J. Hans D. Jensen

German nuclear physicist

• Born: June 25, 1907; Hamburg, Germany
• Died: February 11, 1973; Heidelberg, Germany

In 1949, Hans Jensen discovered that if one allowed for strong spin-orbit coupling among nucleons, the nuclear shell model explained accurately the structure of the nucleus. Jensen and Maria Goeppert-Mayer, who independently discovered this at the same time, shared half of the 1963 Nobel Prize in Physics for the achievement.

Also known as: Johannes Hans Daniel Jensen; Hans Jensen

Primary field: Physics

Specialties: Nuclear physics; theoretical physics; atomic and molecular physics

Early Life

Johannes Hans Daniel Jensen, better known as Hans Jensen, was born in Hamburg, Germany, on June 25, 1907. His father, Karl Friedrich Jensen, and his mother, Helene Auguste Adolphine Ohm, were married in Constantinople (now Istanbul) in 1898. They supported an orphanage for Armenian Christian children in central Turkey but had to leave the Ottoman Empire soon after their wedding. In Hamburg, Jensen’s father worked first as Baptist church custodian and then as a gardener for the Hamburg Botanical Garden. Hans was the third of his parents’ children.

Jensen’s mother died in 1922, the year he received his certificate of secondary education. Unusual in Germany for the time for a pupil with such a basic degree, he was permitted to continue his schooling as an exceptionally gifted student. His father died in 1923, and his older sister Lisbeth took care of him. In 1925, Jensen graduated from high school.

In 1926, Jensen enrolled at the University of Hamburg to study physics, mathematics, physical chemistry, chemistry, and philosophy. He also studied at the University of Freiburg im Breisgau but returned to Hamburg to earn his Ph.D. in physics in 1932. He was appointed research assistant at the Institute for Theoretical Physics at the University of Hamburg. Around the same time, Jensen married.

After Adolf Hitler’s rise to power in 1933, the University of Hamburg demanded of its faculty membership in the National Socialist (Nazi) Party or its affiliations. Although he disliked National Socialism, Jensen was pressured: His wife had supported a left-wing student group and was in danger of being expelled from the university. In 1933, Jensen reluctantly joined the National Socialist German University Lecturers League.

Life’s Work

In 1936, Jensen passed his state postdoctoral lecture qualification. In 1937, he became an assistant professor of physics at the University of Hamburg. That year, he submitted to official pressure and joined the Nazi party. However, he never embraced Nazi ideology. With colleagues, Jensen saved a Jewish physicist from deportation to a death camp. After the war, Dutch forced laborers testified to having survived only because of the care given by his wife, Dr. Elisabeth Jensen.

Jensen’s interest in theoretical and nuclear physics and international scientific exchange brought him in further conflict with Nazi ideology in the late 1930s. The Nazis rejected quantum physics in favor of their idea of Deutsche Physik (German physics), which was well behind the times.

Jensen was drafted into the army in August 1939 and served as a Wehrmacht officer in the meteorological service until his discharge in 1940. In 1941, he was appointed associate professor in physics at the Technical University of Hannover. At this time, his colleague Paul Harteck, chair of the physical chemistry department at the University of Hamburg, brought Jensen into the Uranverein (uranium club), which pursued military applications of nuclear fission.

With Harteck, Jensen began working to separate heavy uranium 238 isotopes from the lighter uranium 235 isotopes to obtain nuclear fission and weapons-grade material. Their first efforts at a diffusion approach proved impractical. By December 1941, Harteck and Jensen changed to isotope separation through a double centrifuge. Progress was reported by February 1943 in the top-secret internal papers of the Uranverein. The experimental centrifuge, moved to the central German town of Celle in 1944, was captured by Allied forces at the end of World War II in May 1945.

Cleared of active collaboration with the Nazis, in 1946, Jensen became a full professor at Hannover. He focused his research on the structure of atomic nuclei. He became an avid reader of international journals, which were rare in post–World War II West Germany. On his first postwar trip to Copenhagen in 1948, Jensen came across the August copy of the American journal Physical Review. In it, he read a paper by Maria Goeppert-Mayer. She cited strong experimental evidence for the reality of the so-called magic numbers at which isotopes of an atom were especially stable.

Inspired, Jensen pursued his own research into the atomic nucleus. In fall of 1948, Jensen became a full professor of theoretical physics at Heidelberg University. There, he began development of the Institute for Theoretical Physics. With two colleagues from other German universities, Otto Haxel from Göttingen and Hans Suess from Hamburg, Jensen discovered how the nuclear shell model could explain the structure of the atom’s nucleus. The key was to allow for strong spin-orbit coupling of the protons and neutrons. In spring of 1949, Haxel, Jensen, and Suess published their findings in three separate contributions to Germany’s journal Die Naturwissenschaften (The natural sciences). Their findings were sent also to Physical Review, received on April 18, 1949. On February 8, 1949, Physical Review had received a paper from Goeppert-Mayer, who independently reported the same theoretical discovery. Both papers were published in volume 75 of Physical Review in 1949.

Haxel, Jensen, and Suess published a longer paper on their theory in Germany’s Zeitschrift für Physik (now known as European Physical Journal) in 1950. With this, their collaboration on the subject ended. However, Jensen contacted Goeppert-Mayer. The two scientists explored their theory together, their cooperation aided by Jensen’s position as a visiting professor at five major American universities from 1951 to 1953. In 1955, Jensen and Goeppert-Mayer published their coauthored book Elementary Theory of Nuclear Shell Structure. That year, Jensen and Haxel became coeditors of the Zeitschrift für Physik.

In Germany, Jensen’s international fame persuaded officials at the University of Heidelberg to buy a house for the Institute for Theoretical Physics and to increase its faculty. Jensen took care of the institute’s garden and had personal quarters there. Rather than just hiring assistants, as was common, Jensen appointed other scientists at his own rank at the institute.

While Jensen and Goeppert-Mayer fine-tuned their nuclear shell model, Jensen became interested in the breaking of symmetry during beta-ray decay. He continued to publish with Goeppert-Mayer and on his own. In 1963, for their nuclear shell model, Jensen and Goeppert-Mayer were awarded half of the Nobel Prize in Physics. The other half of the prize was awarded to Hungarian American physicist Eugene Wigner.

Jensen continued his research into nucleus structure and other issues in nuclear physics. He fostered a strong scientific community at his institute. His last publications dealt with issues in quantum mechanics. Jensen died unexpectedly in Heidelberg on February 11, 1973. His last paper, “Radiation Reaction of a Rotating Rigid Surface-Charged Sphere,” with J. Daboul, was published in volume 265 of Zeitschrift für Physik after Jensen’s death.

Impact

The nuclear shell model as developed by Jensen and Goeppert-Mayer has exerted a tremendous impact on the study of nuclear physics. As the Nobel Prize committee noted in 1963, the pair’s discovery constituted a major advance in understanding the relationship of nuclear properties. Their model became crucial for further experimental and theoretical research of atomic nuclei.

As Israeli nuclear physicist Igal Talmi has expressed it, the nuclear shell model has been influential because its predictions have been proven by experiments. Although there is no theoretical explanation for it yet, the nuclear shell model has become generally accepted by nuclear scientists.

As a professor dedicated to the education of young scientists and an active promoter of scientific collaboration, Jensen helped turn the Institute for Theoretical Physics at the University of Heidelberg into a premier research center. He also worked hard to reintegrate the German scientific community in the free world and to achieve reconciliation with Jewish and Israeli scientists. Even though the American Manhattan Project used the method of diffusion to enrich uranium to become weapons-grade material, Jensen and Harteck’s alternative idea was used later. Despite experimental failures, their idea to use centrifuges would, in altered form, prove the most common method of enriching uranium.

Bibliography

Hoffmann, Dieter. “Between Autonomy and Accommodation: The German Physical Society during the Third Reich.” Physics in Perspective 7.3 (2005): 293–329. Print. Reviews the position of German physics during the Nazi era. Also looks at the reluctant role played by Jensen.

Powers, Thomas. Heisenberg’s War: The Secret History of the German Bomb. 1993. Cambridge: Da Capo, 2000. Print. Examines the covert role German scientist Werner Heisenberg played in thwarting Nazi plans for the atomic bomb; analyzes Jensen’s role in and contributions to Germany’s secret nuclear research project.

Simonsohn, Gerhard. “The German Physical Society and Research.” The German Physical Society and the Third Reich. Ed. Dieter Hoffmann and Mark Walker. New York: Cambridge UP, 2012. 187–245. Print. Examines Jensen’s opposition to the Nazi demand to cut all ties to German expatriate scientists.

Talmi, Igal. “Historical Survey.” Simple Models of Complex Nuclei: Shell Model and Interacting Boson Model. Langhorne: Harwood, 1993. 21–32. Print. Outlines Jensen and Goeppert-Mayer’s development of the nuclear shell model, describing key papers by both scientists that highlighted their discoveries and led to the model’s broad acceptance.