The knowledge of the Earth's natural radiative natural environment (NRE) and its dynamics is an important issue for controlling the risks of malfunctioning of advanced technologies, dosimetric risks (biological effects), and in assessing the impact of albedo neutrons escaping from the upper atmosphere to the radiation belts. The latter contribute on the one hand to the population of the proton radiation belt (Salammbô models) and on the other hand induce instrumental background noise on all equipment operated in low or high altitude orbit, including in particular instruments dedicated to astronomy or to the study of the globe. For their part, cosmic rays (essentially composed of protons and helium nuclei) interact with atoms in the upper atmosphere either by losing part of their energy through ionization or for the most energetic particles, by causing nuclear reactions. These secondary reactions in turn cause cascade reactions and up to ground level, the result being the generation of secondary particles of the neutron, proton, electron or muon type, whose spectra will vary according to altitude, longitude, latitude, atmospheric conditions and solar activity.
The objective of this thesis is to characterize secondary particles of atmospheric RNE from adapted nuclear transport codes (GEANT4, MCNPx, FLUKA or Corsika) based on a 3D model of the atmosphere. In addition, this 3D approach will allow to characterize the angular distribution of secondary particles and quantify the components escaping from the upper atmosphere to the radiation belts. The impact of cosmic radiation modulation as a function of the solar cycle on the population of atmospheric neutrons and albedo will be quantified. The model results will be validated with the neutron spectrum measurements performed by ONERA/DPHY at Concordia (Antarctica). In addition to its high neutron detection level (altitude 3223m, cut-off rigidity ~ 0GV), this measurement site is characterized by a scene that can be easily modelled thanks to its stable water conditions. Additional comparisons may be made from other spectrometers and/or instruments (neutron monitors). The model results will also be compared with instrumental background noise measurements from several space missions. In the long term, this model could be extended to study the impact of solar flares on their contributions to neutron generation in the Earth's atmosphere and its albedo component (SPAND). Another perspective will be to apply this cosmic shower modeling to the case of Jupiter, whose atmospheric and magnetic conditions are very different from the terrestrial case.
The bibliographic work will cover the fields of cosmic rays, atmospheric modelling and radiation-matter interactions (cosmic showers). An important task will be to orient the choice of a nuclear transport tool (GEANT4, MCNPx, FLUKA or Corsika) according to the problem and the relevance of the physical models. The thesis work will be divided into several steps:
1) Development of a 3D model of the atmosphere (latitude, longitude and altitude) based on the state of the art.
2) Characterization of secondary particles of atmospheric RNE from adapted nuclear transport codes based on the 3D model of the atmosphere (angular properties of secondary particles integrating the component escaping from the upper atmosphere to the radiation belts). 
3) Validation of 3D modelling with neutron spectrum measurements performed by ONERA/DPhIEE at Concordia (Antarctica). Additional comparisons may be made from other spectrometers and/or instruments (neutron monitors). 
4) Valuation of this new physical description in the "Salammbô 3D" and "MUSCA SEP3" themes. 
5) Assessment of expected background noise in low orbit for different space missions and comparison with flight data (ESA/Integral for gamma albedo, JAXA/Hitomi for neutron albedo); application to space missions in preparation (CAS/Einstein Probe, ESA/Theseus).
 Natacha Combier, Arnaud Claret, Philippe Laurent, Vincent Maget, Daniel Boscher, Alfredo Ferrari, and Markus Brugger, "Improvements of FLUKA Calculation of the Neutron Albedo", IEEE Transaction on Nuclear Science, VoL. 64, NO. 1, January 2017.
 G. Hubert and A. Cheminet, "Radiation effects investigations based on atmospheric radiation model (ATMORAD) considering GEANT4 simulations of extensive air showers and solar modulation potential", Radiation Research, Vol. 184, No. 1, pp. 83-94, July 2015.
 G. Hubert, S. Duzellier, C. Inguimbert, C. Boatella-Polo, F. Bezerra, and R. Ecoffet, "Operational SER calculations on the SAC-C orbit using the Multi SCAles Single Event Phenomena Predictive Platform (MUSCA SEP3)", IEEE Trans. Nucl. Sci., Vol. 56, No.6, pp. 3032-3042, Dec. 2009.