The Mechanism of Nuclear Fission

The potential energy associated with any arbitrary deformation of the nuclear form may be plotted as a function of the parameters which specify the deformation, thus giving a contour surface which is represented schematically in the left-hand portion of the figure.<br /> The pass or saddle point corresponds to the critical deformation of unstable equilibrium. To the extent to which we may use classical terms, the course of the fission process may be symboiized by a ball lying in the hollow at the origin of coordinates (spherical form) which receives an impulse (neutron capture) which sets it to executing a complicated Lissajous figure of oscillation about equilibrium. If its energy is sufficient, it will in the course of time happen to move in the proper direction to pass over the saddle point (after which fission will occur), unless it loses its energy (radiation or neutron re-emission). At the right is a cross section taken through the fission barrier, illustrating the calculation in the text of the probability per unit time of fission occurring.

Niels Bohr and John Archibald Wheeler
On the basis of the liquid drop model of atomic nuclei, an account is given of the mechanism of nuclear fission. In particular, conclusions are drawn regarding the variation from nucleus to nucleus of the critical energy required for fission, and regarding the dependence of fission cross section fo’r a given nucleus on energy of the exciting agency. A detailed discussion of the observations is presented on the basis of the theoretical considerations. Theory and experiment fit together in a reasonable way to give a satisfactory picture of nuclear fission.
Read more at https://journals.aps.org/pr/pdf/10.1103/PhysRev.56.426
Phys. Rev. 56, 426 – Published 1 September 1939

John Archibald Wheeler: A Biographical Memoir

John Wheeler as a postdoc of Niels Bohr in
Copenhagen, 1934

Kip S. Thorne
John Archibald Wheeler was a theoretical physicist who worked on both down-to-earth projects and highly speculative ideas, and always emphasized the importance of experiment and observation, even when speculating wildly. His research and insights had large impacts on nuclear and particle physics, the design of nuclear weapons, general relativity and relativistic astrophysics, and quantum gravity and quantum information. But his greatest impacts were through the students, postdocs, and mature physicists whom he educated and inspired.
He was guided by what he called the principle of radical conservatism, inspired by Niels Bohr: base your research on well established physical laws (be conservative), but push them into the most extreme conceivable domains (be radical). He often pushed far beyond the boundaries of well understood physics, speculating in prescient ways that inspired future generations of physicists.
After completing his PhD with Karl Herzfeld at Johns Hopkins University (1933), Wheeler embarked on a postdoctoral year with Gregory Breit at NYU and another with Niels Bohr in Copenhagen. He then moved to a three-year assistant professorship at the University of North Carolina (1935-37), followed by a 40 year professorial career at Princeton University (1937-1976) and then ten years as a professor at the University of Texas, Austin (1976-1987). He returned to Princeton in retirement but remained actively and intensely engaged with physics right up to his death at age 96.
Read more at https://arxiv.org/ftp/arxiv/papers/1901/1901.06623.pdf