Cool use of gamma-ray imaging technology! The underlying technology is a coded aperture to give a random response on the detector that can be used to reconstruct the direction of the gamma-ray source. I think they are using a moving mask with a single detector in the middle. They probably didn't know if the source had broken apart and caused more contamination when they found the general area. So the imaging allows you to know from a distance if the source has been distributed without going into a potentially loose contamination area.

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