  We have measured the neutron spin asymmetry $A_1^n$ with high precision at
three kinematics in the deep inelastic region at $x=0.33$, 0.47 and 0.60, and
$Q^2=2.7$, 3.5 and 4.8 (GeV/c)$^2$, respectively. Our results unambiguously
show, for the first time, that $A_1^n$ crosses zero around $x=0.47$ and becomes
significantly positive at $x=0.60$. Combined with the world proton data,
polarized quark distributions were extracted. Our results, in general, agree
with relativistic constituent quark models and with perturbative quantum
chromodynamics (pQCD) analyses based on the earlier data. However they deviate
from pQCD predictions based on hadron helicity conservation.
