Pair Spectrometer

Energy resolution of 24 FSF (top) and 6 WSF (bottom) counters estimated with the GEANT simulation. The converter thickness corresponds to $5x10^{{-3}}X_{0}$ . The contribution from the beam spot size (~ 4 mm at the converter) to the energy resolution dominates for the high-energy bins of the WSF counters.

Photon energy spectrum measured by the pair spectrometer. The plot is obtained by requiring a coincidence between the FSF and WSF counters and contains 24 x 6 = 144 energy bins. The coincident rate corresponds to a high-luminosity run ( $10^{8}$ photons/sec in the coherent peak ) using Aluminum converter with the thickness of $5x10^{{-3}}$ radiation lengths.