A detector using a gas-flow technique to measure the in-core fast neutron flux was designed, developed, and experimentally tested. Based on the 9Be(n,α)6He reaction of neutrons with beryllium, the method uses a flowing gas (helium) to bring the 6He beta emitter to an ex-vessel beta detector to achieve measurements independent of the gamma irradiation of the in-vessel neutron detector. In tests at the TRIGA reactor of Washington State University, the system demonstrated a fast neutron sensitivity of 3.7 × 10−10 counts/sec per n/(cm2 sec). Thus, using appropriate counting techniques, the technique could have an operational range of 109 to 1016n/(cm2 sec).