Since atoms of solids are relatively immobile within
the material, the mechanism for signal generation by
these materials is dierent from that of an air-ioniz-
ation meter. In solids, the energy levels of outer, or
valence, electrons of an atom cannot be associated
with a single atom or molecule but are associated with
the matrix of atoms and molecules that make up the
solid. The energy levels of these outer electrons tend to
organize themselves into bands of energy levels separ-
ated by gaps or ``forbidden'' zones