Principles, Production, and Interaction of Neutrons with Matter

Nuclear reactions and the behaviour of subatomic particles are basic to core design. Here, the main purpose is to provide details at the review level. We treat such processes as radioactivity decay, neutron scattering, radiative capture, and fission. In general, these processes are associated with the emission of subatomic particles and radiations as well as their interaction with matter. We use the general term 'radiation' to include both material particles and true electromagnetic radiation.

Production of Neutrons

Alpha (a) particles are highly energetic positively charged particles, which are emitted by radioactive substances, and the amount of their positive electricity is equal to two units of electronic charge. The mass of the a particle is equal to that of a helium atom. They are emitted during the radioactive disintegration of certain heavy elements like uranium or radium, etc. When mono-energetic alpha particles emitted from a radioactive substance are allowed to pass through a very thin metal foil, they are found to be scattered in different directions with respect to the direction of the collimated beam of the incident particles. Though, by far, a great majority of the particles are scattered at small angles (greater than 90°).

Such large-angle scattering cannot be explained on the basis of the Thomson model of the atom. The electrostatic repulsive force between the positive charge of the scattering atom and a-particle depends inversely upon the square of the distance (q) of the a-particle from the centre of the charge of the latter and directly upon the portion of the positive charge of the scattering atom contained within the sphere of the radius q. The angle of scattering increases with the increasing effective charge of the atom repelling the a-particle and decreases with the increasing distance from the centre of the atom at which the a-particle passes by the atom. When the a-particle passes by at a relatively large distance from the centre of the atom (~10-10 m near the periphery of the atom), this distance becomes so large that the angle of scattering is quite small, even though the entire positive charge of the atom repels it.

On the other hand, when the a-particle passes by at a relatively close distance from the centre of the atom, the effective charge repelling it is so small that the angle of scattering is again quite small.

In 1911, Rutherford proposed a new model of the atom. According to him, the entire positive charge of an atom and almost its entire mass are contained within a very small sphere near the centre. This positively charged core is known as the atomic nucleus. When an a-particle passes by very close to the centre of the atom, it feels a strong electrostatic repulsive force due to the entire positive charge of the atom and hence is scattered at a relatively large angle.

However, the Rutherford model of the atom has one serious drawback. As such, the electromagnetic theory of light predicts that the revolving electrons (due to their centripetal acceleration) should continually emit electromagnetic radiation so that they move spirally inwards and ultimately plunge into the nucleus. In 1913, Niels Bohr suggested a way out of the difficulty, which however involve entirely new concepts and that were at variance with some of the fundamental concepts of classical mechanics and Maxwell's electromagnetic theory of light.

This is known as Bohr's quantum theory. The quantum theory, in a more developed form at present, constitutes the theoretical basis of all subatomic phenomena.

In 1920, James Chadwick has determined the nuclear charge of several elements on the basis of Rutherford's theory of the a-particle scattering. To increase the number of scattered a-particles, Chadwick used a narrow ring-shaped scattering foil mounted on a suitable frame for the experiment. Since the atom as a whole is electrically neutral, the nuclear charge should be equal to the atomic number. Chadwick's determination of nuclear charge for the different elements confirmed this most conclusively.

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