The COMPTEL instrumental line background

The instrumental line background of the Compton telescope COMPTEL onboard t he Comptoa Gamma Ray Observatory is due to the activation and/or decay of m any isotopes. The major components of this background can be attributed to eight individual isotopes, lamely D-2, Na-22, Na-24, Al-28, K-40, Mn-52 Ni- 57, and Tl-208. The identification of instrumental lines with specific isot opes 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 o f the event rate due to a specific isotope depends on its life-time, orbita l parameters such as the altitude of the satellite above Earth, and the sol ar cycle. A detailed understanding of the background contributions from ins trumental lines is crucial at MeV energies for measuring the cosmic diffuse gamma-ray background and for observing gamma -ray line emission in the int erstellar medium or from supernovae and their remnants. Procedures to deter mine the event rate from each background isotope are described, and their a verage activity in spacecraft materials over the first seven years of the m ission is estimated.