August 08 Issue - Employee Monthly Magazine

Milagro retires

Decommissioned components
to be recycled for new project

A diver prepares to retrieve photomultiplier tubes from the depths of the 5-million-gallon water reservoir at the Laboratory's Milagro facility at Fenton Hill. Photo by Sandra Valdez,

Miracles are hard to come by. And this "miracle" —milagro in Spanish —is no exception. Tucked away in the heart of the Jemez Mountains, accessible only by an unmarked dirt road, is the Lab's very own "Milagro": a facility where, until earlier this year, scientists mapped the galaxy at tera-electron volt (TeV) energies, revealed gamma-ray sources, and discovered possible acceleration sites of cosmic rays.

After seven years, the Milagro experiment ceased operation. Its results have been truly miraculous. By discovering new sources of TeV and charting the Northern Hemisphere, Milagro helped usher in a new era for ground-based gamma-ray astrophysics and inspired further research by demonstrating that many more yet undiscovered sources of gamma rays exist.

"Cosmic and gamma rays are always raining down on the Earth, but most of the time, we don't know from where," said Jim Linnemann, a professor at the Department of Physics and Astronomy at Michigan State University and one of many scientists from around the nation and world who have worked at Milagro.

"Milagro has allowed us to study extreme phenomena in the universe," said Brenda Dingus of Neutron Science and Technology, who has been involved in the project since 1997.

The core of the Milagro experiment was formed by an extensive air shower (EAS) detector that "saw" the universe at high energies. The particles of light, called gamma-ray photons, that Milagro perceived are about 1 trillion times more energetic than visible light. While these photons are similar to ones making up visible light, they behave differently because of their higher energy levels.

Milagro pioneered water-Cherenkov technology to detect these particles. This technique involves placing a series of light-sensitive photomultiplier tubes (PMT) in a large volume of water, the detector. When a high-energy photon strikes Earth's atmosphere, it causes a particle cascade, which in turn, results in a scatter of secondary particles. When these secondary particles enter the detector, they emit light that is sensed by the PMTs. Scientists are able to deduce the direction of the original particle by recording the location and time each PMT was struck, allowing them to continuously monitor the sky for gamma-ray emission of energies up to about 100 TeV.

Milagro was unique in that it was sensitive over its entire area. Traditionally, EAS arrays had been composed of isolated sets of small detectors, spread over large areas, which were typically active over only 1 percent of the enclosed area and unable to detect gamma rays from astrophysical sources. In addition, Milagro was able to sense photons at ground level because the water covering the tubes was of sufficient depth to convert gamma rays to detectable charged particles.

The Milagro detector, operated by the Laboratory in partnership with the National Science Foundation and the Department of Energy Office of Science, used a central, covered reservoir holding about 5 million gallons of water. The reservoir was outfitted with 728 PMTs arranged in two layers. One hundred seventy-six outrigger water tanks, each outfitted with one tube, surrounded the central reservoir and let scientists identify air showers over a larger area, as well as accurately pinpoint the location of the shower core.

Having fulfilled its mission, Milagro is slowly and carefully being dismantled. In June, the tarp covering the huge pond was lifted from the water by air-blowing fans, allowing researchers and divers to enter the resulting bubble chamber. Workers piloting small dinghies in the near blackness probed the reservoir's depths to cut free and retrieve hundreds of dimly glowing photomultiplier tubes.

Removal of the photomultiplier tubes from the pond has been completed, although PMTs still are being retrieved from the outrigger tanks. The reservoir's vast supply of water, which Dingus said is constantly monitored for cleanliness and purity, is being put to good use. On-site personnel are using the water, dispersed through enormous hoses and sprinklers, to gently dampen the surrounding forest area, rendering it less vulnerable to potential fire hazards. Dingus added that final disposition of the pond is, as yet, unclear.

The facility's electronics and data acquisition system currently is being recalibrated and upgraded at the Los Alamos Neutron Science Center. Together with the PMTs and other components salvaged from Milagro, it will be used in a new joint United States–Mexico experiment: the High Altitude Water Cherenkov (HAWC) telescope located at Volcàn Sierra Negra, Mexico, near the site of the Large Millimeter Telescope.

"The first $1 million have just been approved [for HAWC]," Dingus said.

The scientist explained that, at about 13,450 feet above sea level (compared with Milagro's 8,530-foot elevation) and with a larger detector, HAWC will be about 15 times more sensitive than Milagro.

—Tatjana Rosev