Measurement and analysis of the $^{241}$Am(n,$\gamma$) cross section at the CERN n_TOF facility.

In the context of the current nuclear technology, the radiotoxicity of the spent fuel of a typical PWR reactor is dominated by minor actinides for times greater than 104 years. In particular, 241Am and its 432 years half-life is responsible for about half of the minor actinide content of a PWR spent fuel. This thesis work consisted in measuring and analysing the 241Am(n, ) cross section at the CERN n TOF facility. After selecting exclusively the events obtained with lead shielding in front of the C6D6 detectors, the amplitude-energy calibration has to be adjusted with time, by using a photon coming from the 27Al(,,p)30Si reaction. Histogram extraction included applying a weighting function (obtained by MCNP simulation), a dead time correction, and a normalization to the compound nucleus excitation energy. The background corrected spectra were normalized relatively to the 4.9 eV resonance on 197Au. Finally, the resonance analysis was performed using the SAMMY code. The extracted thermal value is 678±68 barns, the uncertainty being mostly due to the large background level. The resolved range was extended from 150 eV to 320 eV, with a total of 192 resonances that had to be added of heavily modified. The unresolved region was analysed up to 150 keV, yielding a larger average cross section than previously evaluated below 20 keV. 124