alt Hacker News> probably an imaging radiation detector with LIDAR.
Sorry - had to laugh at "LIDAR" - the source bounced off a truck in W.Australian Pilbara and the Eastern States specialists rolled along the road in a van picking up the lost source by direction.
The environment would be best described as flat. Very flat. Road, flat ground. No buildings, no reflections or refraction, not a lot for a LIDAR to do.
I'm still impressed with the direction to source capabilities over what we started with many years ago; crystals stacked with lead plate separating top crystal from lower crystals .. to get a sense of what was cosmic and what was not.
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.