A gas recoil fast neutron spectrometer

Abstract A gas recoil fast neutron spectrometer of thick radiator type was constructed and its performances were investigated. The spectrometer was composed of three methane-filled proportional counters arranged longitudinally with one common center wire. An insulator disk with many holes was inserted between counters A and B as a proton collimator. Signals from the cathodes of counters A and B were used as coincidence signals, while signals from counter C were used as anti-coincidence signals. Center wire signals give the neutron spectrum to be measured. No summing circuit was required, because the center wire produces sum signals of counters A and B directly. Energy resolution is estimated by the collimator opening and higher resolution is obtainable by the sacrifice of efficiency. The collimator with 2 mm dia. holes was prepared corresponding to 10% resolution. The spectrometer is expected to have a high efficiency because it has many collimator holes (800–1000) and one collimating hole forms one counter telescope of the ordinary case. Higher transparency of the collimator also means higher counter efficiency. The collimator has 38% transparency. Efficiency and pulse height distribution of the spectrometer were studied experimentally and theoretically. In the point of view of coincidence technique, the spectrometer has a possibility of use of faster coincidence than usual because of its geometrical condition. In the spectrometer, signals A and B to be coincident have nearly the same time jitter and rise time, because in such a case proton tracks in counters A and B are located at nearly the same distance from the center wire.