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Astron. Astrophys. 328, 107-120 (1997)

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4. Gamma-ray line production

A given gamma-ray line can be obtained through several nuclear reactions, including direct excitation from protons and [FORMULA] particles, and production in an excited state by a spallation reaction. For each channel ([FORMULA]) with cross-section [FORMULA], we have:

[EQUATION]

We use a wide sample of nuclear reactions based on the compilation by Ramaty et al. (1979) (hereafter RKL79), with more than 70 gamma-ray lines and almost 200 cross-sections (updated when possible; Tatischeff et al. 1996). However, only a few of these lines have significant contributions to the total emitted flux.

In the context of gamma-ray line spectroscopy, one has to distinguish between broad line and narrow line production mechanisms. The energy dispersion of the emitted gamma-rays reflects the velocity distribution of the excited nuclei with respect to the observer, that is, to a good approximation, with respect to the ISM. In the case of the interaction of light EPs (protons or [FORMULA] particles) with heavy ISM nuclei, the heavy excited nuclei, because of their large inertia, acquire a recoil velocity much lower than that of the projectile. The resulting de-excitation lines are then rather narrow in this case (width ranging from a few tens of keV to around 100 keV). We shall refer to this `light-onto-heavy' process as the `direct process'. Conversely, `heavy-onto-light' interactions lead to heavy excited nuclei with velocities close to those of the incident EPs and produce in turn broad de-excitation lines (from a few hundreds of keV to an MeV). This will be refered to as the `inverse process'.

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© European Southern Observatory (ESO) 1997

Online publication: March 24, 1998

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