From: Alberto Fasso' (fasso@SLAC.Stanford.EDU)
Date: Wed Feb 11 2004 - 17:51:03 CET
On Wed, 11 Feb 2004, Thomas Otto (TIS-RP, CERN) wrote:
> Dear colleagues,
> there are 3 different types of iron available as regards the low-energy
> neutron transport properties (LOW-MAT command):
> Iron self-shielded
> Iron shield.
> Can you explain what the particularities of these different materials
> are and in which situations one should use them ?
> At the moment I need to estimate shielding for an electron accelerator
> with energies above the neutron production threshold.
> Thank you,
> Thomas Otto
I report here a text which will appear in the next version of the Manual
The group structure is necessarily coarse with respect to the resonance
structure in many materials. A resonance in a material present in a
dilute mixture or as a small piece cannot affect much a smooth neutron
flux (case of so-called "infinitely dilution") but if an isotope is
very pure and is present in large amounts, it can act as a "neutron
sink", causing sharp dips in the neutron spectrum corresponding to
each resonance. This effect, which results in a lower reaction rate
sigma*phi, is called "self-shielding" and is necessarily lost in
the process of cross section averaging over the width of each energy
group, unless a special correction is made. Such corrected cross
section sets with different degrees of self-shielding have been
included in the FLUKA library for a few important elements (Fe, Cu, Pb):
but it is the responsibility of the user to select the set with
the degree of self-shielding most suitable in each different case.
It is worth stressing that non-self-shielded materials are
perfectly adequate in most practical cases, because the presence of even
small amounts of impurities is generally sufficient to smooth out the
effect. On the other hand, in regions of non-resolved resonances
the multigroup approach is known to give very good results anyway.
I hope that the above answers your question.
-- Alberto Fassò SLAC-RP, P.O. Box 4349, MS 48, Stanford CA 94309 Phone: (1 650) 926 4762 Fax: (1 650) 926 3569 firstname.lastname@example.org
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