recall the flux needed for detectable activation, but the fusors themselves were capable of over 10^6 neutrons/second. A few inches of water made a dandy moderator, as well as a few blocks of paraffin. There were some specific studies done on the thicknessof moderator needed. Beryllium would work as a moderator, but it's expensive and toxic.

[Richard Hull]

As every one has said, Beryllium is just out of the question as a moderator. No one here could afford even two or three pounds of the stuff and that is not enough to do anything with anyway. Beryllium is much better when used as a reflector.

Water is easy to use and is used to moderate neutrons for both activation work and detection of the fast neutrons. With paraffin you have to cast a flammable substance which is still fairly easy if you are careful. Polyethylene is really nice, but a bit expensive and must be milled or machined to shape.

Richard Hull

[Alex Aitken]

Carl also did an MCNP study on moderators and the important result was density of protons is all that matters. Higher atomic weight elements/isotopes like deuterium, berylium or carbon may be better moderators by virtue of lower capture crosssection.but with a point source you fight the inverse squared law as much as you fight moderator efficiency.

[pfostini]

Would Be Oxide ceramic work?

[Richard Hester]

BeO would work OK as a moderator, but if I had that much of the stuff hanging around, I'd use it for something else, given its good thermal conductivity and insulating properties (and high cost). Water/paraffin/HDPE are good enough for our purposes, and are a lot easier to procure and deal with...

[Alex Aitken]

If you were building a reactor, BeO would work. If you are trying to do activation studies or neutron detector work with a fusor it'd be useless. Density of the thermal neutrons on the surface of the moderator is what is important, not the total number over the whole area.

[dlsworks]

also remeber, that Beryllium is an effeciently transparent and refractive material for x-rays.

Darius

[Alex Aitken]

Transparent I agree with, by virtue of its low atomic number, refractive? I dont think so somehow, though with single crystals its probably diffractive to an extent.

[dlsworks]

well it has a refractive index such that it is tantamount ot being able to lens x-rays..infact one of the only substances known to man

http://66.102.7.104/search?q=cache:mLHq ... rays&hl=en

my only consideration, as I don't think anybody here actually wants to lens x-rays it would be of some minor concern if not a major one if certain arrangments were made convinient for them.....rather, x-rays.