[en] Double ionization of an atom by a single photon is the simplest and most fundamental many-electron process. The ejection of two electrons following the absorption of one photon is strictly prohibited in an independent electron approximation. Thus determining the probability of double photoionization alone is already a challenging test of the understanding of electron-electron correlation. Furthermore, in the slow breakup of a bound system into three charged particles, the final state wave function must represent a high degree of few-body Coulomb correlation involving the simultaneous interaction of all three particles. The case of double photoionization is again particularly well suited to study this problem as the energy and the angular momentum delivered to the system can be very well controlled. Helium, as the most basic three body system, has been the target of extensive studies over the past decades. The purpose of this project has been to study double and single ionization using cold target recoil ion momentum spectroscopy (COLTRIMS). This technique has been widely applied within the area of ion-atom collisions to study the dynamics of energy and momentum transfer in collisions between few-electron systems, and the entire technical machinery has been transferred to photon-atom collisions. The technique uses space- and time-imaging of He⁺ and He⁺⁺ recoil ions created in photon-He collisions to measure the full momentum vector of each ion produced. Event-mode recording is used and a solid angle of nearly 4π is realized, allowing an extremely high data-collection efficiency. In order to reduce the initial momentum spread of the He target a precooled supersonic He jet is used