noise or background levels are greatly reduced due to the sub nanosecond timing resolution of the noble liquid detectors rapid imaging for snm 50 can take place by simply assuming that the center of a struck panel is the terminus of the gamma a series of one nanosecond separated arcs extending from each of the struck detectors 533 544 555 and 566 form a grid given the relative timing of the detectors a series of grid lines are formed the intersection of which locates the snm as illustrated by fig5 random coincidences will uniformly populate the cargo volume where as snm will appear at a single point within the volume the snm may be located within voxels of one cubic foot 0 035 m 3 or larger or smaller within the shipping container operationally requiring a 4 panel coincidence does reduce the signal from fissile materials in comparison to a 3 panel coincidence however the nuclear cross section for thermal induced fission is hundreds of times larger than the nuclear cross section for fast neutron induced fission making this mode of operation feasible the combination of fast and slow neutron modes allows for the differentiation of u 235 from u 238 both u 235 and u 238 have similar cross sections at 14 mev 2 barn and 1 barn respectively the fast neutron mode will yield a signal if either u 235 or u 238 is present on the other hand.