Figure 25. Schematic cross-sectional view of the DDA instrument 

The DDA method uses the difference in die-away (or decay) time between epithermal (medium energy) and thermal (low energy) neutrons to characterize the amount of fissile material in spent nuclear fuel assemblies.  DDA uses a neutron generator to induce fissions in the spent fuel.  He-3 tubes in polyethylene moderator detect neutrons.  A cadmium liner around the polyethylene is used for some of He-3 tubes to absorb thermal neutrons and make the Cd-lined He-3 positions sensitive to only fast (high energy) and epi-thermal neutrons.   Without the presence of fissile material in the spent fuel, epithermal and fast neutrons die-away very quickly while thermal neutrons die-away more slowly.  As the amount of fissile material and neutron production from fission increases, the die-away time of epi-thermal neutrons gets closer to that of the thermal neutron population.  The shift in the epi-thermal neutron population die-away time is proportional to that amount of fissile material in the fuel. Figure 25 shows a diagram of the DDA system.

Source: V. Henzl, M.T. Swinhoe, S.J. Tobin, H.O. Menlove, Measurement of the Multiplication of a Spent Fuel Assembly with the Differential Die-away Method Within the Scope of the Next Generation Safeguards Initiative Spent Fuel Project," Journal of Nuclear Materials Management 40:3 (2012).