The principal goal of this thesis is to highlight a possible "neutron skin" effect in the nuclear reaction mechanisms between heavy ions at energies around 6 MeV/nucleon. To this end, the reactions 58Ni+122Sn at 354 and 375.5 MeV and 64Ni+116Sn at 382.5 MeV were carried out at the CYCLONE accelerator in Louvain-la-Neuve. The projectile Ni and the target Sn were selected because of their magicity in proton. This characteristic implies the existence of a proton core more bound and of a neutron peripheral distribution in the isotopes 64Ni and 122Sn, rich in neutrons. The first goal was, to observe the possible influence of these neutrons on the fusion probability between the projectile and the target. In addition, the two types of reactions were selected in order to produce the same compound nucleus (180Pt) with the same excitation energy or with the same angular momentum. Thus, the effect of a neutron skin could be observed from the point of view of the production as from the point of view of the de-excitation of the compound nucleus. This de-excitation can proceed by evaporation of light particles or by fission. So, on the one hand, the fusion, fusion-evaporation and fusion-fission cross sections, and, on the other hand, the multiplicities of the light particles (neutrons, protons or alpha) emitted by the compound nucleus or by the fission fragments should have been compared. The experimental realization of this project required the use of the neutron detection system DEMON made up of 90 liquid scintillators laid out on a 4 m in diameter sphere which contain an aluminium chamber under vacuum. The chamber contained two multi-wire proportionnal counters XY intended for the detection of the fission fragments, two systems of Silicon junctions coupled with microchannels intended for the detection of evaporation residues and finally six triple Silicon telescope intended for the detection of the light charged particles (protons and alpha). At the end of the analysis, it was shownthat the cross sections of fusion are identical, within the error bars (about 10 %), for the two types of studied reactions.Also, it was observed that the possible compound nucleus de-excitations are independent of its production, as well on the competition between fission and evaporation as on the light particle emission. No effect of neutron skin was thus highlighted in the nuclear reaction mechanisms leading to fusion of the target and the projectile at energy around 6 MeV/nucleon. An effect seems nevertheless to appear in the nucleons transfer reactions in which the compound nucleus does not amalgamate. ------------------------------------------------ L'objectif principal de cette thèse est de mettre en évidence un éventuel effet de "peau de neutrons" dans les mécanismes des réactions nucléaires entre ions lourds à des énergies de l'ordre de 6 MeV/nucléon. Dans ce but, les réactions 58Ni+122Sn à 354 et à 375.5 MeV et 64Ni+116Sn à 382.5 MeV ont été réalisées auprès de l'accélérateur CYCLONE de Louvain-la-Neuve. Le projectile Ni et la cible Sn ont été choisis en raison de leur magicité en proton. Cette caractéristique implique l'existence d'un coeur de protons plus liés et d'une distribution plus périphérique des neutrons dans les isotopes 64Ni et 122Sn, riche en neutrons. Il s'agissait donc, dans un premier temps, d'observer l'influence possible de ces neutrons excédentaires sur la probabilité de fusion du projectile et de la cible. D'autre part, les deux types de réactions ont été choisis afin de produire le même noyau composé 180Pt de même énergie d'excitation ou de même moment angulaire. Ainsi, l'effet de peau de neutrons pouvait être observé tant du point de vue de la production que du point de vue de la désexcitation du noyau composé. Cette désexcitation peut se faire par évaporation de particules légères ou par fission. Plus précisement, il s'agissait de comparer, d'une part, les sections efficaces de fusion, de fusion-évaporation et de fusion-fission, et, d'autre part, les multiplicités des particules légères (neutrons, protons ou alpha) émises par le noyau composé ou les fragments de fission. La réalisation expérimentale de ce projet a nécessité l'utilisation du système de détection de neutrons DEMON constitué de 90 scintillateurs liquides disposés sur une sphère de 4 m de diamètre au centre de laquelle une chambre en aluminium était mise sous vide. Celle-ci contenait deux chambres à gaz et à multi-fils XY destinés à la détection des fragments de fission, deux systèmes de jonctions au Silicium couplées à des galettes à micro-canaux destinés à la détection de résidus d'évaporation et enfin six triple-téléscopes au Silicium destinés à la détection des particules chargées légères (protons et a). Au terme de l'analyse des données récoltées, il s'avère que les sections efficaces de fusion du projectile et de la cible sont identiques, dans une barre d'erreur de l'ordre de 10 %, pour les deux types de réactions étudiées. De même, il a été observé que les désexcitations possibles du noyau composé sont indépendantes de sa production, tant sur la compétition entre fission et évaporation que sur l'émission de particules légères. Aucun effet de peau de neutrons n'a donc été mis en évidence dans les mécanismes des réactions nucléaires impliquant la fusion du projectile et de la cible. Un effet semble néanmoins apparaître dans les réactions d'échanges de nucléons dans lesquelles les noyaux ne fusionnent pas.

Heavy atomic nuclei are thought to have proton and neutron radial distributions which have different extents. This difference is usually quantified in terms of a neutron skin (rnp), defined as the difference between the root mean square radii of the neutron and proton radial distributions (rnp = rn - rp). The nature, or even existence, of the neutron skin is currently not well established for many nuclei. Different nuclear theories give different predictions for the neutron skin thickness ranging for a typical heavy nucleus from 0.05 to 0.35 fm. Accurate measurement of the properties of the neutron skin would be a powerful constraint to differentiate between models of nuclear structure, improving our knowledge of the basic Equation Of State (EOS) for neutron rich matter. Particularly, the rate at which the neutron skin thickness changes across an isotopic chain of nuclei gives a tight constraint on the EOS and is also amenable to experimental determination with small systematic error. Improving our knowledge of the EOS for neutron rich matter is a crucial step towards gaining a deeper understanding of nuclear structure and nuclear matter in general. These results will also impact our knowledge of compact astrophysical objects such as neutron stars. This thesis describes the first measurement of neutron skin thicknesses along an isotopic chain using an electromagnetic probe. The neutron skin is measured through the study of the coherent photoproduction of neutral π mesons emitted from nuclei. This experiment was carried out in the A2 hall of the MAMI facility in Mainz, Germany in October 2012. The incident photon beam comprised of energy tagged photons in the range of Eγ=150-800 MeV with an intensity of 10⁸ photons per second. Experimental data was obtained for three different tin targets, 116Sn, 120Sn and 124Sn. The products from the resulting photoreactions were measured in the Crystal Ball detector and in the TAPS calorimeter systems, with track and particle identification information for charged particles provided by a multi wire proportional chamber (MWPC) and a particle identification detector (PID). The experiment provides the first information on the evolution of the neutron skin thickness along an isotopic chain using an electromagnetic probe. The results are compared with a range of theoretical models and previous data from strongly interacting probes. The new data will provide an important new experimental constraint on the basic properties of the EOS in atomic nuclei.