10 Discoveries During the Atomic Age

1896 – The discovery of radioactivity

Photographic plate made by Henri Becquerel showing effects of exposure to radioactivity.
Becquerel’s photographic plate showing the exposure to radiation from uranium salt.
Credit: Wikimedia Commons // Public Domain

In 1896, Henri Becquerel discovered radioactivity while testing uranium salts during phosphorescence experiments. Unlike other salts, the uranium salts blackened the photographic plate he used, leading him to remark: “these rays, whose effects have a great similarity to the effects produced by the rays studied by M. Lenard and M. Röntgen, are invisible rays emitted by phosphorescence and persisting infinitely longer than the duration of the luminous rays”. Further research and experiments – by Becquerel, Ernest Rutherford, Paul Villard, and Pierre and Marie Curie – showed that this radioactivity was different from the X-Rays discovered by Wilhelm Röntgen.

1898 – Marie Curie finds more radioactive elements

Pierre and Marie Curie in the laboratory, demonstrating the experimental apparatus used to detect the ionisation of air, and hence the radioactivity, of samples of purified ore which enabled their discovery of radium.
Pierre and Marie Curie in a laboratory, circa 1904.
Credit: Wikimedia Commons // Public Domain

Marie Curie experimented with uranium following Becquerel’s discovery, finding that the activity around uranium depended on quantity rather than activity. This led to her hypothesis that radiation came from the atom itself, not due to the interaction of molecules. She studied pitchblende and torbernite, two uranium minerals, and found that they were more active than uranium itself, so must contain another, stronger radioactive mineral. In 1898, after testing numerous substances, she found thorium was also radioactive. Working with Pierre, they also announced the existence of polonium and radium, and officially gave radioactivity its name.

1903 – Ernest Rutherford discovers gamma rays

Ernest Rutherford in 1905.
Ernest Rutherford in 1905 at McGill University.
Credit: Wikimedia Commons // CC BY 4.0

In 1899, Ernest Rutherford coined the terms ‘alpha ray’ and ‘beta ray’ to describe two known types of radiation. Four years later, he studied Paul Villard’s findings on radium radioactivity and added ‘gamma ray’ as a third radiation type. Rutherford noted that the penetrating power of the radioactivity showed greater strength than beta rays, as gamma rays were not easily deflected by magnetic fields, unlike the other types.

1910s – Discovery of isotopes

Frederick Soddy first theorized the existence of isotopes in 1913, noting that uranium decays to radium and a radioactive element can have more than one atomic mass while the chemical properties remain identical. In the same year, Soddy and Kazimierz Fajans individually discovered that atoms moved lower in atomic number by two places with alpha decay and higher by one place with beta decay. This became known as the law of radioactive displacement or Fajans’ and Soddy’s law. Others built off of Soddy’s theory to discover more stable and radioactive isotopes.

1928 – Creation of the Geiger counter

Geiger Counter
An early Geiger counter made by Hans Geiger later used by James Chadwick.
Credit: Wikimedia Commons // CC BY-SA 2.0

The Geiger counter (or Geiger-Müller counter) is one of the most well-known radiation detectors still in production. In 1928, Hans Geiger built on a concept of a detector (supervised by Rutherford in 1908) with his student Walther Müller. Together they devised a sealed tube to use experimental ionization principles. This became the Geiger-Müller counter, capable of detecting alpha, beta, and gamma radiation.

1932 – Neutrons are discovered

Ernest Rutherford’s model of an atom was devised in 1911 and consisted of protons and electrons. In 1932, physicist James Chadwick discovered the neutron, a nuclear particle with no electrical charge. An experiment by Frédéric and Irène Joliot-Curie claimed that they had managed to knock protons from paraffin wax using beryllium and polonium, but Rutherford and Chadwick disagreed as protons were too heavy. Chadwick concentrated on proving the existence of the neutron by performing experiments – only two weeks later he submitted a letter on “Possible Existence of a Neutron”. 

1938 – The discovery of nuclear fission

Lise Meitner seated second from the far left at the 7th Solvay Conference in 1933.
Lise Meitner seated second from the far left at the 7th Solvay Conference in 1933.
Credit: Wikimedia Commons // Public Domain

Physicists Lise Meitner and Otto Robert Frisch worked with chemists Otto Hahn and Fritz Strassman to discover nuclear fission. Hahn and Strassman bombarded uranium with slow neutrons in their experiments and reported the production of barium to Meitner. She theorized with Frisch that the uranium nuclear had split and proved it in the next year. Frisch termed it nuclear fission after the concept in the biology of fission, the division of one cell into two. Hahn was awarded the 1944 Nobel Prize in Chemistry for “his discovery of the fission of heavy atomic nuclei”, despite Meitner’s contributions having led to it.

1939 – Einstein and Szilárd fear the creation of atomic bombs

Einstein - Roosevelt Letter
The Einstein-Szilárd letter sent to President Roosevelt.
Credit: Wikimedia Commons // Public Domain

Leó Szilárd, in partnership with Enrico Fermi, began to work on a nuclear reactor using natural uranium in the 1930s. Following a letter by Siegfried Flügge on exploiting nuclear energy in 1939, Szilárd and fellow physicist Eugene Wigner decided to warn Belgium, who had the best source of uranium ore, by communicating with Albert Einstein. When they explained to Einstein the possibility of atomic bombs, Einstein confessed he hadn’t thought of that, and agreed to sign a letter to the Belgian Ambassador and President Roosevelt. This ultimately resulted in the creation of the Manhattan Project to develop the first atomic bombs.

1941 – Plutonium is discovered

Glenn T. Seaborg and John F. Kennedy in 1961
Glenn T. Seaborg while Chairman of the United States Atomic Energy Commission with President Kennedy in 1961.
Credit: Wikimedia Commons // Public Domain

Plutonium was first officially produced, isolated, and identified chemically by Glenn T. Seaborg, Edwin McMillan, Emilio Segrè, Joseph W. Kennedy, and Arthur Wahl. They bombarded uranium with deuterons which initially produced neptunium before beta-decay formed a new element. It was found that plutonium was fairly stable but undergoes alpha-decay – they also discovered that it was fissile, leading to its use in the Manhattan Project.

[IMAGE]https://commons.wikimedia.org/wiki/File:Marker_at_Site_A.jpg Caption: A commemorative boulder at Site A, where the first nuclear reactor was rebuilt.[/IMAGE]

1942 – The first artificial nuclear reactor is created

A commemorative boulder at Site A, where the first nuclear reactor was rebuilt.
Credit: Wikimedia Commons // Public Domain

As part of the Manhattan Project, the first nuclear reactor was developed under the leadership of Enrico Fermi using natural uranium. It was initially theorized by Szilárd, who worked with Fermi on the reactor, to be possible in 1933. There was no radiation shielding or cooling system, with the experiments remaining at low power. It was rebuilt in a less populated area the following year to be kept running and experimented on. This first nuclear reactor proved the feasibility of nuclear reactors and the use of nuclear energy.


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