The DN system was integrated with the DDA to make use of greater sensitivity of delayed neutron count rate to U-235 concentration.  Delayed neutrons are emitted by very short lived fission products.  Delayed neutrons emission and detection lacks the coincident neutrons which are simultaneously emitted and detected for prompt fission neutrons.  Thus, delayed neutrons are counted by the detection system as single neutron events minus any background.  U-235 fission emits approximately 2.6 times as many delayed neutrons as Pu-239, so the signal is more dependent on the U-235 concentration.  The DN system works by repeatedly pulsing with a neutron source and counting a spent fuel assembly to produce a steady emission rate of delayed neutrons.  The amount of delayed neutrons emitted is proportional to the amount of fissile material in the spent fuel.  Due to the shared need for a strong neutron source, DN was integrated with DDA from the beginning of design.  The two He-3 tubes without a surrounding Cd liner in DDA system of Figure 25 are used for delayed neutron counting.  Given the greater dependence of the DN signal on U-235, the combined DN and DDA system may be able to resolve individual quantities for U-235 and Pu-239.

Source: P. Blanc, H.O. Menlove, S.J. Tobin, S. Croft, A. Favalli, "An Integrated Delayed-neutron Differential-Die-away Instrument to Quantify Plutonium in Spent Nuclear Fuel," Journal of Nuclear Materials Management   40:3 (2012).