Since the discovery of nuclear fission by Otto Hahn and Fritz Strassmann in Germany 77 years ago, the world has never been the same. From nuclear reactors for generating electricity to new tools for diagnosis and treatment of diseases to nuclear weapons capable of mass destruction, the entire social and geopolitical profile of the world has changed.
It all began with Lord Rutherford, who in early 20th century, showed that chemical elements can be transformed from one to another by bombarding them with charged particles like the alpha particle — a form of modern alchemy. The target
element absorbed the incoming particle, rearranged itself to a new element,
generally the next one in the periodic table. These were called ‘nuclear transmutations’ as against chemical reactions, in which the nature of the element remained unchanged. The big question then was what would happen if uranium, the heaviest of all the naturally occurring elements, is bombarded by such particles. Would it generate transuranium elements? This could not be immediately tested since the high positive charge on its nucleus repelled the bombarding particle (with a positive charge), and prevented it from penetrating.
This led to the discovery of neutrons. In 1932, James Chadwick from England
discovered a new type of highly penetrating radiation, which constituted particles of neutral charge and with mass almost equal to that of protons. He called them neutrons. Enrico Fermi, an Italian scientist was quick to grasp the advantage of
neutrons in transmutation experiments. Owing to the absence of any net charge in neutrons, they could penetrate the target nucleus more readily and bring about
nuclear transformation.
Following this suggestion along with Enrico, two more groups of scientists – the Curies in France and Otto Hahn and Fritz Strassmann in Germany — bombarded uranium with slow neutrons and started analysing the products. The results were almost same in all these experiments. One of the products was identified as an element with atomic number 93 (a transuranium element). Uranium (atomic number 92) itself is weakly radioactive and many of its decay products are also
radioactive. Hence, many of the radioactive products identified in those experiments were assumed to be some of those decay products. This seemed to be a
reasonable assumption based on the known mechanism of the transmutation reactions.
However, there were some products which did not fit into this explanation. Some appeared to have surprisingly low atomic numbers, which couldn’t be
explained by the then existing theories. Otto Hahn and Fritz Strassmann subjected these products to extensive radiochemical analysis and found that some of them could be lanthanum (atomic number 57) and barium (atomic number 56). But then the question was how such low atomic number elements were generated? Was the uranium nucleus “bursting” under neutron bombardment? They were not sure.
On December 22, 1938, Otto and Fritz sent a manuscript for publication to the German science magazine Naturwissenschaften, reporting that they had
discovered barium as one of the products of bombarding uranium with neutrons. Around the same time, Otto also sent a letter to Lise Meitner, a radiochemist with whom he had collaborated earlier, describing the puzzling result. Lise and her cousin Otto Robert Frisch, also a physicist working with Niels Bohr at Copenhagen, pored over the contents of Otto’s letter to find an explanation.
Sometime earlier to all these happenings, Russian physicist George Gamow and Danish physicist Niels Bohr had proposed that nuclei of heavy elements like uranium, containing large number of protons and neutrons are less tightly bound than nuclei of lighter elements and hence, may behave like a liquid drop. Lise visualised that just as a liquid drop can fragment into smaller drops when disturbed, a uranium nucleus, after absorbing a neutron would wobble, become more unstable, elongate and start pinching at the middle to finally break into two parts of approximately equal mass.
Lise estimated that the resulting two nuclei repel each other and gain a total
kinetic energy of about 200 million electron volts (MeV). This is a huge amount of energy when compared to the energy released in radioactive decay (maximum about a few MeV) and chemical reactions (only of the order of a few eV). So, where does this energy come from? Lise worked out that the total mass of the products formed by the splitting of uranium nucleus would be slightly lower than its original mass plus the mass of neutron. This small difference, according to Einstein’s equivalence of mass and energy (E=mc2) could account for the energy released.
Thus, Lise and Otto (Frisch) correctly interpreted Otto (Hahn) and Fritz’s
findings, which is: on absorption of a slow neutron, the uranium nucleus splits into two lighter fragments, releasing in the process, an energy of about 200 MeV. Here was a new type of nuclear transmutation, much different from any known till then. Otto(Frisch) named it ‘nuclear fission’.
If one nuclear fission reaction of uranium could release so much energy, imagine the kind of explosive power millions of such reactions in an instant would generate! Hungarian physicist Leo Szilard suggested that if a fission chain reaction can be set up, the enormous amount of energy released can be harnessed for military and civilian purposes. This was in 1939, the beginning of the Second World War. This possibility soon acquired enormous political significance and the race for harnessing nuclear energy for military purposes started.
What followed next was a mixed bag of development and destruction — the
construction of the first nuclear reactor, Chicago Pile-1 in 1942, testing of the first atomic bomb in New Mexico in July 1945, bombing of Hiroshima and Nagasaki with uranium-235 and plutonium-239 bombs (6th and 9th August 1945) and the subsequent cold wars and finally, the nuclear arms race.
