Optimization aspects of the new nELBE photo-neutron source

The nELBE beamline at Forschungszentrum Dresden-Rossendorf (F'ZD) provides intense neutron beams by stopping primary electrons in a liquid lead target, where neutrons are produced by bremsstrahlung photons via (gamma,n) reactions. With the aim to increase the neutron yield through the enhancement of the electron beam energy (from the current 40 MeV limit up to 50 MeV), as well as to minimize several sources of background that are presently affecting the measurements, a new neutron beam-line and a new, larger neutron experimental room have been designed. The optimization of the neutron/photon ratio, the minimization of the backscattered radiation from the walls and the possibility to have better experimental conditions are the main advantages of the new design. To optimize the beamline, extensive simulations with the particle interaction and transport code FLUKA have been performed. Starting from the primary electron beam, both the photon and neutron 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 both with an independent simulation with the MCNP code and with analytical calculations, obtaining a very satisfactory agreement at the level of few percent. The evaluated radiation fields have been used to optimize the direction of the new neutron beamline, in order to minimize the photon flash contribution. A general overview of the new photo-neutron source, together with all the steps of the optimization study, is here presented and discussed.