The GLAST Instruments
GLAST will employ two instruments to observe the gamma-ray Universe. The primary instrument is the Large Area Telescope, or LAT. It has a wide field-of-view, allowing it to see large areas of the sky at the same time. It will detect gamma rays with 10 million to 150 billion times the energy of the light detected by the human eye (in technical terms, it detects gamma-ray energies of 20 million electron volts to greater than 300 billion electron volts, where the energy of visible light is roughly 2 electron volts).
The Large Area Telescope (LAT)
The LAT will detect gamma rays by using a technique known as pair-conversion. When a gamma ray slams into a layer of tungsten in the detector, it creates a pair of subatomic particles (an electron and its anti-matter counterpart, a positron). These particles in turn hit another, deeper layer of tungsten, each creating further particles and so on. The direction of the incoming gamma ray is determined by tracking the direction of these cascading particles back to their source using high-precision silicon detectors. Furthermore, a separate detector counts up the total energy of all the particles created. Since the total energy of the particles created depends on the energy of the original gamma ray, counting up the total energy determines the energy of that gamma ray. In this way, GLAST will be able to make gamma-ray images of astronomical objects, while also determining the energy for each detected gamma ray.
» Learn More
The GLAST Burst Monitor (GBM)
The secondary instrument onboard is the GLAST Burst Monitor, or GBM. The GBM is designed to observe gamma ray bursts, which are sudden, brief flashes of gamma rays that occur about once a day at random positions in the sky. These bursts are still a mystery to astronomers; no one knows what causes them, or what physical forces are at work. All that is known is that they are among the most powerful explosions in the Universe. The GBM has such a large field-of-view that it will be able to see bursts from over 2/3 of the sky at one time, providing locations for follow-up observations of these enigmatic explosions. The GBM is composed of two sets of detectors - 12 sodium iodide (NaI) scintillators and two cylindrical bismuth germanate (BGO) detectors. When gamma rays interact with these crystalline detectors, they produce flashes of visible light, which the detector can use to locate the gamma-ray burst on the sky. The GBM works at a lower energy range than the LAT, so together they provide the widest range of energy detection in the gamma-ray regime for any satellite ever built.
» Learn More