Expected to be completed by late 1997 or early 1998, Milagro will be sensitive to gamma-rays in the energy range of 100 GeV (1011 eV) to 100 TeV (1014 eV). Milagro will stare continuously at the sky from horizon to horizon, day and night, acting like a camera whose shutter is always open.
As high energy gamma-rays reach Earth, they strike air molecules in the upper atmosphere. The initial collisions produce showers of sub-atomic particles and lower-energy photons that avalanche groundward as a thin, radially expanding disk. Each air shower either peters out in the atmosphere or, at high elevations, intercepts the ground. Milagro's detectors will sense the arrival of these air showers and record information for reconstructing the point on the sky from which the original gamma-ray came.
Unfortunately, the gamma-ray signals would be hidden within a downpour of cosmic rays, high-speed protons that whip across the galaxy and strike Earth from all directions at all times, generating their own air showers. Because of the cosmic rays, this type of astronomy is akin to doing optical astronomy in the daytime. Milagro is designed to sort out gamma-rays from the "bright" cosmic ray background. Milagro's pool acts as a big collector, a camera lens bigger than a football field. When a gamma-ray generated air shower strikes the pool, charged particles in the shower create a faint trail of light, Cherenkov radiation, as they move through the water.
A cover on the pool will prevent outside light from reaching the water, but energetic particles in the air shower will pass through the cover easily. The faint trail of Cherenkov radiation will stand out to the array of sensitive PMTs submerged about 2 meters below the surface of the dark pool. The pool water will be filtered and purified to keep it crystal clear.
Each trail of light will be seen by different PMTs at slightly different times. By measuring the arrival times of the light at the various PMTs, UCI reseachers will be able to calculate trajectories for the air showers and from those the directions from which the original gamma-rays came from in the sky.
To help distinguish events generated by gamma-rays from those generated by cosmic rays, Milagro will incorporate another array of PMTs placed near the bottom of the 8 meter deep pool. The bottom array of detectors will register Cherenkov light from cosmic ray generated particles called muons; gamma-rays, in general, don't generate muons. If any events are seen simultaneously by the shallow and deep detectors, Milagro will reject those as being created by cosmic rays. Milagro is expected to see about 1,000 cosmic ray events each second and only one or fewer gamma-ray events each second. Sorting through the data and peeling off the events of interest will be one of the challenges the Milagro team faces.
To improve the resolution of their telescope, the Milagro scientists will place scintillation detectors around the pool to intercept more particles from each shower. The scintillation detectors look for a tiny spark of light formed when part of a high-energy air shower penetrates the device and strikes a special plastic in its base. A PMT looks down at the plastic and registers any scintillation light within the detector's pitch-black interior. Including data from the shmoos effectively increases the size of the Milagro telescope, which will in turn increase its resolution, or ability to pinpoint the locations of celestial sources of gamma-rays.
Procurement, testing, and assembling of the 800 PMTs used in the Milagro pond is the responsibility of UCI Professor Gaurang Yodh and Researcher Anthony Shoup. They are assisted by graduate students Scott Hugenberger and Isabel Leonor. Recently, this research team has shipped 250 PMTs to the Fenton Hill site to be installed in the first construction phase called Milagrito. This phase should start taking data in the late Fall of this year.
Milagro will be investigating previously unexplored regions and thus has the potential for exciting new discoveries. No other experiment will be "looking" at the whole sky at these gamma-ray energies. Other experiments have looked at previously "known" sources, but Milagro's wide view of the sky and around-the-clock operation make it the prototype for a future generation of cosmic gamma-ray detectors.