Dear Bo Yang,
First of all, you want to optimize shielding in the lateral direction or
in the forward direction, or both? These are two very different problems.
Then, yours is an interesting question, but I am not aware of any existing
optimization. You could try it yourself: I am sure that your results would
be very appreciated by many people.
Some information has been given by J.D. Cossairt in his radiation Course
(Fermilab Report TM-1834, p. 160), but for concrete instead of
polyethylene:
The first excited state of 56 Fe, the dominant isotope in natural iron
(92% abundance), is at 847 keV. Due to the inefficiency of the
transfer of energy by means of elastic scattering, neutrons having
kinetic energies above 847 keV in a given spectrum will be
slowed by inelastic scattering to E n ¡Ö 847 keV only to build up at
energies just below this value. Amplifying this effect, when one
considers the dose equivalent external to such shields, is the fact
that the quality factor for neutrons as a function of energy also has its
maximum value at about 500 keV (see Fig. 1.3). Thus, pure iron
shields are rather ineffective in attenuating neutrons in this energy
region.
.....................................................................
In general, an iron shield "capped" or "backed" by ... a concrete
shield will be an efficient use of space. It has been determined that
about 60 cm of concrete is the most efficient thickness to use for
this purpose.
Just three pieces of advice:
1) as warned by P. Neuhold, be careful of the high energy Fe capture gammas
2) be careful of the "window" in the iron cross section explained above
3) remember that the density of iron can vary a lot: from 7.0 to 7.8 g/cm3
Alberto
On Mon, 20 Jul 2015, Ñ wrote:
>
> Hi,every Fluka user
>
> Recently I want to use FLUKA to design a compact shielding for a 200MeV proton loss point. Due to existence
> of high-energy neutron, I try to use iron plus polyethylene as a shielding. But I wonder if there is a
> optimized ratio between iron thickness and polyethylene thickness to effectively shield the high-energy
> neutron. If anybody can give me some suggestion or reference, thanks very much.
>
> Bo yang
>
>
>
>
>
>
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Received on Mon Jul 20 2015 - 19:40:52 CEST