Physicists Uncover Forgotten 1938 Fusion Breakthrough That Could Power the Future

Physicists confirm DT fusion insights from a 1938 experiment. The findings connect past theory with current fusion efforts.

A team at Los Alamos National Laboratory has successfully recreated a significant yet largely overlooked physics experiment: the first recorded observation of deuterium-tritium (DT) fusion. Their updated version of the 1938 experiment, recently detailed in Physical Review C, reaffirms the pivotal role of University of Michigan physicist Arthur Ruhlig. Ruhlig’s original work likely laid the foundation for a fusion process that continues to influence both nuclear energy development and national security programs.

“Ruhlig’s key insight was proposing that DT fusion occurs with a very high probability when deuterium and tritium are brought into close proximity,” explained Mark Chadwick, associate Laboratory director for Science, Computation and Theory at Los Alamos. “By replicating his experiment, we were able to revisit his original conclusions and appreciate how accurate they were. His intuition had a lasting impact on the direction of nuclear fuel research.”

The DT fusion reaction remains fundamental to advancing fusion-based technologies, including its critical role in both defense applications and future clean energy solutions. The reaction forms the basis of projects like those at the National Ignition Facility, where researchers are working to achieve controlled fusion. Motivated by a hypothesis that Ruhlig may have originated the concept, Los Alamos scientists designed an experiment to rigorously test and validate the significance of his early work.

Tracking down the origin of DT fusion

In 2023, Mark Chadwick and his colleagues, including theoretical physicist Mark Paris, were compiling a detailed history of early fusion research. A notable point in that timeline involves a suggestion made by physicist Emil Konopinski during a July 1942 physics conference in Berkeley, led by J. Robert Oppenheimer, who would later direct the Manhattan Project. At that meeting, Konopinski proposed that among several possible fusion reactions, deuterium-tritium (DT) fusion held particular promise for use in conjunction with fission-based weapons.

Curious about how Konopinski arrived at that conclusion so early in the project—just months after the Manhattan Project had formally begun—Chadwick and his Los Alamos team began investigating. Selecting DT fusion as the most viable option among many potential reactions proved to be a pivotal and insightful decision.

One evening, while searching through archives at the National Security Research Center, Chadwick discovered a 1986 audio recording of Konopinski discussing his rationale for pursuing DT fusion. (The recording has since been shared on YouTube.) In the recording, Konopinski reflects on the early days of nuclear research and repeatedly credits his interest in DT fusion to what he called “pre-war” studies.

Tritium, a key component in DT fusion, was first discovered in 1934 by a research team led by experimental physicist Ernest Rutherford. Rutherford was a central figure in early atomic theory, collaborating with Niels Bohr and supervising James Chadwick in the discovery of the neutron. Starting from the year tritium was discovered, Paris combed through physics publications and eventually found a 1938 letter to the editor in Physical Review written solely by Arthur Ruhlig. The letter described a gamma-ray experiment and hinted at something more.

In the experiment, Ruhlig investigated deuterium-on-deuterium reactions by firing a beam of deuterons at deuterium and analyzing the resulting gamma-ray emissions. (A deuteron is the nucleus of a deuterium atom, consisting of one proton and one neutron.) In a brief but intriguing comment in the letter’s final paragraph, Ruhlig reported detecting high-energy protons, which he believed originated from secondary interactions. He concluded that these were caused by neutrons from tritium-deuterium fusion scattering protons from a thin cellophane film placed inside a cloud chamber. Ruhlig referenced a private discussion with physicist Hans Bethe as part of his reasoning. He concluded that DT fusion “must be an exceedingly probable one,” and estimated that about one in every 1,000 energetic protons resulted from such reactions.

And there the matter dropped; Ruhlig’s paper was infrequently cited, with the few citations bearing mostly on the gamma-ray issues. But Konopinski appears to have remembered the work.

Paris and Chadwick put together the pieces: As it happens, Ruhlig and Konopinski were both University of Michigan students, overlapping in their doctoral studies in the 1930s. Ruhlig’s thesis adviser, Richard Crane, was a colleague of Bethe, and Konopinski served on a research fellowship overseen by Bethe. They also shared a mentor in University of Michigan physicist George Uhlenbeck, co-discoverer of electron spin. And though Ruhlig’s paper was not often cited, that does not necessarily mean it was unknown — the journal would have been part of many physicists’ regular reading.

“The evidence for Konopinski interpreting and taking up Ruhlig’s suggestion of the probability of DT fusion is circumstantial, but nonetheless strong,” Paris said. “We’re left to ask, what did Ruhlig actually observe? Are his conclusions consistent with what we would arrive at with a computational approach and an understanding of modern cross sections? Ultimately, the way to answer those remaining questions is to replicate the experiment.”

Chadwick mentioned the Ruhlig paper, and their theories about the 1938 experiment’s role in the development of DT fusion, to Lab Director Thom Mason, who insisted on the team conducting an experiment — not just a simulation — to validate their conclusions.

Replicating the experiment

The team collaborated with experimental physicists from Duke University, based at the Triangle Universities Nuclear Laboratory in North Carolina, to replicate Ruhlig’s work with a modern, rigorously executed duplication of the original experiment. The reproduction would be accompanied by theoretical and computational analysis.

The team used the laboratory’s Tandem accelerator at its lowest operating power, producing a 3.5-mm deuteron beam. They paired that beam with a thin, cobalt-alloy foil between the accelerator vacuum and target that effectively duplicated as best as possible Ruhlig’s 500 keV beam. As in 1938, the beam was directed at a target of deuterated phosphoric acid, with a liquid scintillator neutron detector tracking the neutrons of interest to gauge the secondary reactions.

“In contrast to fusion experiments such as in the inertial confinement fusion efforts at the National Ignition Facility, we were able to perform, for the first time at a low-energy nuclear physics facility, a DT fusion experiment as a secondary reaction following the initial deuterium-deuterium interaction which provides the tritium,” said Werner Tornow, Duke University physicist for the Triangle Universities Nuclear Laboratory. “This work helps answer some intriguing questions about physics history, but it’s also impactful in extending our ability to work with DT fusion in a considerably more challenging environment.”

Confirming Ruhlig’s essential observations

In analyzing their results, the modern experiment did observe secondary DT reactions, although it also suggests that Ruhlig overestimated the ratio at which he was seeing excess neutron production, the products of fusion; the researchers detected a much smaller ratio. As Ruhlig’s 1938 letter describing the experiment provides only sparse details as to how he arrived at his determination, though, it is ultimately difficult to decisively gauge the Michigan physicist’s accuracy against the modern results. The team’s calculated value using modern methods did agree with the measure value gleaned from the replicated experiment.

Importantly, the measurements derived from the experimental techniques employed by Ruhlig and re-tested by the Los Alamos and Triangle Universities Nuclear Laboratory researchers can be applied to active fusion efforts such as at NIF.

“Regardless of the inconsistency of Ruhlig’s rate of fusion against our modern understanding, our replication leaves no doubt that he was at least qualitatively correct when he said that DT fusion was ‘exceedingly probable,’” Chadwick said. “Ruhlig’s accidental observation of DT fusion, together with subsequent Manhattan Project cross section measurements, contributed to the peaceful application of DT fusion in tokamaks focused on energy projects and in inertial confinement fusion experiments like NIF. I think we’re all proud to lift Arthur Ruhlig up again out of history as an important contributor to ongoing, vital research.”

Notably, the team published its results in Physical Review — the same journal that published Ruhlig’s first observation of DT fusion in 1938.

References: “Modern version of the uncited 1938 experiment that first observed DT fusion” by W. Tornow, S. W. Finch, J. B. Wilhelmy, M. B. Chadwick, G. M. Hale, J. P. Lestone and M. W. Paris, 20 June 2025, Physical Review C.
DOI: 10.1103/PhysRevC.111.064618

“A lost detail in D–T fusion history” by Mark W. Paris and Mark B. Chadwick, 1 October 2023, Physics Today.
DOI: 10.1063/PT.3.5317

“Early Nuclear Fusion Cross-Section Advances 1934–1952 and Comparison to Today’s ENDF Data” by M. B. Chadwick, M. W. Paris, G. M. Hale, J. P. Lestone, S. Alhumaidi, J. B. Wilhelmy and N. A. Gibson, 17 April 2024, Fusion Science and Technology.
DOI: 10.1080/15361055.2023.2297128

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