Abstract: A prompt responding, coaxial, self-powered neutron flux detector assembly is provided wherein the emitter electrode is at least of the order of 0.9 mm in diameter and is of metal selected from iron, nickel, titanium, chromium or alloys based on these elements so that at least 90% of the total electrical current induced in the detector is prompt and will be derived from (n,.gamma.,e) interactions in said metal, the tubular collector electrode is of one of these materials with zirconium or zirconium alloys included in the group and in use these electrodes are connected directly to the measuring instrument or are connected to the measuring instrument via a lead cable. When a lead is used having conductors of similar metals to the detector electrodes, then for(i) a coaxial lead cable the emitter electrode diameter is .gtorsim.