The radiation and ionization energy loss are presented for single arm Monte Carlo simulation for the GDH sum rule experiment in Hall-A at Jefferson Lab. Radiation and ionization energy loss are discussed for 12 C elastic scattering simulation. The relative momentum ratio ∆p p and 12 C elastic cross section are compared without and with radiative energy loss and a reasonable shape is obtained by the simulation. The total energy loss distribution is obtained, showing a Landau shape for 12 C elastic scattering. This simulation work will give good support for radiation correction analysis of the GDH sum rule experiment.