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

Abstract

International audienceThe 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

Directionality and head-tail recognition in the keV-range with the MIMAC detector by deconvolution of the ionic signal

“…The AMANDE facility [43] of the French Institute for Radiation protection and Nuclear Safety (IRSN) produces mono-energetic neutron field between 2 keV and 20 MeV. In this section, the AMANDE facility makes use of the nuclear reaction 45 Sc (p, n) to produce neutron fields with kinetic energy of 8.12 ± 0.01 keV or 27.24 ± 0.05 keV [28] depending on the energy of the proton beam that activates one resonance or another. A MIMAC chamber specially designed for neutron spectroscopy, MIMAC-FastN [44], is placed in front of the 45 Sc target, at a distance of 33 cm, such that the proton beam is parallel to the Z-axis of the detector.…”

“…Photons are also produced during the nuclear reaction on the 45 Sc target with a fluence about 20 times larger than the neutron's one [28]. The electron-recoil discrimination represents then a central element of our analysis in order to extract the proton recoils from the γ background.…”

“…At 27 keV, we follow the same procedure than in our previous analyses [45] in training a Boosted Decision Tree (BDT). We perform a "background only" run for which the energy of the AMANDE proton beam is sufficiently decreased to remain out of the neutron resonance: the proton beam, only 20 keV less energetic at the same current level [28], keeps producing the same γ rate on the target while no neutron gets produced. This "background only" run is combined with the measurements (corresponding to a "signal + background" run) in order to train and test a BDT thanks to the TMVA software [46].…”

“…We have already demonstrated that MIMAC can access directionality and reconstruct a neutron spectrum at intermediate energies (127 keV [27] and 27 keV [28]) when operating at low gain with a gap of 256 µm. However, we are interested in accessing directionality in the keVrange for which operating at high gain (> 10 4 ) is mandatory.…”

Neutron spectroscopy from 1 to 15 MeV with Mimac-FastN, a mobile and directional fast neutron spectrometer and an active phantom for BNCT and PFBT

“…The gas mixture and the pressure can be modified depending on the application energy range [11,12]. Fast neutron detection is performed through the tracking of the nuclear recoils that result from nuclear elastic scattering between incident fast neutrons and the gas nuclei.…”