When a uranium nucleus was bombared by neutrons, it was found that the products of the reaction were having a number of different half lives. It was assumed that in these reactions, we are producing trans uranium elements. If we attribute these different half lives to the successive disintegration of the same atom, one can explain the possibility of a single nucleus emitting several β–particles one after another. Obviousely each emission increases the atomic number of resultant nucleus by one and a series of new transuranium elements can be stipulated. These result aroused tremendous interest in the physical world. O. Hahn and F. Strassmann then tried to identify the products of the reaction when uranium was barbarded will neutrons chemically and found to surprise of every one that these products were a number of different elements nearly all of which are in the centre of periodic table. They thus concluded that the uranium nucleus after the capture of a neutron seemed to be spilting apart into two nearly equal fragments as illustrated in the figure. These results were supported by Miss Lise Meitner and S. Frisch. They suggested that the neutron initiated a decomposition of the uranium nucleus into two nuclei of roughly equal size. The phenomenon was termed as nuclear fission.
Fission
In general, fission fragments are not stable but contain an excess number of neutrons. Hence they either ejects neutrons or convert them into protons by ejecting β –particles. We list below some examples
Further experiment showed that uranium after neutron capture divides itself into many possible fragments each corresponding to elements near the centre of periodic table. We list below some of the possible fragment pairs.
Q in all the above reactions represents the energy released in the fission reaction. The value of Q in the fission reaction is about 200 MeV. This energy release from a single atom is formidable contribution. Hence the energy release can be enormous if a very large number of atoms undergo fission.
This is the underlying principal of a fission bomb. We illustrate a chain reaction in the following diagram.
The explosive release of nuclear energy by fission is used in the design of bombs. In a bomb, the material is chosen and arranged in such a way that each neutron has the greatest possible probability of producing another fission. The arrangment ensures that on the average one fission leads to more than one subsequent fission, a rapidly increasing number of fissions occur each acompained by the release of about 200 MeV energy. This heats the bomb and causes an explosion of infamous violence.
