Fast Neutron Transient Detection Hodoscope for Reactor Fuel Meltdown Studies

A hodoscope sensitive to penetrating fast fission neutrons has been designed for the test reactor TREAT. Discrete portions of an imaginary 15 × 23 matrix centered on the fuel plane are scanned by individual fast neutron detector channels. Located outside the reactor core, 13 ft from the test pin, this hodoscope is able to record the movement of fuel with a 0.15-in. horizontal resolution and close to 1-msec time resolution. Instrumentation was optimized with regard to cost. Each neutron signal is translated to a digital positive pulse, several microseconds duration, which gates a 6977 fluorescent vacuum tube into conduction. An array of 334 lamps is photographed by a fast framing camera at 1000 frames/sec, making the readout system, in effect, an inexpensive two-parameter analyzer with a capability approaching 107 cps.