A new Recoil Proton Telescope for energy and fluence measurement of fast neutron fields

Abstract

The spectrometer ATHENA (Accurate Telescope for High Energy Neutron metrology Applications), is being developed at the IRSN / LMDN (Institut de Radioprotection et de SUrete nucleaire / Laboratoire de Metrologie et de dosimetrie des neutrons) and aims at characterizing energy and fluence of fast neutron fields. The detector is a Recoil Proton Telescope and measures neutron fields in the range of 5 to 20MeV. Since the institute is designated by the LNE (Laboratoire National de metrologie et d'Essais) under the label LNE-IRSN as a reference for the neutron fluence, dose equivalents and kerma quantities in neutron metrology, his telescope is intended to become a primary standard for both energy and fluence measurements. The neutron detection is achieved by a polyethylene radiator for n-p conversion, three 50μm thick silicon sensors that use CMOS technology for the proton tracking and a 3 mm thick silicon diode to measure the residual proton energy. This first prototype used CMOS sensors called MIMOSTAR, initially developed for heavy ion physics. The use of CMOS sensors and silicon diode increases the intrinsic efficiency of the detector by a factor of ten compared with conventional designs. The first prototype has already been done and was a successful study giving the results it offered in terms of energy and fluence measurements. For mono energetic beams going from 5 to 19MeV, the telescope's energy resolution was between 5 and 11% and fluence determination was in agreement with other measurement standards. A second and final prototype of the detector is being designed. It will hold upgraded CMOS sensors called FastPixN. These CMOS sensors are supposed to run 400 times faster than the older version and therefore give the telescope the ability to support neutron flux in the range of 107 to 108cm−2.s−1. The first prototype's results showed that a 50μm pixel size is enough for a precise scattering angle reconstruction. Simulations using MCNPX and GEANT4 are already in place for further improvements. A DeltaE diode will replace the third CMOS sensor and will be installed right before the silicon diode for a better recoil proton selection. The final prototype with its new geometry will increase the telescope's efficiency by a factor of 1.5. It will also cover some of the most important points in metrology; repeatability, reproducibility and sustainability.