  Using a small amount of radioactive 111Cd atoms implanted in cerium lattice
to probe the hyperfine electric quadrupole interaction, we have detected a
quadrupole electron charge density component in alpha-Ce, which is absent in
gamma-Ce. The appearance of the quadrupole density in alpha-Ce predicted by the
quadrupole scenario of the phase transition demonstrates that the spacial
symmetry is lowered at the gamma->alpha phase transition. This finding makes
cerium the first element where the symmetry change is driven exclusively by the
valence electron degrees of freedom while the atomic centers of mass (cerium
nuclei) occupy the face centered cubic positions. We also report on nuclear
quadrupole interactions in other high pressure phases of cerium: alpha' (C2/m
space symmetry) and alpha' (alpha-U structure).
