Galactic diffuse gamma-ray measurements were done at energies below few GeV by detectors on satellites (EGRET) and at TeV energies by Milagro. Only upper limits were provided above 500 GeV (Whipple, HEGRA, TIBET). The subject of this Ph. D. is the study of the Galactic diffuse gamma-ray emission around 100 GeV with the CELESTE experiment.

A gamma-ray entering the atmosphere creates an electromagnetic shower which produces Cherenkov light in the blue-UV wavelengths which can be detected as a pool of several tens of metres on the ground. CELESTE was the first gamma-ray atmospheric Cherenkov telescope which detected signal from the Crab Nebula and the blazars Mrk 421 and Mrk 501 with an energy threshold below 100 GeV. Located in the French Pyrénnées, 1650 m a.s.l., CELESTE used 40 to 53 of the 54-square metre mirrors of the former solar plant of Thémis and the 100 m-tower where secondary optics, photomulipliers and a fast acquisition electronics were located. Because of the very strong hadronic isotropic background coming from the showers induced by cosmic protons and ions, data was taken by pointing the telescope on the source direction (ON) and away from the source sky region (OFF). Gamma-ray signal is then extracted by substracting ON and OFF source region data after analysis cuts on variables based on the geometric and timing properties of the Cherenkov wavefront, which allows to dicriminate between signal and background events.

In the view of detecting Galactic diffuse gamma-rays mostly located at low Galactic latitudes, we perform the analysis of some CELESTE OFF source data, comparing data taken near the Galactic plane and data taken away from the Galactic plane. Since models and extrapolations from fluxes measured at lower energies predict a rather low signal, we use an analysis method based on a composed estimator. It aims to improve the sensitivity of the instrument, which is limited by the hadronic background. Preliminary results on the systematic effets study give a favorable perspective for an upper limit on the diffuse Gamma-ray emisson with CELESTE.

Another part of this thesis is devoted to the study of a large set of Monte Carlo simulations to identify signatures and composition of the Cosmic Rays (hadrons, electrons) and the diffuse gamma-rays detected by the CELESTE telescope.

In addition, the Ph. D. student have actively paticipated in CELESTE data taking on the Thémis site till its closure in June 2004.