Re: problem with neutron shielding with borated polyethylene

Dear En-Min,

borated polyethylene is a good shielding for thermal and epithermal neutrons,
but if you inject 2 GeV electrons on borated polyethylene, you don't get
immediately thermal neutrons. You get gammas, which produce neutrons
of about 2 MeV (and the number of neutrons produced is not the same if
the polyethylene is 5% borated or 30% borated!) These neutrons are partly
escaping, and partly slowed down until some of them are becoming thermal
and are captured by the boron. It is a complex situation: therefore, it is not
at all obvious that the total effect would be to reduce the neutron
fluence. It depends also on the size of the polyethylene shield.

Such a complex situation is difficult to assess with simple considerations:
and that is the reason you are using Monte Carlo!In-Reply-To: <201112200743.pBK7hbDn027370_at_smtp1.mi.infn.it>
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Try some modifications on your problem: for instance let the electron beam hit
a target instead of the polyethylene, so that the neutron production happens
outside the polyethylene, and the latter acts only as a shield and not
also as a neutron-producing target.

Alberto

On Mon, 19 Dec 2011, b97901029_at_ntu.edu.tw wrote:

> Dear FLUKA experts,
>
> I found some problems when I was trying to shield neutron
> generated from electron beam by 5% borated polyethylene and 30%
> borated polyethylene. The electron beam is injected in the borated
> polyethylene. I had plotted the spread of neutron fluence, but much to
> my surprise, the spread of neutron in 30% borated polyethylene is
> larger than that in 5% borated polyethylene. Does that make sense? I
> think 30% borated polyethylene should have better shielding effect and
> can't explain what's wrong with the result. I would really appreciate
> if someone can tell me what's wrong with the result.
>
> Attached are the input files and the neutron fluence plots.
>
> Thanks in advance,
> En-Min
Received on Wed Dec 21 2011 - 13:58:42 CET