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 γ 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 γ 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 γ 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 γ-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.

Jörn Kleemann is an experimental physicist specializing in nuclear structure and dynamics.

He received his doctoral degree from the Technical University of Darmstadt (Darmstadt, Germany), where he conducted his research at the Institute for Nuclear Physics under the supervision of Prof. Dr. Dr. h.c. mult. Norbert Pietralla.

His research focuses on the experimental study of the shapes and collective excitations of atomic nuclei through photonuclear reactions, in particular nuclear resonance fluorescence.

During his doctoral research, he conceived and successfully implemented a novel experimental method to systematically investigate the γ-decay behavior of the giant dipole resonance, providing new insights into this fundamental nuclear excitation.

Committed to open science, he has made the experimental dataset and his analysis framework publicly available in an open research data repository, ensuring the transparency and reproducibility of his doctoral work.