Neutron Energy Reconstruction and Fluence Determination at 27 keV With the LNE-IRSN-MIMAC MicroTPC Recoil Detector

The aim is to characterize the energy distribution of neutron fluence in the energy range 8 keV–5 MeV based on a primary standard: the LNE-IRSN/MIMAC microTPC. The microTPC is a time projection chamber. Time projection chambers are gaseous detectors able to measure charged particles energy and to reconstruct their track. The gas is used as a (n, p) converter in order to detect neutrons down to few keV. The neutron energy is reconstructed event by event thanks to proton scattering angle and proton ionization energy measurements. The scattering angle is deduced from the 3-D track. The proton energy is obtained by charge collection measurements, knowing the ionization quenching factor. The fluence is reconstructed thanks to the detected events number and the simulation of the detector response. The microTPC is a new reliable detector able to measure energy distribution of the neutron fluence without unfolding procedure or prior neutron calibration contrary to usual gaseous counters. The microTPC is characterized at the AMANDE facility, with neutron energies going from 8 keV to 565 keV. This work shows the first direct reconstruction of neutron energy and fluence, simultaneously, at 27.2 keV in a continuous irradiation mode.