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note: because important websites are frequently "here today but gone tomorrow", the following was archived from on July 21, 2005. This is NOT an attempt to divert readers from the aforementioned website. Indeed, the reader should only read this back-up copy if it cannot be found at the original author's site.
15 - Dawn of the Atomic Age
Last changed April 1997
A New Century - A New Universe
The turn of the Century marked a profound revolution in the development of Science and our understanding of the fundamental principles of the natural world.
During the 19th Century, Classical Physics (the laws of motion, electromagnetic fields, and thermodynamics) had reached an advanced state of development. Chemistry had also reached a considerable degree of sophistication but on a largely empirical basis. The fundamental basis of chemistry remained mysterious. Much had been learned about the Earth and solar system as well. Estimates of the age of the Earth had risen from about 6,000 years in the late 18th Century to tens-or-hundreds of millions-of-years. And the view that Life, the Earth, and the rest of the Solar System had arisen in a single great upheaval in recent times had been replaced by the idea of gradual change over eons.
To some, it seemed that Science (especially Physics) was reaching such a state of maturity that few fundamental principles remained to be discovered. But there were problems. Essentially nothing was known about the fundamental structure of matter that gave rise to the Periodic Law and other chemical behaviors. The very existence of atoms was largely conjectural. Geology and Astronomy seemed in serious conflict since the apparent age of the geologic record could not be reconciled with the only power source for the Sun then conceivable -- gravitational contraction -- which would exhaust itself in mere millions of years. An important part of classical thermodynamics was stubbornly resisting resolution -- the properties of blackbody radiation. In fact, by the end of 1900s it had become clear that within the existing framework of physics no solution of the blackbody problem was possible (the untenable prediction made by existing physics was termed the "ultraviolet catastrophe"). Something important was missing.
Advancing experimental technique in the seemingly well-understood field of Electricity and Magnetism gave the first clues to the new universe. In 1895, Wilhelm Konrad Roentgen at the University of Wurzburg discovered X-rays. He had been conducting experiments involving high-voltage currents in evacuated tubes. The penetrating radiation that he discovered was wholly new and unexpected.
The following year (1896) -- by serendipitous accident while investigating X-rays -- HenriBecquerel (at the Museum of Natural History in Paris) discovered radioactivity in a piece of uranium salt. This discovery provided for the first time direct evidence of the fundamental structure of matter and also revealed the existence a totally new source of energy independent of the Sun's rays or of chemical fuels. And vastly more concentrated than either.
Discoveries followed rapidly. Marie Skladowska Curie and her husband Pierre Curie immediately began isolating sources of radiation from uranium ore. This led to the discovery of Polonium in 1896 and Radium in 1897. Different types of radioactive emissions were soon identified. In 1899, Becquerel found that at least some of the radiation emissions were electrically-charged. Ernest Rutherford further distinguished 2 types of charged emissions -- alpha and beta rays. Paul Villard identified neutral gamma rays.
During this time, another key discovery was in the making -- the development of Quantum Theory. 2 threads led to the foundation of the theory: one theoretical and one experimental. The theoretical development was by Max Planck at the University of Berlin. In pursuing the perplexing problem of blackbody radiation, he developed a theory announced in 1900 that successfully predicted the observed blackbody spectrum. This theory postulated that matter could only absorb or emit energy in arbitrary units or "quanta".
In 1898, J.J. Thomson detected the emission of electrons when a metal surface is illuminated by ultraviolet light -- the photoelectriceffect. The properties of this phenomenon could not be explained -- particularly a metal-dependent frequency threshold for the emissions.
Albert Einstein united these threads with his theory of the photoelectric effect in 1905 which proposed the existence of the photon -- quantized light (for which he received the Nobel Prize). Also in 1905, Einstein formulated his Special Theory of Relativity, one aspect of which (the equivalence of mass and energy) began to give some insight into the origin of the atomic energy that had been revealed by the discovery of radioactive decay.
These developments had also greatly extended the understanding of the Earth and Sun. In 1905, Rutherford and Boltwood used the ratio between radioactive isotopes and their decay products to date a rock to 500 million years old. This great age sharpened the conflict with classical theories of solar development. But radioactivity also offered a resolution. Perhaps some atomic transformation process -- not then understood -- was the source of the Sun's brilliance and longevity.
The New Universe Explored
With the hints given by these new discoveries and the powerful new probes of matter offered by the newly-discovered ionizing radiations, more discoveries followed swiftly.
Rutherford soon demonstrated that alpha particles were, in fact, Helium atoms minus their electrons.
In 1906, Rutherford began a series of experiments at McGill University where he was now professor and continued at the University of Manchester. In these experiments, he studied how alpha rays were scattered by thin layers of mica and gold.
The age of the Earth jumped again in 1907 when Boltwood identified a piece of uraninite as being 1.64 billion years old.
In 1911, Rutherford published his conclusions drawn from the alpha scattering experiments -- that nearly all of the mass of the atom is concentrated in a tiny positively charged region in the center called the nucleus.
J.J. Thomson discovers isotopes of Neon in 1912 -- showing that the atoms of the same element could have different masses.
Although it was realized late in the 19th Century that the identities of chemical elements were related to the number of electrons that each atom contained (the atomic number), it was difficult to determine this number accurately for most elements. In 1913, H.G.J. Moseley demonstrated that by studying X-ray emissions, the atomic number could be easily measured.
It was now possible to study the relationship between the atomic charge (the atomic number) and the atomic mass. Evidence began to accumulate that there were 2 principal contributors to the mass of the atom and the nucleus -- one that was positively-charged (later called the proton) and one that was neutral (the neutron).
Also in 1913, Niels Bohr made a key theoretical breakthrough. He devised the "Bohr atom" -- a planetary model of the hydrogen atom with the electron orbiting the positively-charged nucleus -- that explained studying the spectrum of light emitted by hydrogen atom. This model was based on the quantum theory and was consistent with the atomic structure observed by Rutherford.
Although Physics and Science continued to advance (Einstein completed the General Theory ofRelativity during this period, for example), there was a temporary doldrum in key discoveries about the structure of matter lasting for several years. This is partly explainable by the calamity of the First World War that disrupted all of Europe. Some of the destructive effects of the war on Science were quite direct. The young genius Moseley perished in the trenches of Gallipolli.
Discovery of the Neutron
On June 3, 1920, Ernest Rutherford gave his second Bakerian Lecture in London. In the course of this lecture, he speculated on the possible existence and properties of the neutron. This is apparently the earliest public proposal of the idea of positive and neutral particles composing the atomic nucleus.
In 1921, the American chemist H.D. Harkins coined the term "neutron" in a proposal of nuclear structure. Rutherford published further work on the idea in this same year. Little progress was made on developing the idea or proving its existence for the next several years.
In 1930, two German physicists -- W. Bothe and H. Becker -- observed unusually penetrating radiation being emitted from beryllium metal when it was bombarded by alpha particles. On December 28, 1931, Irene Joliot-Curie (Marie and Pierre's daughter) reported on these same emissions but -- like Bothe and Becker -- believed them to be energetic gamma rays. Joliot-Curie discovered that these emissions produced large numbers of protons when they passed through paraffin or other hydrogen containing materials -- something never observed (and apparently impossible to explain) with gamma rays.
Over a 10-day period from February 7-to-17, 1932, James Chadwick conducted a series of experiments that conclusively demonstrated that these unusual emissions were actually neutrons. Using this new potent new tool, rapid progress on the structure of matter began to be made.
Although radioactive decay releases an enormous amount of energy compared to chemical processes, this energy release is gradual and cannot be modified to any significant degree. The possibility of "atomic energy" as a source of human-controlled power thus came into existence as a concept but without any known means of bringing it about -- even in theory. On September 12, 1933, this changed.
On that day, the brilliant Hungarian physicist Leo Szilard conceived the idea of using a chainreaction of neutron collisions with atomic nuclei to release energy. He also considered the possibility of using this chain reaction to make bombs. These insights predate the discovery of an actual chain reaction process (fission) by more than 6 years.
Invention and Discovery: Atomic Bombs and Fission
Last changed April 1997
Leo Szilard and the Invention of the Atomic Bomb
It would be logical to assume that the discovery of fission preceded the invention of the atomic bomb. It would be normal also to expect that no single individual could really claim to be "the inventor" since the possibility sprang naturally from a physical process and required the efforts of many thousands to bring it into existence. Many descriptions of the origin of atomic bombs can be found that logically and normally say exactly these things. But they are not correct.
The idea of "invention" does not usually require the physical realization of the invented thing. This fact is clearly recognized by patent law which does not require a working model in order to award a patent. It is common for inventions to require additional discoveries and developments before the actual thing can be made. In these cases, an invention may fairly have more than one inventor -- the originator of the principle idea and the individual who actually made the first workable model.
In the case of the atomic bomb, there is clearly one man who is the originator of the idea. He was also the instigator of the project that led ultimately to the successful construction of the atomic bomb and was a principal investigator in the early R&D both before and after the founding of the atomic bomb project -- making a number of the key discoveries himself. By any normal standard, this man is the inventoroftheatomicbomb.
This man is Leo Szilard.
On September 12, 1932 -- within 7 months of the discovery of the neutron and more than 6 years before the discovery of fission -- Leo Szilard conceived of the possibility of a controlledrelease of atomic power through a multiplying neutron chain reaction. He also realized that if such a reaction could be found, then a bomb could be built using it.
On July 4, 1934, Leo Szilard filed a patent application for the atomic bomb. In his application, Szilard described not only the basic concept of using neutron-induced chain reactions to create explosions but also the key concept of the critical mass. The patent was awarded to him, making Leo Szilard the legally-recognized inventor of the atomic bomb.
Szilard did not patent this prescient and tremendously important idea for personal gain. His motive was to protect the idea to prevent its harmful use for he immediately attempted to turn the idea over to the British government for free so that it could be classified and protected under British secrecy laws.
On October 8, 1935, the British War Office rejected Szilard's offer. But a few months later in February 1936, he succeeded in getting the British Admiralty to accept the gift. Szilard's actions in attempting to restrict the availability of the atomic bomb are also the earliest case of nuclear armscontrol. Later when the possibility of a German atomic bomb had been shown to be nonexistent, Szilard campaigned vigorously against the use of the bomb and went on to help found The Bulletin of Atomic Scientists and The Council for a Livable World.
The Discovery of Fission
With the discovery of the neutron by James Chadwick in February 1932, a scientific "gold rush" ensued to discover what effects would be produced by bombarding different materials with this new particle. Over the next several years, teams of researchers in several countries (especially one headed by Enrico Fermi in Rome) bombarded every known element with neutrons and recorded scores (even hundreds) of new radioactive isotopes.
On May 10, 1934, Fermi's research group published a report on experiments with neutron bombardment of Uranium. This was the first such investigation to be reported on. Several radioactive products are detected, but positive identifications were not made. Interpreting the results of neutron bombardment of Uranium became known as the "Uranium Problem" since the large number of different radioactivities produced defied rational explanation. The dominant theory was that a number of trans-Uranic elements never before seen were being produced. But the chemical behavior as well as the nuclear behavior of these substances were unexpected and confusing.
The first statement of the correct resolution of the Uranium Problem was published by German chemist Ida Noddack in September. Her letter in "Zeitshrift fur Angewandte Chemie" argued that the anomalous radioactivities produced by neutron bombardment of uranium may be due to the atom splitting into smaller pieces. No notice of this suggestion was taken.
Fermi discovered the extremely important principle of neutron behavior called "moderation" on October 22, 1934. Moderation is the phenomenon of enhanced capture of low-energy neutrons as when they are slowed down by repeated collisions with light atoms.
December 1935 -- Chadwick won the Nobel Prize for discovery of the neutron.
November-December 1938 -- Otto Hahn and Lise Meitner correctly unravel the Uranium Problem. Hahn determines conclusively that one of the mysterious radioactivities is a previously known isotope of Barium. Working with Meitner, they develop a theoretical interpretation of this demonstrated fact. On December 21, 1938, Hahn submits a paper to "Naturwissenschaften" showing conclusive evidence of the production of radioactive barium from neutron-irradiated Uranium (i.e., evidence of fission).
In the first few weeks of January, word of the discovery traveled quickly in Europe.
January 13, 1939 -- Otto Frisch observed fission directly by detecting fission fragments in an ionization chamber. With the assistance of William Arnold, he coins the term "fission".
By mid-January, Szilard heard about the discovery of fission from Eugene Wigner and immediately realized that the fission fragments -- due to their lower atomic weights -- would have excess neutrons which must be shed. The multiplying neutron chain reaction that he had postulated had finally been discovered.
January 26, 1939 -- Niels Bohr publicly announces the discovery of fission at an annual theoretical physics conference at George Washington University in Washington, DC. This announcement was the principal revelation of fission in the United States.
January 29, 1939 -- Robert Oppenheimer hears about the discovery of fission. Within a few minutes, he realized that excess neutrons must be emitted and that it might be possible to build a bomb.
February 5, 1939 -- Niels Bohr gained a crucial insight into the principles of fission -- that U235 and U238 must have different fission properties that U238 could be fissioned by fast neutrons (but not slow ones) and that U235accounted for observed slow fission in uranium.
At this point, there were too many uncertainties about fission to see clearly whether or how self-sustaining chain reactions could arise. Key uncertainties were:
1. The number of neutrons emitted per fission, and
2. The cross-sections for fission and absorption at different energies for the uranium isotopes.
For a chain reaction, there would need to be both a sufficientexcess of neutrons produced and the ratio between fission to absorption averaged over the neutron energies present would need to be sufficiently large.