Whether a nucleus is stable depends on the balance between protons and neutrons. Hahn’s experiments showed that when a heavy element such as uranium is struck by a neutron, the nucleus splits into two fragments of roughly equal size. The emitted neutrons can then split other nuclei, producing an exponential chain reaction. If this chain reaction is controlled, it produces heat for electricity; if it is uncontrolled, it releases an enormous explosive energy. Although Hahn’s work was part of basic radioactivity studies, it quickly became central to wartime military projects. The same principle later enabled production of medical isotopes and radiation therapy that save lives. However, nuclear waste and safety issues remain serious challenges. Hence the discovery is an excellent case study for the relationship between science, technology and ethics.

At the end of 1938 Hahn and Fritz Strassmann conducted experiments in Berlin in which they bombarded uranium dioxide with neutrons and, through meticulous radiochemical separation, detected isotopes of barium with intermediate mass numbers. This observation required a radical interpretation—that the uranium nucleus had split—and Hahn consulted Lise Meitner and Otto Frisch for explanation. Meitner and Frisch used Bohr’s liquid-drop model to introduce the concept of nuclear fission and estimated an energy release of roughly 200 MeV. Subsequently, Fermi, Bethe and colleagues formulated the critical conditions for a self-sustaining chain reaction, leading to the Piles memorandum and eventually the Manhattan Project. As a chemist, Hahn’s great contribution was the rigorous demonstration of the phenomenon using precision radiochemical methods. His techniques—daughter-nuclide identification, thermal-ionization mass spectrometry and distribution-coefficient catalogues—laid the foundation of modern nuclear chemistry. In contemporary nuclear power plants, the heat generated by fission is extracted in a primary coolant loop to drive steam turbines. Next-generation reactors such as fast breeders and small modular reactors inherit Hahn’s insights into neutron economy and fuel cycles. Nuclear-fission fragment data are also applied to cosmic-ray production studies and geochemical dating. Thus, the science of fission forms an interface linking fundamental physics, energy engineering and environmental science.

In Hahn and Strassmann’s 1938 experiment, β− decay chains following the U(n,f) reaction were quantified in the Cs–Ba region, and production yields of Ba-139 and Ba-140 indicated an asymmetric mass split. Using barium-chloride precipitation and X-ray spectrometry, they achieved isotope identification at a sensitivity of 10^−11 g. The Meitner–Frisch liquid-drop model analyzed the competition between Coulomb repulsion and surface energy, calculating the fission barrier and Q value. The subsequent Bohr–Wheeler formalism treated the cross-section via quantum tunneling and laid the groundwork for the concept of multimodal fission. Modern nuclear data libraries such as ENDF/B and JENDL include fragment yields Y(A,Z,E) and delayed-neutron spectra, which are incorporated into reactor physics codes. Hahn’s radiochemical quantifications, together with extraction-coefficient data, continue to be cited in nuclear-fuel reprocessing design. In THORP and PUREX processes, distribution coefficients for actinides and yttrium measured by Hahn serve as initial parameters for stage optimization. In nuclear forensics, prompt-dose characteristics and other fission parameters are calibrated against the baseline provided by Hahn’s original data.