[en] In this thesis the low-energy dipole response of ${}^6$He and ${}^8$He is investigated using relativistic Coulomb excitation in inverse kinematics. The dipole strength distributions of ${}^6$He and ${}^8$He are extracted and presented up to 15 MeV excitation energy. This is the first complete measurement of the low-energy dipole response of ${}^8$He. An experiment is presented that has been performed to measure the decay of ${}^6$He and ${}^8$He after electromagnetic excitation with high precision and statistics. For the first time a four neutron decay channel is successfully measured and analyzed using the invariant-mass method. The combination of the neutron detectors new Large Area Neutron Detector (NeuLAND) and Neutron Detection System for Breakup of Unstable Nuclei with Large Acceptance (NEBULA) at the Superconducting Analyzer for multi-particle from Radio Isotope Beams (SAMURAI) setup at the RIKEN Nishina Center in Japan made this possible. The full setup is presented and the measurement principles are explained. The calibration procedures and analysis techniques applied in this work are described in detail. This includes an improved procedure to identify and subtract the contribution by nuclear excitation reactions from the measured cross section. For the momentum reconstruction of the high energy neutrons, a new reconstruction algorithm is developed. The setups acceptance, efficiency and relative energy resolution is characterized using GEANT4 simulations. The cross talk rejection performance of the developed reconstruction algorithm is determined and correction procedures are developed. The results obtained on the dipole response of ${}^6$He and ${}^8$He are compared to previous results from experiment and theory. Furthermore, correlations in the ground state are investigated in a cluster approach. The final-state correlations are investigated in Jacobi coordinates and by comparing to a phase-space decay. New results on the ${}^5$He and ${}^7$He ground-state resonances are presented and discussed as a side topic.