During the dynamic compression of a subcritical object that is simultaneously receiving a neutron pulse, its fission 𝛾-ray signal can be measured and the die-off in its time-response is directly correlated with peak reactivity and neutron multiplication within. Hence, the signal measured by a time-of-flight (TOF) gamma detector array operating in currentmode is a convolution of the incident neutron pulse, detector time response, and fission signal from the object. Accurate determination of the true fission 𝛾-ray emission rate from the object requires a detector that is both highly sensitive (to preserve statistics) and fast (to minimize distortion of the signal shape from the object). In collaboration with scientists at Mission Support and Test Services (MSTS) and the Nevada National Security Site (NNSS), a TOF 𝛾-ray detection system was designed at Los Alamos National Laboratory (LANL) to meet these experimental objectives. This system consists of a 3-m-diameter array of ∼ 150 hexagonal detector pixels, each operating in current mode. Each pixel consists of a large-volume, fast-plastic scintillator coupled to a 5-in photomultiplier tube via a plastic light guide that uses total internal reflection for optical transport to the photocathode. Development details of this pixel design using statistical and time response metrics, laboratory measurements of full pixels and photomultiplier tubes, and high-fidelity GEANT4 simulations, are given. In closing, fielding considerations and expected performance capabilities for the full detector array are also described.
