  For understanding an anomalous nuclear effect experimentally observed for the
beryllium-9 nucleus at the Thomas Jefferson National Accelerator Facility
(JLab), clustering aspects are studied in structure functions of deep inelastic
lepton-nucleus scattering by using momentum distributions calculated in
antisymmetrized (or fermionic) molecular dynamics (AMD) and also in a simple
shell model for comparison. According to the AMD, the Be-9 nucleus consists of
two alpha-like clusters with a surrounding neutron. The clustering produces
high-momentum components in nuclear wave functions, which affects nuclear
modifications of the structure functions. We investigated whether clustering
features could appear in the structure function F_2 of Be-9 along with studies
for other light nuclei. We found that nuclear modifications of F_2 are similar
in both AMD and shell models within our simple convolution description although
there are slight differences in Be-9. It indicates that the anomalous Be-9
result should be explained by a different mechanism from the nuclear binding
and Fermi motion. If nuclear-modification slopes d(F_2^A/F_2^D)/dx are shown by
the maximum local densities, the Be-9 anomaly can be explained by the AMD
picture, namely by the clustering structure, whereas it certainly cannot be
described in the simple shell model. This fact suggests that the large nuclear
modification in Be-9 should be explained by large densities in the clusters.
For example, internal nucleon structure could be modified in the high-density
clusters. The clustering aspect of nuclear structure functions is an unexplored
topic which is interesting for future investigations.
