  NMR measurements of the electron spin polarization have been performed on a
2D electron system at and around half-filled lowest Landau level. Comparing the
magnetic field and the temperature dependence of the spin polarization to
models of free and interacting composite fermions (CF), we confirm the
existence of a Fermi sea and show that residual interactions are important.
Independent measurements of the CF effective mass, g-factor and Fermi energy
are obtained from the thermal activation of the spin polarization in tilted
fields. The filling factor dependence of the spin polarization for $2/5 < \nu <
2/3$ reveals a broken particle-hole symmetry for the partially polarized CF
Fermi sea.
