Gamma-ray astrophysics is located at a crossroad of cosmology, astronomy, nuclear physics, particle physics, and cosmic ray physics. Astronomical objects have been shining in the gamma-ray band, ever waiting to be discovered by a new and better detector. On the instrumentation side, it was particle physics and cosmic ray physics that have provided new detectors to the field. The early SAS2 and COSB as well as the most recent EGRET instrument detected gamma ray by using a spark chamber, an invention by Drs. Fukui and Miyamoto that revolutionized accelerator experiments in 1960s. Since then many new detector technologies have emerged in particle physics. Among them, the silicon strip detector stands out in the trajectory recorder category: it does not require gas circulation or high voltage supply, it allows micrometer position resolution, and it is read-out by low power compact integrated circuits. The spark chamber technology gave birth to gamma-ray astronomy and the silicon strip detector technology is about to bring it to the front page of astrophysics and particle physics. Replacement of the spark chamber by a stack of silicon strip detectors grants us a much wider field of view and a finer angular resolution. GLAST's field of view covers about 20% of the entire sky, which allows surveying the sky every 90 minutes.

High sensitivity gamma-ray observations by GLAST will offer tremendous opportunity for discovery in cosmology, astronomy, nuclear physics, particle physics, and cosmic ray physics. The most recent instrument, EGRET, has detected 271 point sources and the diffuse Galactic emission. The sources include new classes of gamma-ray blazers, radio-quiet gamma-ray pulsars, and hints of gamma-ray emission from millisecond pulsars, radio galaxies, supernova remnants and X-ray binaries. However, 170 of 271 sources remain still unidentified, implying existence of new classes of gamma-ray emitting objects. The most enigmatic and most exciting phenomenon of present astrophysics is Gamma-Ray Bursts (GRB). The delayed gamma-ray emission from one gamma-ray burst detected by EGRET hints operation of a new powerful particle acceleration mechanism a few hours after the burst. GLAST will have 50-100 times superior sensitivity and allow us to detect 100 times more point sources and observe 20% of all transient phenomena from their onset. It will mark a very important history similar to Einstein Observatory in X-ray astronomy.

The universe is transparent to gamma ray and allows us to reach far back to the early universe and deeper to cores of dense clouds. Interaction of gamma ray with matter and radiation is very well studied in laboratories. Thus gamma ray flux measurement can be used to measure the matter density distribution in our Galaxy and infrared radiation density in early universe. Through these measurements, gamma-ray astronomy will become closely related to radio and infrared astronomy.



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