This is a concept of a gamma generation chamber on a Halo installation. Theoretically there are several of these present along the internal face of the ring, and fire simultaneously towards the center of the ring to create a concentrated horizontal beam of radiation capable of eradicating all life with sufficient biomass to sustain the Flood.
The first 6 photos are from this map, the rest are from other projects I've done previously.
Thank you for looking!
Take a look.
[Edited on 03.06.2012 7:33 PM PST]