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10.1.1} Possible artefacts

 The multigroup scheme adopted in FLUKA is reliable and much faster than
 any possible approach using continuous cross sections. However, it is
 important to remember that there are two rare situations where the
 group approximation could give bad results.

 One of such situations may occur when each neutron is likely to
 scatter only once (e.g. in a very thin foil) before being scored: an
 artefact then is possible, due to the discrete angular distribution.
 In practice the problem vanishes entirely, however, as soon as there is
 the possibility of two or more scatterings: it must be kept in mind, in
 fact, that after a collision only the polar angle is sampled from a
 discrete distribution, while the azimuthal angle is chosen randomly from
 a uniform distribution. In addition, the 3 discrete angles are different
 for each g --> g' combination and for each element or isotope. Thus, any
 memory of the initial direction is very quickly lost after just a few
 collisions.

 The second possible artefact is not connected with the angular but
 with the energy structure of the cross sections used. 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
 fluence (case of so-called "infinite 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 libraries for a few important elements (Al, Fe, Cu, Au,
 Pb, Bi): 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.

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