A new neutron spectrometer, called ATHENA (Accurate Telescope for High Energy Neutron metrology Applications), is under development at the LNE-IRSN. This detector, based on the Recoil Proton Telescope (RPT) principle, is dedicated to the characterization of mono energetic neutron fields produced at the AMANDE facility. In order to be considered as a primary measurement standard, ATHENA will measure, at the level of metrological quality, energy and fluence of the neutrons fields in the range [5 MeV;20 MeV]. Thanks to an innovative tracking of the proton recoils, using CMOS pixel sensors, and to a large acceptance, the RPT will be able to reconstruct both trajectories and energies of the recoil protons with high accuracy. The first prototype of the RPT has allowed validation of the detector principle. The expected performances, in terms of energy resolution and fluence are under investigation. Monte Carlo simulations, based on a MCNPX calculation, have been made. These simulations have allowed to compare theoretical predictions to experimental data obtained at the AMANDE facility. An excellent reconstruction of the 14 MeV neutron is already demonstrated. The results suggest that the RPT will measure energy and fluence of the AMANDE neutron's fields with a good precision and is aimed to become a measurement standard. © 2012 IOP Publishing Ltd and Sissa Medialab srl.