Uranium was discovered in 1789 by
Martin Klaproth, a
German chemist, and named after the planet
Uranus.
Ionising radiation was discovered by
Wilhelm Rontgen in
1895, by passing an electric current through an evacuated glass tube and producing continuous X-rays. Then in 1896
Henri Becquerel found that pitchblende (an ore containing radium and uranium) caused a photographic plate to darken. He went on to demonstrate that this was due to beta radiation (electrons) and alpha particles (helium nuclei) being emitted.
Villard found a third type of radiation from pitchblende: gamma rays, which were much the same as X-rays. Then in 1896
Pierre and Marie Curie gave the name 'radioactivity' to this phenomenon, and in 1898 isolated polonium and radium from the pitchblende.
Radium was later used in medical treatment. In 1898
Samuel Prescott showed that radiation destroyed bacteria in food.
In 1902
Ernest Rutherford showed that radioactivity as a spontaneous event emitting an alpha or beta particle from the nucleus created a different element. He went on to develop a fuller understanding of atoms and in
1919 he fired alpha particles from a radium source into nitrogen and found that nuclear rearrangement was occurring, with formation of oxygen.
Niels Bohr was another scientist who advanced our understanding of the atom and the way electrons were arranged around its nucleus through to the
1940s.
By
1911 Frederick Soddy discovered that naturally-radioactive elements had a number of different isotopes (radionuclides), with the same chemistry. Also in 1911,
George de Hevesy showed that such radionuclides were invaluable as tracers, because minute amounts could readily be detected with simple instruments.
In 1932
James Chadwick discovered the neutron. Also in 1932 Cockcroft and
Walton produced nuclear transformations by bombarding atoms with accelerated protons, then in 1934
Irene Curie and
Frederic Joliot found that some such transformations created artificial radionuclides. The next year
Enrico Fermi found that a much greater variety of artificial radionuclides could be formed when neutrons were used instead of protons.
Fermi continued his experiments, mostly producing heavier elements from his targets, but also, with uranium, some much lighter ones.
At the end of
1938 Otto Hahn and
Fritz Strassmann in
Berlin showed that the new lighter elements were barium and others which were about half the mass of uranium, thereby demonstrating that atomic fission had occurred.
Lise Meitner and her nephew
Otto Frisch, working under Niels Bohr, then explained this by suggesting that the neutron was captured by the nucleus, causing severe vibration leading to the nucleus splitting into two not quite equal parts. They calculated the energy release from this fission as about
200 million electron volts.
Frisch then confirmed this figure experimentally in
January 1939.
This was the first experimental confirmation of
Albert Einstein's paper putting forward the equivalence between mass and energy, which had been published in
1905.
These
1939 developments sparked activity in many laboratories.
Hahn and Strassmann showed that fission not only released a lot of energy but that it also released additional neutrons which could cause fission in other uranium nuclei and possibly a self-sustaining chain reaction leading to an enormous release of energy. This suggestion was soon confirmed experimentally by Joliot and his co-workers in
Paris, and
Leo Szilard working with Fermi in
New York.
Bohr soon proposed that fission was much more likely to occur in the uranium-235 isotope than in
U-238 and that fission would occur more effectively with slow-moving neutrons than with fast neutrons, the latter
point being confirmed by Szilard and Fermi, who proposed using a 'moderator' to slow down the emitted neutrons. Bohr and
Wheeler extended these ideas into what became the classical analysis of the fission process, and their paper was published only two days before war broke out in 1939.
Another important factor was that
U-235 was then known to comprise only 0.7% of natural uranium, with the other 99.3% being U-238, with similar chemical properties. Hence the separation of the two to obtain pure U-235 would be difficult and would require the use of their very slightly different physical properties. This increase in the proportion of the U-235 isotope became known as 'enrichment'.
- published: 04 Apr 2016
- views: 3