Calibration of the HAWC observatory
Abstract
The HAWC (High AltitudeWater Cherenkov) observatory is a 2nd generation, high-sensitivity gamma ray observatory building on experience from its predecessor, the Milagro observatory. In both observatories, water-cherenkov photons produced by air shower particles are detected and their arrival times at the photo multipliers are used to reconstruct the shower plane and direction. In HAWC, 300 7.3 meter diameter × 4.5 meter deep water tanks are each filled with water and placed in close proximity (∼1 m separation). Each tank has 3 upward-looking, 8"-diameter PMTs positioned ∼0.5 meters from the bottom of the tank. The reconstruction of the initial gamma ray direction is highly dependent on the relative time differences between different tanks and PMT channels. In this paper, a calibration system is presented, modeled on the Milagro calibration system, using a pulsing laser with 300 ps long pulses. Pulses are directed into fiber optic cables, which are split and directed to 1/15th of the tanks in the array at a time. For measuring PMT slewing times, neutral density filters will provide a range of laser intensities across 6 orders of magnitude. This system is planned to run autonomously in two different modes, for slow(∼5 Hz) laser pulsing at one light intensity, and for fast (∼200 Hz) laser pulsing across several different intensities. We will report on recent studies made in Spring 2011 using the HAWC prototype water Cherenkov detector at Colorado State University to evaluate the performance of the calibration system.