NaI detector effective area model. Panel (a) shows the NaI detector effective area measurements (red crosses), for zero incidence angle, reported in M09. The pink line shows our adopted interpolation. Panel (b) shows the measured effective area (M09) for different photon incidence angles at three reference photon energies (black circles: 32 keV; blue squares: 279 keV; red triangles: 662 keV). The solid lines show our best-fitting model. Panel (c) shows the best-fit values of parameters c₁, c₂ and c₃ of our model (Eq. C.5) for the angular dependence of the effective area at the three reference energies (coloured circles). The dashed lines show our assumed behaviour of c_i as a function of the photon energy.