This book represents a significant experimental advancement for the field of nuclear physics as it enhances our understanding of the giant dipole resonance (GDR) of atomic nuclei, a fundamental nuclear excitation inherent to virtually all nuclei, dominating their photo responses. In particular, the work focuses on the experimental investigation of ? decay of the GDR, which, despite being a key property of this collective mode and despite decades of research on the GDR, has remained poorly characterized until now. Key highlights of the thesis include:• Development of a novel experimental approach to systematically study ? decay of the GDR using artificial MeV-photon sources. • The first experimental application of this method to the GDRs of both spherical and deformed nuclei, yielding unprecedented data on the ? decay of this fundamental resonance from a Bayesian spectra-deconvolution analysis. • Interpretation of the data in the framework of the macroscopic, geometrical model of the GDR, resulting in a groundbreaking validation of this elementary picture of the mode. • Derivation of precise nuclear shape information from the ?-decay data allowing to furthermore test modern nuclear structure theory calculations of these observables, in particular nuclear triaxiality. This book has a clear and accessible structure, introduces the basic physics and concepts underlying the experimental method, data analysis, and interpretation in detail and provides a thorough and reproducible description of the performed experiment and every step of the analysis. This makes it understandable even to non-specialists and a valuable reference for future experimental work.
