Increasing Nuetron Count Using Neutron Reflectors?

[Conner Ruhl]

Would using plates if neutron reflecting material surrounding the vacuum chamber increase neutron count at a gap in the plates? Sorry if I am confusing, here is a diagram that better explains my idea: http://i.imgur.com/d2BSb.jpg.

Comments, critisisms, and concerns are invaluable to me.

[Carl Willis]

Yes, every little bit helps. Neutron reflection and moderation is not particularly efficient, so it is still important to get the activation sample as close to the source as possible and choose your moderator materials wisely.

-Carl

[Conner Ruhl]

Would it help if the plates were aimed towards the "activation chamber?"

[Doug Coulter]

So, Carl, what reflects neutrons the best (that I can get? - uranium in quantity is probably not something I can get). Any idea of the scale of improvement? 1%? More? I already have my "active area" coated with 1/8" lead except for a hole for the neutron moderator "oven" for example - mainly to keep the X rays down in the lab. One wonders that with all the isotopes of lead there are if it isn't a net absorber, for example. At any rate, plenty of fast neutrons make it through to operate my hornyak detector outside the lead.

Would you just be going for something high atomic weight that didn't like to absorb neutrons of various speeds?

Interesting he showed fissionable material there. I kind of looked at that idea, but we're just at the threshold for fast fission with 238 (still low probability I think) and there's not much 235 around in what you can get - I'd think net loss there due to resonance capture in the 238 at just over thermal energies.

Not sure we'd want to be breeding even microscopic amounts of Pu, that would create some negative feelings around the regulators, no?

[Richard Hull]

If you are thinking neutron reflection is like a mirror then that is wrong. Efficient neutron specific reflectors...(that term is to be taken with a grain of salt), can be expenisve.

A simple moderator can act as a reflector of sorts, but only in the sense that it keeps neutrons changing direction to remain in the neighborhood, some few going back in the direction they came from. Every little bit helps, but don't expect anything fabulous happening.

Richard Hull

[Conner Ruhl]

I know that they don't operate as mirrors. I was just wanting to know if the slightly increased probablility that the neutrons would hit the moderating nucleai at favorable angle would get them into the right area. Upon reflection it seems to me that the gains would be minuscule compared to the losses from gaps created. Feel free to correct me if I am being nonsensical (which is always highly probable).

[Chris Bradley]

Think of a moderator like a sponge that you inject a gas into the dead centre of.

As the gas goes in, it'll randomise and come out of the surface of the sponge per unit area. So the 'reflection' backwards can never exceed 50% of the input.

If the sponge is too thin, then some of the gas will just shoot straight through the other side of the sponge. If it is too thick, then most of the gas will leak out of the edges rather than come back towards the source. You want the sponge just thick enough that the 'leak-through' is minimised to the 'leak-out'.

[Conner Ruhl]

I knew that neutrons went into a moderator and collided multiple times, slowing down and losing energy in the process. Your analogy helped clarfify what I was thinking and made it easier to understand. Thank you for that!

[Carl Willis]

Hi Doug,

The following paper discussing the neutron albedo of various materials may be the kind of information you are looking for:

http://www.dtic.mil/dtic/tr/fulltext/u2/616613.pdf

About your lead: it's most certainly a net absorber at 2.5 MeV. Actually most lead has antimony in it in various proportions as a hardener. Unless lead is known to be particularly pure, I would regard it as an unknown and potentially significant neutron absorber.

I don't think there's any regulatory interest in amateur production of trace quantities of plutonium. Saner heads all understand that it's a non-issue. When one of us builds a critical graphite pile I suspect there may be more regulatory curiosity in whether that individual's interest is appropriately focused in a non-threatening direction and there will be a big push to make sure that the relevant licensing is sought. Other than that...playground's open.

-Carl

[Doug Coulter]

Thanks, Carl. The linked paper covers low energy neutrons pretty well. Is there an analogous one for 2.5 mev neutrons?

The reason I ask - I am running my cylinder fusor in a 6" by 6" pipe. I have wrapped the 8" flanges at both ends with lead except for a hole I put my "neutron oven" into, and blow forced air through all this to keep from melting the HDPE moderator too much. Most of the area around the pipe has no moderator, since I need the space for the cooling air, so what I am looking for is a way to reflect those fast neuts back up toward the "oven" I activate thing in.

I know in the case of the lead (1/8") - plenty of fast N's get through to run my "fast neutron only" hornyak detector, and to light up all my moderated slow neutron detectors at various ranges, so I'm neither reflecting or stopping all that many with the lead. In fact, a fair amount of gammas get out too - there's more going on than just the power supply energy X rays. I can live with that - square law helps the operator.

I'm guessing that no one has more than a percent or a fraction of one reflecting fast neutrons - particularly at angles near 180 deg. Would that be a good guess?

I didn't see anything like this in "Fast Neutron Physics" or "Progress in fast neutron physics" either.
But who knows - maybe we've learned a trick since the '50s?