The comptel instrumental line background

The instrumental line background of the Compton telescope COMPTEL onboard the Compton Gamma- Ray Observatory is due to the activation and/or decay of many isotopes. The major components of this background can be attributed to eight individual isotopes, namely 2 D, 22 Na, 24 Na, 28 Al, 40 K, 52 Mn, 57 Ni, and 208 Tl. The identication of instrumental lines with specic isotopes is based on the line energies as well as on the variation of the event rate with time, cosmic-ray intensity, and deposited radiation dose during passages through the South- Atlantic Anomaly. The characteristic variation of the event rate due to a specic isotope depends on its life-time, orbital parameters such as the altitude of the satellite above Earth, and the solar cycle. A detailed understanding of the background contributions from instrumental lines is crucial at MeV energies for measuring the cosmic diuse gamma-ray background and for observing -ray line emission in the interstellar medium or from supernovae and their remnants. Procedures to determine the event rate from each background isotope are described, and their average activity in spacecraft materials over the rst seven years of the mission is estimated.