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.
The French Institute for Radiation protection and Nuclear Safety (IRSN), designated by the French Metrology Institute (LNE) for neutron metrology, is developing a time projection chamber using a ...micromegas anode: microTPC. This work is carried out in collaboration with the Laboratory of Subatomic Physics and Cosmology (LPSC). The aim is to characterize the energy distribution of neutron fluence in the energy range 8 keV-5 MeV with a primary procedure. The time projection chambers are gaseous detectors able to measure charged particles energy and to reconstruct their track if a pixelated anode is used. In our case, the gas is used as a (n, p) converter in order to detect neutrons down to few keV. Coming from elastic collisions with neutrons, recoil protons lose a part of their kinetic energy by ionizing the gas. The ionization electrons are drifted toward a pixelated anode (2D projection), read at 50 MHz by a self-triggered electronic system to obtain the third track dimension. The neutron energy is reconstructed event by event thanks to proton scattering angle and proton energy measurements. The scattering angle is deduced from the 3D track. The proton energy is obtained by charge collection measurements, knowing the ionization quenching factor (i.e. the part of proton kinetic energy lost by ionizing the gas). The fluence is calculated thanks to the detected events number and the simulation of the detector response. The μTPC 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 still being developed and measurements have been carried out at the AMANDE facility, with neutrons energies going from 8 keV to 565 keV. After the context and the μ-TPC working principle presentation, measurements of the neutron energy and fluence at 27 keV and 144 keV are shown and compared to the complete detector response simulation. This work shows the first direct reconstruction of neutron energy and fluence, simultaneously, at 27.2 keV in a continuous irradiation mode.
The Archaean crust of eastern Finland is composed of three main lithological units: ‒ Gneissic basement. It was originally composed of tonalites, trondhjemites and granodiorites emplaced at 2.86 G.a. ...and 2.62 G.a. and subsequently transformed into grey gneisses. The parental magmas originated from the mantle in a two-stage process: (i) formation of a basaltic crust, (ii) melting of this crust transformed into garnet-bearing amphibolite. Field, petrological and geochemical data argue in favour of a subduction-like process rather than an underplate accretion model. ‒ Greenstone belt (Kuhmo‒Suomussalmi). The lower volcanic sequence of this greenstone belt is composed of mafic and ultramafic lavas with komatiitic and tholeiitic affinities and was emplaced at about 2.65 G.a. The uprising of a mantle diapir initiated the breaking of the preexisting sialic crust (grey gneisses) and induced the emplacement of the greenstones in a proto-oceanic rift geodynamic environment. ‒ Calc-alkaline magmatism. The calc-alkaline magmas were emplaced as volcanic rocks in the greenstone belt 2.5 G.a. ago (Luoma acid volcanics), or as K-feldspar phenocryst granodiorites in the immediate vicinity of the Kuhmo‒ Suomussalmi greenstone belt 2.5 G.a. ago (Suomussalmi and Arola augen gneisses). The younger plutons were emplaced as pink leucogranites 2.41 G.a. ago. The origin of this magmatism is thought to be correlated to the late tectonic evolution of the Kuhmo‒Suomussalmi greenstone belt. The high density (d ~ 3.3) lavas of the lower volcanic sequence were emplaced over a low density (d ~ 2.7) sialic crust. This created an inverse density gradient and generated a gravity instability that initiated the subsidence (sagduction) of the greenstone belt with respect to its basement. This later was carried down in the vicinity of the belt and underwent partial melting, thus generating the calc-alkaline magmas. It must be pointed out that, in the course of time, petrogenetic processes have changed from ensimatic to ensialic, implying a major reworking of the preexisting crustal materials.
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