It's a timing thing - Australia used a "we built it ourselves" detector with shielding that rotated about the crystal pack to find that missing mining source that hit the news last month.
The detector part (doped crystals + scintillation sensors) is fundamentally undirectional - those pesky gamma come in from any direction.
One side has to be shielded to bias the reading (use Tungsten perhaps) .. and by rotating the entire satellite the shielding direction changes - you now have a bulk data stats analysis challenge, do particular orientations align with counts (at various energy levels) rising or falling.
As alway, have a fiddle with a ground based setup first.
