  In two-dimensional electron systems confined to wide AlAs quantum wells,
composite fermions around the filling factor $\nu$ = 3/2 are fully spin
polarized but possess a valley degree of freedom. Here we measure the energy
needed to completely valley polarize these composite fermions as a function of
electron density. Comparing our results to the existing theory, we find overall
good quantitative agreement, but there is an unexpected trend: The measured
composite fermion valley polarization energy, normalized to the Coulomb energy,
decreases with decreasing density.
