Scientists and Society

The Development of Nuclear Physics, leading to the Atomic Bomb: Timeline

1913 Niels Bohr published important papers on the atomic structure, applying quantum theory.
1919 Ernest Rutherford created oxygen by shooting an alpha particle (nucleus of Helium) at nitrogen.
1920 Rutherford suggested the existence of "the third element" (neutron) of an atom at the Royal Society.
1930 Ernest Lawrence invented cyclotron, a device for accelerating a particle.
1932 James Chadwick discovered the neutron (envisaged by Rutherford).

Leo Szilard obtained the idea that a chain reaction might be set up if an element could be found that would emit two neutrons when it swallowed one neutron. This idea became a classified British patent in 1935 (before the nuclear fission was discovered).

1934 Artificial radioactivity discovered by Frederic and Irene Joliot-Curie (bombardment with alpha particles). Around the same time, Enrico Fermi was studying bombardment with neutrons, and he was led to similar discovery. Fermi discovered "slow" neutrons, but he made a mistake: he conjectured that transuranic elements were produced (instead of considering the possibility of nuclear fission, the possibility quite hard, at this stage, for most people to conceive) by bombardment.
1935 Uranium 235 was discovered (for the "efficient" nuclear fission, this, rather than uranium 238, must be used).
1938 In December, Otto Hahn and Fritz Strassmann found the phenomenon which strongely suggested the nuclear fission of uranium by bombardment of (slow) neutrons. Hahn communicated this to Lise Meitner (former colleague then in Sweden), and asked her opinion. Meitner speculated on this with Otto Frisch; they concluded that Hahn's phenomenon was due to the nuclear fission of uranium and that this would release a high energy.

Hahn-Strassmann paper was published, and Meitner-Frisch paper (theoretical) was written.The news was brought by Bohr to the United States, before the publication of their paper. Frisch confirmed Hahn-Strassmann experiment and wrote another paper.

Bohr pointed out that the uranium fission by slow neutrons can occur only with U235.

August-- Szilard, Eugene Wigner, and Edward Teller urged Einstein to write a letter on the possibility of a uranium weapon; Roosevelt received the letter on October 11, 1939 from Sachs.


Frisch and Rudolf Peierls (in the Britain) envisaged the feasibility of the atomic bomb by means of uranium 235 (and fast neutrons)---"Frisch-Peierls Memoranda". And MAUD committee was formed for the development of atomic bomb.


Plutonium (another material for the atomic bomb) was discovered by E.M.McMillan and G.T.Seaborg (and Lawrence was eager to pursue the possibility of plutonium bomb).

MAUD Report was finished, and then transmitted to the United States. This Report concluded that (1) "the scheme for a uranium bomb is practicable and likely to lead to decisive results in the war"; that (2) "this work continue on the highest priority and on the increasing scale necessary to obtain the weapon in the shortest possible time"; and that (3) "the present collaboration with America should be continued and extended".

In October, Vannever Bush (director of Office of Scientific Research and Development) met President Roosevelt and presented the content of the MAUD Report, and the President took it; thus the project of the atomic bomb started.

Many physicists, including Lawrence, Oppenheimer, Fermi and others, began to be involved in this project.


Arthur Copmpton founded "Metallurgical Laboratory" in the University of Chicago; and Fermi and Seaborg, among others, joined. There, the first nuclear reactor was constructed, and the control of chain reaction eventually succeeded on December 2.

Oppenheimer began to organize a group of physicists for studying theoretical problems involved in making the atomic bomb; J.H.Manley, E.M.McMillan, Robert Serber, Hans Bethe, Edward Teller, among others.

In the meantime, the military organization "Manhatttan Engineer District" for the development of nuclear weapons was created in August, and General Leslie Groves began to direct the whole project. Groves met Oppenheimer in October and was impressed by his brilliant performance; within a month, Oppenheimer was nominated to the Director of a new laboratory for manufacturing the atomic bomb. Thus the Los Alamos Laboratory was born, and it was opened in April, 1943.


Army constructed huge factories for separating U235 from U238 in Oak Ridge, Tennessee; and for plutonium production piles, Hanford, Washington was chosen. All of these plants were to have an unprecedented scale.

For U235 separation, both the electromagnetic method (due to Lawrence) and the gaseous-diffusion method (Harold Urey was in charge of this, and its design was due to John Dunning) were used.

December-January 1944--British team (under Chadwick, including Frisch and Peierls) of scientists joined Los Alamos. Niels Bohr also visited USA, and Los Alamos in particular.


Two methods were pursued at Los Alamos for ignition of the atomic bomb: gun method and implosion (explosion toward the center of the bomb). The gun research for uranium bomb was in a favorable condition; but there arised grave difficulties for implosion. And worse, it turned out that gun method cannot be used for plutonium bomb. Thus Oppenheimer was forced a hard decision to concentrate on implosion method which had progressed little. But von Neumann designed "implosion lens system", and George Kistiakowsky manufactured and tested it; its design was improved by R. Christy and used for "Fat Man" (tested at Trinity and dropped on Nagasaki).

December--Plutonium production at Hanford started. The plants at Oak Ridge were, by the end of 1944, capable of producing a sufficient amount of enriched uranium.


April--experimental research both on uranium bomb and plutonium bomb were almost finished at Los Alamos.

President Roosevelt died on April 12; and New President Harry Truman was informed of the project of the atomic bomb. Target Committee (under Groves's authority) began considerations for choosing targets from Japanese major cities. In May, the Interim Committee (under President, for consulting the use of the atomic bomb) was formed; the committee added Scientific Panel consisting of Arthur Compton, Ernest Lawrence, Robert Oppenheimer, and Enrico Fermi.

In the meantime, some people in the Manhattan Project began to raise objections against the use of the bomb against Japan. Franck Report, June11; Leo Szilard's petition to President, July 17.

After a number of various crises, the Trinity experiment was "successfully" performed on July 16.

Potsdam Declaration, July 26; and Japanese government refused to accept that.

August 6, Little Boy exploded on Hiroshima. And on August 9, Fat Man on Nagasaki.

We also have to remember that toward the end of the war, atrocity increased in the form of regional (and indiscriminate) bombing; to mention a few, Luebeck, Hamburg, Dresden, Tokyo and many other cities in Japan.

Important Sites in Manhattan Project

Chicago, Illinois: Metallurgical Laboratory, University of Chicago, the first nuclear reactor

Hanford, Washington: Plutonium production complex

Oak Ridge, Tennessie: U235 separation plants

Los Alamos, New Mexico: Los Alamos National Laboratory, research, design, and construction of the atomic bomb

Trinity, New Mexico: the site of the first experiment of the (plutonium) bomb "Fat Man"

Richard Rhodes, The Making of the Atomic Bomb, Simon and Shuster, 1986.

In order to have a glimpse of the misery of the atomic bomb, visit the Maruki Gallery (Hiroshima Panels).


Last modified Oct. 3, 2010. (c) Soshichi Uchii