Page 15 - Neutrons for Sciences and Society

Chapter 1 - Pre-history
lead to a chain reaction. The origin of the energy evolved was the loss of mass occurring during the fission reaction. This mass
m is transformed into energy E following Einstein’s equation:
E = mc2
where c is the speed of light.
Such a reaction could lead to
a new source of energy. The
first practical realisation of the
controlled chain reaction was
achieved at the end of 1942 by
Enrico Fermi in Chicago, USA. At that time the US was at war, and was working on the development of the atomic bomb, a more brutal way to implement this same chain reaction. All this work, including Fermi’s was hidden with the greatest secrecy under the codename “Manhattan Project”.
The neutrons accompanying fission have energies of the order of 1 million electron volts (MeV); it was quickly understood that when these particles hit nuclei of other atoms, notably those of small atomic mass, the neutron lost energy and slowed. After several collisions the neutrons formed a sort of gas in thermal equilibrium with the medium in which they found themselves. These neutrons are known as thermal neutrons. The chain reaction is maintained by the fission being more favourably
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Fig. 1.2: The instrument of discovery used for the discovery of the neutron. In those days a very simple set-up could be enough to make a great discovery. The tube contained an alpha emitter, a beryllium target and an ionisation detector.