  A search for neutral heavy leptons (NHLs) has been performed using an
instrumented decay channel at the NuTeV (E-815) experiment at Fermilab. The
decay channel was composed of helium bags interspersed with drift chambers, and
was used in conjunction with the NuTeV neutrino detector to search for NHL
decays. The data were examined for NHLs decaying into muonic final states (mu
mu nu, mu e nu, mu pi, and mu rho); no evidence has been found for NHLs in the
0.25 - 2.0 GeV mass range. This analysis places limits on the mixing of NHLs
with standard light neutrinos at a level up to an order of magnitude more
restrictive than previous search limits in this mass range.
