Energy measurement of fast neutron fields with a Recoil Proton Telescope using active pixel sensors

a The spectrometer ATHENA (Accurate Telescope for High Energy Neutron metrology Applications), in development at the LNE-IRSN, characterizes energy and fluence of fast neutron fields. The detector is a Recoil Proton Telescope and measures neutron energies in the range of 5 to 20 MeV. The system is intended to become a primary standard for both energy and fluence measurements. The most innovative part of ATHENA is made of three CMOS pixel sensors, thinned down to 50 microns thickness, allowing an accurate tracking of the recoil. The use of CMOS sensors and a thick silicon diode increase the intrinsic efficiency of the detector by a factor of ten compared with conventional designs. In this paper, we demonstrate the ability of the detector to measure neutron energies in the range of 5 to 20 MeV. Experimental investigations, using mono-energetic neutron fields produced by the AMANDE facility, indicate a good reconstruction of neutron energies. The present design is still under development. The accuracy of measuring 5 MeV and 14 MeV neutron energies was 15% and 5%, respectively.