- Neutron interrogation is one way of measuring quantities of fissile
material and is useful for rapid and routine measurement.
- The sample (1 m diameter drum) is irradiated by a pulse of fast
neutrons from a neutron generator. These fission neutrons are
moderated, scattered into the sample and stimulate fission in any
fissile material.
- Fast neutrons released by fission are detected by cadmium shielded 3He
detectors which are placed in polyethylene around the sample
positions. Thermalised neutrons then scatter into the sample and
initiate fission.
- The initial interrogating pulse will not have disappeared by time C,
but the newly born fission neutrons die away over a longer time scale
of a couple of milliseconds. Thus by counting fast response neutrons
in the time slot C - D we obtain a signal that is proportional to the
quantity of fissile material present.
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