Calibration and testing of a large-area fast-neutron directional detector

We have developed a new directional fast-neutron detector based on double proton recoil in two separated planes of plastic scintillators with continuous position-sensitive readout in one of two dimensions. This method allows the energy spectrum of the neutrons to be measured by a combination of peak amplitude in the first plane and time of flight to the second plane. The planes are made up of 100-cm long, 10-cm high paddles with photomultipliers at both ends, so that the location of an event along the paddle can be estimated from the time delay between the optical pulses detected at the two ends. The direction of the scattered neutron can be estimated from the locations of two time-correlated events in the two planes, and the energy lost in the first scattering event can be estimated from the pulse amplitude in the first plane. The direction of the incident neutron can then be determined to lie on a cone whose angle is determined by the kinematic equations. The superposition of many such cones generates an image that indicates the presence of a localized source. Setting upper and lower limits on time of flight and energy allows discrimination between gamma rays, muons and neutrons. Monte Carlo simulations were performed to determine factors affecting the expected angular resolution and efficiency. These models show that this design has a lower energy limit for useful directional events at about 250 keV, because lower energy neutrons are likely to scatter more than once in the first plane.