From 1936 to 1945, Gentner was a staff assistant at
Walther Bothe's Institut für Physik at the
Kaiser-Wilhelm Institut für medizinische Forschung (KWImF, Kaiser Wilhelm Institute for Medical Research; today, the
Max-Planck Institut für medizinische Forschung), in Heidelberg. One of his areas of specialization was in nuclear photoeffects (
Kernphotoeffekt). In 1932, Walther Bothe had succeeded
Philipp Lenard as director of the
Physikalische und Radiologische Institut (Physics and Radiological Institute) at the University of Heidelberg. When
Adolf Hitler became Chancellor of Germany on 30 January 1933, the concept of
Deutsche Physik took on more favor as well as fervor;
deutsche Physik, was
anti-Semitic and anti-theoretical physics, especially modern physics, including
quantum mechanics and both atomic and nuclear physics. As applied in the university environment, political factors took priority over the historically applied concept of scholarly ability, even though its two most prominent supporters were the
Nobel Laureates in Physics Philipp Lenard and
Johannes Stark. Supporters of
deutsche Physik launched vicious attacks against leading theoretical physicists. While Lenard was retired from the University of Heidelberg, he still had significant influence there. In 1934, Lenard had managed to get Bothe relieved of his directorship of the Institute of Physics at the University of Heidelberg, whereupon Bothe was able to become the director of the
Institut für Physik of the KWImF, replacing Karl W. Hauser, who had recently died.
Ludolf von Krehl, director of the KWImF, and
Max Planck, President of the
Kaiser-Wilhelm Gesellschaft (KWG, Kaiser Wilhelm Society, today, the
Max-Planck Gesellschaft), had offered the directorship to Bothe to ward off the possibility of his emigration. When it came time for Gentner to submit his
Habilitationsschrift,
Die Absorption, Streuung und Sekundärstrahlung harter Gamma-Strahlen (
The absorption, scattering and secondary hard gamma rays), the relations between the KWImF and the University of Heidelberg were so strained that
Habilitation was not possible there. So, Gentner completed the requirements at the University of Frankfurt, in 1937, and became a
Privatdozent (lecturer) there. This necessitated making trips by train between the facilities, which soon became a burden. By the end of 1937, the rapid successes Bothe and Gentner had with the building and research uses of a
Van de Graaff generator had led them to consider building a cyclotron. By November, a report had already been sent to the president of the
Kaiser-Wilhelm Gesellschaft (KWG, Kaiser Wilhelm Society; today, the Max Planck Society), and Bothe began securing funds from the
Helmholtz-Gesellschaft (Helmholtz Society; today, the
Helmholtz Association of German Research Centres), the
Badischen Kultusministerium (Baden Ministry of Culture),
I.G. Farben, the KWG, and various other research oriented agencies. Initial promises led to ordering a magnet from
Siemens in September 1938, however, further financing then became problematic. In these times, Gentner continued his research on the nuclear photoeffect, with the aid of the Van de Graaff generator, which had been upgraded to produce energies just under 1 MeV. When his line of research was completed with the 7Li (p, gamma) and the 11B (p, gamma) reactions, and on the nuclear isomer 80Br, Gentner devoted his full effort to the building of the planned cyclotron. After the armistice between France and Germany in the summer of 1940, Bothe and Gentner received orders to inspect the cyclotron
Frédéric Joliot-Curie had built in Paris. While it had been built, it was not yet operational. In September 1940, Gentner received orders to form a group to put the cyclotron into operation. Hermann Dänzer from the University of Frankfurt participated in this effort. According to author Robert Jungk in his landmark work,
Brighter Than A Thousand Suns, Gentner only agreed to take over Joliot-Curie's laboratory after he had received Joliot-Curie's express consent - and the two men crafted a secret agreement that the laboratory would not complete work that supported the German war effort. While in Paris, Gentner intervened personally to free both Frédéric Joliot-Curie and
Paul Langevin after they were arrested and detained. At the end of the winter of 1941/1942, the cyclotron was operational with a 7-MeV beam of
deuterons.
Uranium and
thorium were irradiated with the beam, and the byproducts were sent to Otto Hahn at the
Kaiser-Wilhelm Institut für Chemie (KWIC, Kaiser Wilhelm Institute for Chemistry, today, the
Max Planck Institute for Chemistry), in Berlin. In mid-1942, according to Jungk, Gentner was relieved of his duties because of the "weakness" he had shown. His successor in Paris was Wolfgang Riezler from
Bonn. A next mission of the HWA was the completion of the Heidelberg cyclotron. It was during 1941 that Bothe had acquired all the necessary funding to complete construction. The magnet was delivered in March 1943, and the first beam of deuteron was emitted in December. The inauguration ceremony for the cyclotron was held on 2 June 1944. In 1956, soon after the founding of
CERN, in
Geneva, Gentner was appointed
Direktor der Abteilung Synchrozyklotron (Director of the
Synchrocyclotron Department) and
Direktor der Forschung (Director of Research), positions which he held until October 1958. His department was responsible for the construction of their 600-MeV synchrocyclotron. Parallel to this, he had also been asked by the
Stuttgarter Landesregierung (Stuttgart State Government) to be the first head of the
Kernforschungszentrum Karlsruhe (KfK, Karlsruhe Nuclear Research Centre, today the
Forschungszentrum Karlsruhe), whose construction had just been decided. Gentner declined so as to stay with more fundamental research, rather than applied. The synchrocyclotron at CERN delivered its first beam on 1 August 1957. From 1971 to 1974, he was chairman of the CERN board. At the end of 1957, Gentner was in negotiations with Otto Hahn, President of the
Max-Planck Gesellschaft (MPG, Max Planck Society, successor of the
Kaiser-Wilhelm Gesellschaft), and with the Senate of the MPG to establish a new institute under their auspices. Essentially,
Walther Bothe's Institut für Physik at the
Max-Planck Institut für medizinische Forschung, in Heidelberg, was to be spun off and become a full-fledged institute of the MPG. The decision to proceed was made in May 1958. Gentner was named the director of the
Max-Planck Institut für Kernphysik (MPIK, Max Planck Institute for Nuclear Physics) on 1 October, and he also received the position as an
ordentlicher Professor (ordinarius professor) at the University of Heidelberg. Bothe had not lived to see the final establishment of the MPIK, as he had died in February of that year. In 1959, in collaboration with his Heidelberg colleagues Otto Haxel and
J. Hans D. Jensen, Gentner closed negotiations with the
Heidelberger Gemeinderates (Heidelberg Local Council) to build a 6-Mev tandem-accelerator and a special building for the study of cosmic physics. During his career, Gentner demonstrated his interest in
Kosmochemie und Archäometrie (
cosmochemistry and
archaeometry), which are fields at the intersection of cultural and natural sciences. ==Honors==