Nuclear History archive — page 24

A vintage brass electroscope with a round glass window beside a small porcelain dish of greenish uranium oxide powder and a cherry blossom petal

Nishina's Cyclotrons in Tokyo (1937–1940)

Yoshio Nishina's RIKEN laboratory started a cyclotron in April 1937, and in 1940 his team with chemist Kenjiro Kimura found uranium-237 and symmetric fission by fast neutrons, narrowly missing element 93.

A stack of old typed manuscript pages tied with red ribbon and sealed with a plain red wax seal, beside a fountain pen on a bright desk

The Discovery of Fission

Szilard's Campaign for Voluntary Secrecy (1939–1940)

In spring 1939 Leo Szilard urged physicists to hold back fission results; Joliot refused, but in 1940 a voluntary National Research Council committee led by Gregory Breit kept American fission papers out of print.

A chunk of dark pitchblende uranium ore resting on a blank file folder tied with red ribbon, beside a fountain pen and a teacup on a sunlit oak conference table

The Discovery of Fission

Germany's Uranium Club (1939)

In April 1939 an education-ministry meeting formed Germany's informal Uranium Club while two Hamburg chemists warned the army of a new explosive; after war began, Army Ordnance relaunched the group in September.

A curved glass vacuum tube between two steel magnet poles splits a beam into a wide silver arc and a thin amber arc that end in small brass collector cups, on a bright background

The Discovery of Fission

Bohr, Wheeler and Uranium-235 (1939)

In February 1939 Niels Bohr argued that rare U-235, not U-238, splits with slow neutrons; the Bohr–Wheeler theory published on 1 September 1939 explained why, and Nier and Dunning's separated samples confirmed it in March 1940.

A round glass flask of glowing yellow-green liquid on a white bench, with a thread of light inside branching from one point into two and then many

The Discovery of Fission

One Begets Two: Fission Neutrons, 1939

In spring 1939 Szilard and Zinn, Fermi's team and Joliot's Paris group found that each fission frees more than one neutron (Paris: 3.5 ± 0.7; today about 2.4), opening the road to a chain reaction.

A vintage green-screen oscilloscope showing a single tall spike above a flat trace, connected by a braided cable to a small brass ionization chamber on a pale wooden bench

The Discovery of Fission

Fission Confirmed Around the World, 1939

From Frisch in Copenhagen on 13 January to Columbia, Carnegie, Johns Hopkins, Berkeley and Paris by the end of January 1939, replication turned fission into an accepted fact.

A 1930s-style ocean liner sailing across a bright, calm Atlantic, its glowing wake dividing into two branches

The Discovery of Fission

Bohr Brings Fission to America (1939)

In January 1939 Bohr and Rosenfeld sailed from Gothenburg to New York with the news of fission; Rosenfeld told a Princeton seminar the night they landed, and Bohr and Fermi announced it in Washington on 26 January.

A vintage fountain pen above blank cream paper, with a drop of blue ink pinching in the middle and separating into two droplets

The Discovery of Fission

How "Fission" Got Its Name (1939)

In January 1939 Otto Frisch asked biologist William Arnold what bacteria do when they divide; 'binary fission' became 'nuclear fission', first printed in Nature on 11 February 1939.

Old wooden skis and bamboo poles leaning on a snowy birch beside parallel ski tracks and footprints, with a water droplet stretching into two

The Discovery of Fission

Meitner and Frisch Explain Fission (1938–1939)

Around Christmas 1938 Lise Meitner and Otto Frisch pictured the uranium nucleus as a wobbly liquid drop, estimated about 200 MeV per split from both electric repulsion and E = mc², and Frisch detected the fragments' large pulses on 13 January 1939.

A shallow glass crystallisation dish on a bright lab bench, filled with clear salt crystals, a few of them glowing faintly teal, beside a glass stirring rod

The Discovery of Fission

It Wasn't Radium, It Was Barium

In December 1938 Hahn and Strassmann proved that their supposed radium isotopes behaved exactly like barium, the chemical evidence that the uranium nucleus had split.