NRL's Large Area Telescope Launches Aboard GLAST

6/30/2008 - 44-08r
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The Naval Research Laboratory's Large Area Telescope (LAT) successfully launched aboard NASA's Gamma-ray Large Areas Space Telescope (GLAST) Observatory on June 11. NRL is a member of an international consortium led by Stanford University that was selected by NASA to develop the primary instrument for GLAST, a major space mission to study high-energy astrophysics. GLAST is the successor to the Energetic Gamma-Ray Experiment Telescope (EGRET) experiment on NASA's Compton Gamma Ray Observatory (CGRO), which ended its mission in 2000 after many years of successful observations. GLAST will be 50 times more sensitive than EGRET and will detect gamma rays at high energies ranging from 20 million to 300 billion electron volts. The study of gamma rays is expected to provide essential insight into physical processes driving celestial objects.

Gamma rays are the most penetrating type of electromagnetic radiation, and allow a direct view of the high-energy processes acting in the innermost regions of cosmic accelerators such as pulsars, black holes, and supernova remnants. GLAST will precisely locate hundreds of mysterious gamma-ray bursts, recently shown to be the most powerful and distant explosions in the Universe since the Big Bang. It will be sensitive enough to detect several dozen gamma-ray pulsars, which, scientists say, will reveal new details about the life cycle of massive stars in our Galaxy.

The Large Area Telescope (LAT)

The GLAST Mission carries two scientific instruments: the Large Area Telescope (LAT) and the Gamma Ray Burst Monitor (GBM). The 3000 kg LAT was assembled at the Stanford Linear Accelerator Center (SLAC), but with substantial hardware contributions from partners in France, Italy, Japan, Sweden, and the U.S. SLAC also manages the collaboration. The Principal Investigator is Peter Michelson of Stanford University/SLAC. The U.S. support for LAT is a collaboration between NASA and the U.S. Department of Energy.

The main GLAST instrument, the Large Area Telescope (LAT), is a wide field-of-view imaging telescope, which consists of a tracker that determines the trajectory of the gamma ray being measured, and the NRL-developed cesium-iodide calorimeter. A charged-particle anti-coincidence shield helps filter out unwanted signals, such as those produced by background particles.

Once a gamma ray penetrates the shield, it interacts in the tracker and is converted into electron-positron pairs (matter and antimatter) that multiply and cascade into a "shower" of charged particles and photons (also known as an electromagnetic shower). The calorimeter collects and measures the energy from these showers to determine how much energy is in each gamma ray.

The passage of these particles through the cesium-iodide crystals produce flashes of scintillation light that are photoelectrically converted to voltages, or electrical signals. These signals are then digitized, recorded and relayed to Earth by the spacecraft's onboard computer and telemetry system. Cesium-iodide blocks are arranged in two perpendicular directions, to provide additional positional information about the shower.

The accuracy of the energy measurement is largely determined by the calorimeter's ability to collect all the energy from the shower of particles created by the interaction of the gamma ray with the GLAST instrument. The penetrating nature of these particles requires a massive calorimeter to stop or capture them. In GLAST, the calorimeter is about 50% of the weight of the experiment. GLAST uses 1350 kg of thallium-doped cesium iodide, CsI(Tl), scintillation crystals to form the calorimeter. These crystals, known for their durability and high resistance to thermal and mechanical shock, look much like rock salt, but are polished like lead glass and emit light when gamma rays or charged particles interact with them. GLAST uses over 1500 CsI crystals.

NRL's Role in GLAST Mission

NRL's Space Science Division (SSD) has played a leading role in the development of the GLAST mission for over 13 years when early concepts for the Large Area Telescope (LAT) instrument were first designed and tested. Under the leadership of Dr. W. Neil Johnson in SSD's High Energy Space Environment Branch, NRL led the team from U.S., French and Swedish institutions in the design and manufacture of the calorimeter subsystem for the LAT. The calorimeter's principal job is to measure the energy of the gamma rays captured by LAT, but it is also important in identifying and removing background radiation. NRL completed the delivery of the calorimeter detectors to Stanford Linear Accelerator Center (SLAC) in May 2005 where they would be integrated with other components into the LAT instrument.

In May 2006 the entire LAT instrument came to NRL's Spacecraft Engineering Department in the Naval Center for Space Technology for the final environmental testing prior to the delivery to NASA's spacecraft contractor, General Dynamics-Advanced Information Systems (GD-AIS). The 4-month test program was executed on schedule, and LAT was delivered to GD-AIS in September 2006.

As integration and test of the GLAST Observatory proceeded at GD-AIS, it became apparent that the use of thermal vacuum test facilities at GD-AIS by another program would potentially delay the launch of GLAST by several months. Consequently, in August 2007 NASA's GSFC asked NRL to support the Observatory thermal vacuum testing in NRL's Spacecraft Engineering Department's "Big Blue" chamber. The late decision to move the testing to NRL was a significant challenge for everyone at NRL. However, the 3-month test was completed in March, 2008, and the Observatory departed NRL for the Kennedy Space Center. Both NASA and General Dynamics have expressed their gratitude to NRL for this exceptional effort.

The NRL SSD team of scientists, along with the rest of the international science team, eagerly await the completion of the post-launch checkout and wealth of scientific discoveries which are expected in the 5 - 10 years of scientific observations.

Figure 1: GLAST launch.
Credit: NASA/Sandra Joseph, Kevin O'Connel

Figure 2: GLAST launch
Credit: United Launch Alliance/Carleton Bailie

Figure 3: The first half of the payload fairing is moved into place around NASA's 4,000-kg GLAST Observatory. The LAT instrument is the 3,000-kg silver box on the top of the Observatory. The fairing is a molded structure that fits flush with the outside surface of the Delta II upper stage booster and forms an aerodynamically smooth nose cone, protecting the spacecraft during launch and ascent.
Credit: NASA/Jim Grossman

Figure 4: The GLAST Observatory during integration and test at General Dynamics. The LAT instrument is on top with its silver thermal radiators along two of the sides. Part of the GBM instrument is visible as the array of cylindrical detectors above the gowned technician.
Credit: NASA/General Dynamics

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