The neutron field inside high energy and high intensity proton accelerators can produce activation of air. The neutrons backscattered by the concrete walls contribute largely to production of argon-4l and carbon-14 concentration. Detailed knowledge on the neutron spectrum is necessary for the accurate estimation as the neutrons lying above thermal energy up to around 1.5 MeV, can also produce the 41Ar considerably large amount in addition to the thermal neutrons. Hence, the reflection of neutrons in a typical proton accelerator facility for proton energies in the range 100-500 MeV by concrete walls and the concentration of 41 Ar and 14C has been studied in detail in using FLUKA Monte Carlo code. The neutron spectra inside the facility clearly indicate the dominance of the neutrons of energies within 1.5 MeV. The estimated neutron albedolbackscatter factors for the secondary generated by the 100 Me V proton beam falling on the copper target have been estimated around 11%. The build up of 41Ar and 14C concentration for a 24 hr proton beam irradiation and the decay of the activity for the next 24 hours after the beam shut down are estimated. The Monte Carlo computed saturated 41 Ar concentration values are 2.5 times higher than that of the estimations based on analytical calculations, which assume the activation only by thermal neutrons.