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TOPIC: second part

dafni

Date: Mar 8, 2008

second part	Permalink

The instruments that the Compton Gamma-Ray Observatory carried measured a wide range of wavelengths within the gamma-ray spectrum. Gamma rays are electromagnetic waves with wavelengths smaller than 1 picometer (pm; one picometer equals 1 trillionth of a meter or 4 billionths of an inch). The Imaging Compton Telescope (COMPTEL) instrument measured the energy and direction of incoming mid-wavelength gamma rays, from about 0.00041 to about 0.0016 pm (about 1.6 x 10^-12 to about 6.24 x 10^-12 in). Scientists in the United States, the former West Germany, and The Netherlands designed and developed COMPTEL. The drum-shaped instrument had two arrays of detectors, one above the other. The double array enabled the car-sized telescope to determine both the energy and the direction of incoming gamma rays. The detector, mounted at the center of the top of the satellite, could see an area of sky about 57° wide by about 57° tall at any one time. The Burst and Transient Source Experiment (BATSE) monitored the sky for short, intense bursts of gamma rays. Designed by scientists in the United States, the BATSE consisted of eight identical modules mounted on the spacecraft's corners. The area of sky that each BATSE detector covered overlapped with that of other BATSE detectors. As a result, gamma rays from the same source could hit more than one detector, which helped pinpoint the location of the source. Each BATSE detector contained two scintillation counters (see Particle Detectors: Scintillation Counter), particle detectors that produce a flash of light when a gamma ray hits the detector. Other devices in the instrument detected and analyzed the light that the scintillation counters produced. The detectors were sensitive to wavelengths from the short-wavelength X-ray range through all of the gamma-ray range. The Oriented Scintillation Spectrometer Experiment (OSSE) determined how much gamma-ray radiation came from a source and how much came from background radiation in the sky around the source. OSSE consisted of four identical instruments in a car-sized triangular box mounted at one end of the satellites topside. Each instrument had an independent pointing system, but the detectors usually acted in pairs. In each pair of detectors, one focused on the gamma-ray source while the other took readings on the level of gamma-ray radiation present in the sky around the sourceknown as background radiation. Having these two readings allowed astronomers to subtract the background radiation level from the total radiation amount, thereby obtaining precise measurements of the radiation from the source. The OSSE often used data from the BATSE detectors to decide where to point its instruments. Like the BATSE detectors, the OSSE detectors used scintillation counters. The wavelength sensitivity of the OSSE detectors was much like that of the BATSE instruments, but the OSSE detectors could provide more information about a source than the BATSE detectors could. The drum-shaped Energetic Gamma Ray Experiment Telescope (EGRET) was on one end of GROs topside. EGRET was a large spark chamber, a particle detector in which a gamma ray ionizes gas to produce an electrical signal. EGRET focused on short-wavelength gamma rays, from about 0.0001 to about 0.04 pm (about 4 x 10^-15 to about 2 x 10^-12 in). EGRET collected enough data to compose a map of the high-energy gamma-ray universe. The telescope could pinpoint a sources location five times more accurately than any previous gamma-ray telescope __________________

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