Dear Sérgio @szpigel, most likely your explanation about the 20Ne origin of the 1.6 MeV line is correct. However, you are missing the fact that 20Ne is produced with a quite sizeable cross section (several hundred mb) on heavier oxygen isotopes (17O and 18O), which can compensate for their per mil abundance, as included in the natural composition you selected. (If needed, one can define a mono-isotopic material, see this post to make it right). Do not hesitate to come back in case of not convincing findings. Best