It was probably an imaging radiation detector regardless, the spinny bit would likely be some sort of camera to add context to the radiation image.

Imaging detectors can get a lot fancier than just two planes. One design is basically segmented crystals and you look at the timing between the separate segments light up as a gamma ray passes through each quadrant. You can then work backwards from those sub nanosecond time difference to get the direction, and stack up a bunch of events to make a sort of picture (more like a blobby heat map).

For neutrons there is an even crazier imaging technique called a coded aperture that I won't even try to describe here. Suffice to say it's very clever and unintuitive.