Theory of Oscillating Absorber in a Chain Reactor

The response of a critical chain-reacting pile to a thermal neutron absorber which is oscillated back and forth inside the pile is calculated. It is shown that at frequencies which are low compared to the periods of delayed neutrons, the neutron intensity in the pile rises and falls as a whole, the shape of the stationary distribution always being maintained. As the frequency of the oscillation increases, the nature of the neutron response changes from the over-all fluctuation characteristic at low frequency to a propagated and attenuated neutron wave which emanates from the neighborhood of the oscillator. These neutron waves set up at high frequencies are entirely analogous to the familiar thermal waves which are established in Angstrom's method of measuring thermal conductivities. It is pointed out that since the pile equations are linear, the amplitude of the oscillating response is proportional to the total neutron absorption of the oscillated absorber, and therefore a known and an unknown absorber can be compared by observing the magnitude of the neutron oscillations caused by each.