Nuclear physics and radioactivity

Old black-and-white photograph of a woman and a man standing in a laboratory. — Lise Meitner and Otto Hahn in their laboratory at the Kaiser Wilhelm Institute for Chemistry, Berlin, in 1913. Unknown author, Public domain, source

Christmas 1938, in Sweden. A woman walks in the snow with her nephew and understands, within a few hours, what will change the course of the century.

Her name is Lise Meitner. The Nobel Prize that will reward this fundamental discovery will be awarded seven years later — without her name on it. This is the story of a scientific revolution, and of the institutional injustice that followed it.

First, what is this about?

The heart of an atom, its nucleus, is made up of particles tightly bound together. In most elements, this nucleus is stable: it undergoes no transformation over time.

In others, the structure is unstable. An unstable nucleus eventually transforms spontaneously, at a moment that cannot be predicted, by ejecting a particle or an electromagnetic wave. This phenomenon is called radioactivity. Uranium is one of the best-known examples.

In 1934, in Berlin, a question was occupying the laboratories: what happens when a uranium nucleus is bombarded with a neutron?¹

The work of a team

From the mid-1930s, three researchers worked together, each with a distinct contribution:

Lise Meitner, physicist;
Otto Hahn, seasoned chemist;
Fritz Strassmann, chemist and laboratory assistant.

For several years, they bombarded uranium with neutrons, convinced that they were producing artificial elements heavier than uranium (the transuranium elements). They were on the wrong track, but at the time the whole scientific community shared this error.

The political rupture and exile

In July 1938, the political situation upended the collaboration.²

An Austrian of Jewish origin, Lise Meitner was directly threatened by the racial laws of the Nazi regime following the Anschluss. Her conversion to Protestantism thirty years earlier offered her no protection. With the crucial help of the Dutch physicist Dirk Coster, she fled Germany in secret and took refuge in Sweden.³

Note: Fritz Strassmann would also pay for his firm opposition to the Nazi regime: shut out of university posts, he would work for years on half pay, or even with no pay at all.⁴

The discovery by correspondence

The key explanation took shape over six crucial weeks:

13 November 1938 — Hahn and Meitner met secretly in Copenhagen. She suggested rigorous additional experimental protocols to him.
December 1938 — In Berlin, Hahn and Strassmann carried out these tests and detected the presence of barium, an element roughly half as heavy as uranium. The result was incomprehensible: the physics of the time did not envisage that a nucleus could break in two.
19 December 1938 — Hahn wrote urgently to Meitner asking her to find a theoretical explanation, however outlandish.
Christmas 1938 (Kungälv, Sweden) — Meitner and her nephew, the physicist Otto Frisch, went for a walk in the snow to discuss it: he on skis, she on foot. They sat down on a tree trunk to calculate.⁵

Meitner proposed seeing the nucleus as a drop of liquid. Frisch, who was better at picturing things, sketched what would happen: the drop, struck by a neutron, elongates, pinches in the middle, then splits in two. The two pieces, electrically charged, repel each other violently.⁶

The energy calculation — One question remained, raised by Frisch: where would this energy come from? Meitner calculated from memory that the two daughter nuclei would, together, be lighter than the original nucleus by about one-fifth of the mass of a proton. This missing mass, entered into Einstein’s formula linking mass and energy, gives about 200 million electronvolts.⁷

The two calculations — that of the electrical repulsion and that of the missing mass — arrive at the same number. Everything fits.

January 1939 — Frisch confirmed the experiment in the laboratory in Copenhagen. He adopted the term fission, after learning that biologists used this name for the division of a cell in two.⁸

In their article, Meitner and Frisch write that the uranium nucleus has only small stability of form, and that it may, after neutron capture, divide itself into two nuclei of roughly equal size — the precise ratio depending on finer structural features, and perhaps partly on chance.⁹

Two publications, unequal treatment

6 January 1939: Hahn and Strassmann published the chemical results in the journal Naturwissenschaften. Meitner’s name was left out.
11 February 1939: Meitner and Frisch published the fundamental physical interpretation in the journal Nature.
November 1945: The 1944 Nobel Prize in Chemistry (awarded late) was announced for Otto Hahn alone. Yet the statutes of the Nobel Foundation allowed the prize to be awarded jointly to all three researchers.¹⁰

Three documented facts

49 nominations: Lise Meitner was nominated 49 times for the Nobel Prize (30 times in physics and 19 times in chemistry) without ever receiving it.
What she said about it herself: her position is more nuanced than that of her defenders, and deserves to be quoted. “Surely Hahn fully deserved the Nobel Prize for chemistry. There is really no doubt about it. But I believe that Frisch and I contributed something not insignificant to the clarification of the process of uranium fission—how it originates and that it produces so much energy, and that was something very remote from Hahn.”¹¹

She does not claim Hahn’s prize. She claims her own. Chemistry had found the barium; physics had explained why. Two prizes existed; only one was given.

Late recognition: In 1966, the United States Atomic Energy Commission jointly awarded the prestigious Enrico Fermi Award to Hahn, Meitner and Strassmann — a distinction shared, this time, between chemistry and physics. The 1944 Nobel Prize, for its part, has remained Hahn’s alone.¹²

The aftermath of the discovery: from theory to history

The story does not end with this academic injustice: the discovery at Kungälv set off, at unprecedented speed, a chain of events on a global scale.

The chain reaction — When a uranium nucleus breaks apart, it releases further neutrons, which can in turn split other neighbouring nuclei.
The first reactor (1942) — In December 1942, in Chicago, Enrico Fermi’s team achieved the first controlled chain reaction (Chicago Pile-1).
Military use (1945) — In August 1945, less than seven years after the walk in the snow, two atomic bombs destroyed Hiroshima and Nagasaki.
Civil energy (1951-1954) — The first nuclear-generated electricity was produced in 1951 (EBR-I reactor, USA), and the first power station to supply electricity to a power grid was commissioned in 1954 at Obninsk (USSR).¹³

Methodological note on radioactivity measurements

When dealing with the impact of radioactivity, precision about units is essential to avoid any confusion:

The becquerel (Bq): Measures raw activity, that is, the number of nuclear decays per second.
The sievert (Sv): Assesses the biological impact of radiation on the human body.¹⁴

Caution with figures: The sievert represents an estimate of average risk calculated for a population. It does not predict an absolute individual outcome. Any figure on radioactivity published without its unit and its assessment context is scientifically unusable.

In memory

On Lise Meitner’s grave, in Bramley (England), her nephew Otto Frisch composed this epitaph:¹⁵

“Lise Meitner: a physicist who never lost her humanity.”

Note 1

Upon returning to Berlin she was made head of a physics section at the KWI, where she did research in nuclear physics. […] In 1934 Enrico Fermi bombarded uranium with neutrons, producing what he thought were the first elements heavier than uranium. […] Following Fermi’s work, Meitner and Hahn, along with chemist Fritz Strassmann, also began bombarding uranium and other elements with neutrons and identifying the series of decay products.

Chodos, Alan, “December 1938: Discovery of Nuclear Fission”, APS News, “This Month in Physics History”, 3 December 2007