Fast Neutron Flux Measurement Using Gas-Flow Techniques

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).