Shielding aspects of the new nELBE photo-neutron source

The nELBE beamline at Forschungszentrum Dresden-Rossendorf (FZD) provides intense neutron beams by stopping a primary electron beam in a liquid lead radiator and producing then neutrons by bremsstrahlung photons through (gamma,n) reactions. To increase the neutron yield through the electron energy enhancement (from the current 40 MeV limit up to 50 MeV), to optimize the neutron/photon ratio and to minimize several source of background at the present time, a new neutron beamline with a new, larger experimental room has been designed. Extensive simulations with the particle interaction and transport code FLUKA have been performed to assess all the shielding aspects of the design. Starting from the primary electron beam, both the photon and neutron secondary radiation fields have been fully characterized. To have a cross-check of the results, the calculated values of the neutron yields at different energies of the primary beam have been compared with an independent simulation with the MCNP code, obtaining a very satisfactory agreement at the level of few percent. The photoproduction area has been studied in a mixed field condition, while, for statistical reasons, the penetration of the beam through the collimator and then in the neutron experimental room has been studied by writing a separate source term for the photon and the neutron radiation component. The results for the nELBE shielding, together with some aspects of the optimization of the photo-neutron beamline, are here presented and discussed.