OSSE provides sensitive spectral measurements of individual gamma-ray point sources.

OSSE - Complementing BATSE is the Oriented Scintillator Spectrometer Experiment (OSSE) which provides the capability of detecting sources in the band from 50 keV to about 10 MeV. OSSE is composed of four independent modules, each a collimated scintillator. Collimation refers to the practice of placing a baffle on the detector to reduce the field-of-view.

OSSE

This allows the detector to concentrate on a single source. The independent modules of OSSE can also view a background region simultaneously for comparison with the source region. This comparison allows OSSE to detect sources which are much weaker than those routinely observable with BATSE. In addition, OSSE provides very good spectral capabilities over its entire energy range. This allows for sensitive searches for features which are important diagnostics of source environments. These spectral features are the primary science objectives for OSSE. There are many types of features OSSE can look for. Gamma-ray lines, which are enhancements in the numbers of photons detected around some characteristic energy,

are the main type. An example of such a line is the 511 keV line associated with the pair annihilation of electrons and positrons (the electron's antiparticle). Other types include lines from nuclear decays or interactions, and cyclotron lines which are caused by the effect of very strong magnetic fields on gamma-ray emitting particles. OSSE is sensitive to other spectral features such as changes in the continuum emission from gamma-ray sources. Most gamma-ray sources have a smooth distribution of strength of emission (flux) versus energy. By finding variations in this distribution, a great deal is learned about the source environment. OSSE has the capability to identify such features which in turn are crucial to understanding the basic physics involved.

The COMPTEL instrument has provided the first comprehensive study of the universe in the 1 - 30 MeV energy range.

Unlike BATSE and OSSE, the Imaging Compton Telescope - COMPTEL - is an imaging detector.

This experiment allows scientists to make images of the gamma- ray sky,

COMPTEL

albeit at a reduced resolution as compared to optical astronomy, over the energy range from about 1 to 30 MeV. This experiment employs a novel design where gamma rays are Compton scattered in the top detector and absorbed in the lower detector. By tracing backwards the path of the scattered gamma ray from top to bottom and measuring the total energy deposited in the detectors, the location of the cosmic gamma ray on the sky is found to lie on a circle and not at a point as seen by a traditional detector. The intersection of "event circles" from many photons determines the location of the gamma-ray source in the sky. The COMPTEL energy range is important to nuclear astrophysics. Nuclear reactions occur during cataclysmic

events such as novae, which are nuclear explosions on the surface of a white dwarf or other compact star, or supernovae, which are totally catastrophic stellar explosions. During these events, nuclear reactions fuse together the light elements of helium, carbon, nitrogen, and oxygen into the heavier elements of aluminum, nickel, and iron while releasing gamma rays in the 0.5 - 5 MeV energy range. Many of these heavier elements are then ejected into interstellar space as radioactive isotopes, which then decay into stable isotopes releasing neutrons and gamma rays. This energy range has historically been a very difficult one to work in. COMPTEL does a good job of bridging the gap between more traditional detectors like OSSE and EGRET.