Abstract : This Ph-D thesis was intended to prepare a precise measurement of the alpha ratio of 233U between 1eV and 10keV. This ratio is a key-parameter to calculate the breeding ratio of reactors based on the 232Th/233U cycle. This measurement would be performed, at the LPSC, on the experimental platform PEREN which is composed of a lead slowing-down time spectrometer associated with an intense pulsed neutron generator. Capture and fission rates are measured thanks to 8 scintillators YAP used in coincidence and surrounding a fission chamber. Preliminary tests using 235U resulted in a very low signal to background ratio despite the successive improvements. The different components of the background were identified and quantified experimentally and thanks to simulation tools (MCNP and GEANT4). Nevertheless, the signal to background ratio has still to be increased by about a factor 10 at least, to allow the measurement of the capture of a fissile element with such an experimental setup.