Prompt vs. Delayed NAA

Neutrons with sufficiently long wavelengths can be reflected from some surfaces in the same way as light can be reflected from the interface between two transparent media. Cold neutrons, then, can be guided down cylindrical wave guides without the normal l/r attenuation and can be bent out of the line-of-sight paths followed by other radiation. 

The usual neutron beams from a research reactor are contaminated by fast neutrons and y-rays that originate in the core. Filters, collimators, and shielding can reduce these undesirable components to some extent. However, cold neutron beams have a much lower y- and fast-neutron background. Thus, detectors for capture neutron and basic physics experiments can be placed closer to the sample, increasing sensitivity and making coincidence techniques feasible in many more situations. 

Since guided cold neutron beams provide comparable neutron flux and lower background than thermal neutron beams, the detection limit of neutron absorption experiments, such as PGNAA, is decreased. This decrease in the 

Another major contribution of using cold neutrons is the ability to focus the neutrons to increase the flux on the sample. This will lead to a proportional increase in the reaction rate. Therefore, anal3d ical techniques such as PGNAA which is exclusively dependent on the reaction rate and not on the direction of the incident beam will be enhanced considerably by neutron focusing. 

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