Dear @fatimazahra.er
Unfortunately your approach is not correct. In the simulated spectrum, what you see at 2505 keV is the actual 2505 keV peak from the Co-60 decay (see the decay scheme below), not a summing peak, which FLUKA cannot by construction reproduce because it emits uncorrelated particles. For coincidence effects to emerge in your simulation, you will need to load multiple particles per history (or primary event), as @horvathd suggested earlier.
Furthermore, your theoretical approach to calculate the correction factors from a spectrum is also not fully correct. For example, it does not take into account that a fraction of the 1332 keV line comes directly from the β-decay of 60Co without previous emission of the 1171 keV line etc.
Since this falls outside the scope of this forum, I would advise to take a look at a γ-spectroscopy textbook or search online for how coincidence summing correction factors can be analytically calculated for some simple cases, such as Co-60. See, e.g.:
- Gamma- and X-ray spectrometry with semiconductor detectors (K. Debertin, R. G. Helmer) (Section 4.5.1)
- Practical Gamma‐Ray Spectrometry (G. R. Gilmore) (Section 8.11)
Best regards