4. Gamma-ray line production
A given gamma-ray line can be obtained through several nuclear reactions, including direct excitation from protons and particles, and production in an excited state by a spallation reaction. For each channel () with cross-section , we have:
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 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'.
© European Southern Observatory (ESO) 1997
Online publication: March 24, 1998