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LAT Going to Give us a Broad Representation of Evolution of Objects that are the Highest Energy Accelerators in Universe
:: 27 March, 2008
Deciphering the genetic code of the universe is no easy task. Yet that's just what the Gamma-ray Large Area Space Telescope's Large Area Telescope (LAT) seeks to accomplish. Integrated at SLAC in 2005 and 2006 from hardware fabricated at laboratories all around the world, the LAT will use its 880,000 silicon strips to detect high-energy gamma rays with unprecedented resolution and sensitivity, filling in gaps in understanding left by previous missions and pushing new boundaries in particle physics and astrophysics.
"The LAT will give us a broad representation of the evolution of objects that are the highest energy accelerators in the universe," said SLAC Professor Elliott Bloom.
As GLAST orbits the earth, gamma rays—emanating from jets of plasma streaming from enormous black holes, pulsars and other astronomical sources—will strike the LAT. By determining the time of each gamma ray's arrival, the direction from where it came and energy it carries—the fundamental quantities of astronomy—the LAT will yield a wealth of new data that will offer a glimpse into the fundamental nature of high-energy processes in the universe.
Gamma rays that encounter the LAT will first meet several layers of tungsten metal. Tungsten's massive and highly charged atomic nuclei interact with the high-energy gamma ray in a way that creates a charged pair of particles: one electron and one positron. These particles travel in V-shaped trajectories, with the electron going one way and the positron going another, which are detected by the silicon-strip sensors positioned just below each tungsten layer. Later, these signals are reconstructed by algorithms to obtain the direction and time of the original gamma ray photon.
After traversing through tracking layers, the particles pass into a cesium iodide imaging calorimeter and generate tiny amounts of light—flashes with brightness proportional to the particles' energies.
Through this multi-step process, the LAT will detect gamma rays with unprecedented sensitivity, which will allow detection of thousands of new sources and possibly even new classes of sources.
In the mid-1990s, the instrument's predecessor, the Energetic Gamma Ray Experiment Telescope (EGRET), made the first sky survey in high-energy gamma rays with a sensitivity up to a few giga-electron-volts (GeV) of energy. Not only will the LAT increase that energy range to 300 GeV, it also will provide vastly more sensitivity for faint sources in the universe. An auxiliary instrument, the GLAST Burst Monitor, will scrutinize lower energies for the GLAST mission.
The LAT must be launched into space because the gamma rays that it is designed to detect are blocked by the Earth's atmosphere. The problem of detecting relatively rare gamma rays in the midst of a continual cacophony of cosmic rays is reduced by an additional detector surrounding the LAT’s 16 tracker towers and calorimeters, and then left to sophisticated data analysis, similar to that found in high-energy physics accelerator experiments. The first round of analysis will be performed by flight software on the LAT, which filters out about 80 percent of background signals. This will ensure that fainter sources will stand out more cleanly against the thousands of signals expected every second and will reduce the volume of data sent back to earth. Further, more detailed, analysis will then be performed in ground processing of the LAT data, which will be delivered to SLAC several times per day during the mission.
"When you create an instrument that goes a factor of 100 beyond what you've done previously," said University of California-Santa Cruz Professor Bill Atwood, "the space for discovery becomes enormous."
About Gamma-ray Large Area Space Telescope
The Gamma-ray Large Area Space Telescope, or GLAST, is a future space-based gamma-ray telescope, designed to explore the high-energy Universe. It will study astrophysical and cosmological phenomena such as active galactic nuclei, pulsars, other high-energy sources, and dark matter. GLAST is a joint venture of NASA, the United States Department of Energy, and government agencies in France, Germany, Italy, Japan, and Sweden.
On February 8, 2008 NASA announced it was seeking suggestions for a new name for GLAST that, "Will capture the excitement of GLAST's mission and call attention to gamma-ray and high-energy astronomy."
GLAST includes two scientific instruments, the Large Area Telescope (LAT) and the GLAST Burst Monitor (GBM). The LAT is an imaging gamma-ray detector which detects photons with energy from about 30 million electron volts (MeV) to 300 billion electron volts (GeV). The GBM consists of 14 scintillation detectors which detect bursts of photons from 8 thousand electron volts (keV) to 30 MeV.
General Dynamics Advanced Information Systems (formerly Spectrum Astro) in Gilbert, Arizona built the spacecraft that will carry the instruments. It will travel in a low, circular orbit with a period of about 95 minutes. Its normal mode of operation will maintain its orientation so that the instruments will look away from the earth, with a "rocking" motion to equalize the coverage of the sky. The view of the instruments will sweep out across most of the sky about 16 times per day. The spacecraft can also maintain an orientation that points to a chosen target.
The construction of both instruments is complete. They have undergone environmental testing, being subjected to vibration, vacuum, and high and low temperatures to ensure that they can withstand the stresses of launch and continue to operate in space. They were integrated with the spacecraft at the General Dynamics facility in Gilbert, Arizona.
Data from the instruments will be available to the public through the GLAST Science Support Center web site. Software for analyzing the data will also be available. Scientists with plans for research will be able to apply to the Guest Investigator program.
On 7 February 2008, NASA's Alan Stern, associate administrator for Science at NASA Headquarters, launched a public competition, closing 31 March 2008, to re-name GLAST in a way that would "capture the excitement of GLAST’s mission and call attention to gamma-ray and high-energy astronomy... something memorable to commemorate this spectacular new astronomy mission... a name that is catchy, easy to say and will help make the satellite and its mission a topic of dinner table and classroom discussion."
In figure, Earlier this month, GLAST arrived at the Astrotech payload processing facility near the Kennedy Space Center to begin final preparations for its May 2008 launch.
About Stanford Linear Accelerator Center
The Stanford Linear Accelerator Center (SLAC) is a United States Department of Energy National Laboratory operated by Stanford University under the programmatic direction of the U.S. Department of Energy Office of Science. The SLAC research program centers on experimental and theoretical research in elementary particle physics using electron beams and a broad program of research in atomic and solid-state physics, chemistry, biology, and medicine using synchrotron radiation. The 3.2-kilometer (2.0-mile) long underground accelerator is the longest linear accelerator in the world, and is claimed to be "the world's straightest object." SLAC's meeting facilities also provided a venue for the homebrew computer club and other pioneers of the 1980s home computer revolution, and later SLAC hosted the first webpage in the U.S. The above-ground klystron gallery atop the beamline is the longest building in the United States.
Founded in 1962, the facility is located on 1.72 square-kilometers (426 acres) of Stanford University-owned land on Sand Hill Road in Menlo Park, California—just west from the University's main campus. The main accelerator, a 3.2-kilometer-long RF linear accelerator, which can accelerate electrons and positrons up to 50 GeV, has been operational since 1966. It is buried 10 metres (30 feet) below ground and passes underneath Interstate 280. As of 2005, SLAC employs over 1,000 people, some 150 of which are physicists with doctorate degrees, and serves over 3,000 visiting researchers yearly, operating particle accelerators for high-energy physics and the Stanford Synchrotron Radiation Laboratory (SSRL) for synchrotron light radiation research.
