  We calculate the characteristic length scale of a vortex line in a dilute,
superfluid gas of fermionic atoms, and find that it is in general smaller, and
has a weaker density dependence than the BCS coherence length. Taking this into
account, we find the energy of a vortex line to be larger than has previously
been estimated. As a consequence, the critical frequency for formation of
vortices in trapped fermion gases can be significantly larger than earlier
calculations have suggested.
