Two Active High-Intensity Neutron Detectors

Two neutron detectors were designed to have a high ratio of neutron to gamma ray sensitivity for time resolved measurements of intense neutron pulses. One detector utilized a thin aluminum foil sandwiched between two fission foils in an evacuated chamber. The foils were thick compared to the range of fission fragments. Fission fragments passing through the surfaces created secondary emission electrons, producing a large neutron induced signal compared to the gamma-ray induced response. The second detector utilized a pair of silicon fission fragment detectors exposed to a 235U foil at the opposite end of an evacuated tube. One silicon detector was exposed to both the gamma ray background and the neutron induced fission fragments from the foil, while the second silicon detector was shielded from the fission fragments by a thin aluminum shield, but otherwise was exposed to the same background radiation. Thus, the time resolved neutron signal could be recorded by observing the differential signal from the two silicon detectors with an oscilloscope and camera. The approximate neutron sensitivities of the two detectors were calculated and compared to the experimentally measured values obtained from exposures to thermal and fission spectrum neutrons.