In solar flares with strong γ‐ray emission, several prominent narrow lines can usually be observed emerging from a broad structure in the several hundred keV to several MeV region. They are produced by interactions of energetic protons, 3He and α‐particles with the most abundant isotopes in the solar atmosphere. The underlying continuum is due to electron and positron bremsstrahlung and to the superposition of broad lines from accelerated heavy ions as well as numerous narrow nuclear lines which are too weak to be resolved individually. The properties of the accelerated particles in the October 28, 2003 flare observed with the gamma‐ray spectrometer SPI onboard INTEGRAL were deduced by comparison of prominent narrow line fluence ratios, the line shapes of the 4.4 MeV 12C and 6.1 MeV 16O lines and the total nuclear line emission in the framework of a thick target interaction model.

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