The HAWC design builds upon our experience with the Milagro detector. Milagro is the first large, uniformly instrumented, air shower array using water Cherenkov technology. The Milagro pond is instrumented with 2 layers of PMTs, a shallow layer for triggering and shower angle reconstruction and a deep calorimetric layer used for hadron rejection. Surrounding the central pond is an array of plastic, outrigger tanks (1 m deep by 3 m diameter) used for core position and shower angle reconstruction. In contrast, the HAWC design utilizes a single deep layer of PMTs with wider separation than used in Milagro. This configuration gives HAWC a much larger active area than Milagro for the same photo-cathode area.
HAWC will re-use the 900 8" Hamamatsu PMTs from Milagro, and them in a 4.0 m deep by 7.3 m diameter commercial corrugated steel water tank. The tanks will be deployed in a dense pattern that provides more than 70% coverage of the 150m x 115m instrumented area. Each tank will contain three 8" baffled upward-facing PMTs anchored to the bottom.