Neutron Reactions and Radiation
The nature of the radiation emitted from a radioactive substance was investigated by Rutherford in 1904, by applying electric and magnetic fields to the radiations. It is observed that on applying the field, the rays emitted from the radioactive substances are separated into three types, called a, p and у rays. The a rays are deflected in a direction that shows that they carry a positive charge; the p rays are deflected in the opposite direction, showing that they carry a negative charge and у rays are not deflected at all, showing that they carry no charge. Furthermore, p rays are deflected to a much greater extent than a rays, which indicates that particles in the p rays are much lighter than those in the a rays. The properties of each of these rays have been studied in detail and briefly described next (Barik et al. 2011).
Properties of a Ray
- • The direction of deflection of the a rays in the electric and magnetic fields shows that they carry a positive charge. The charge and mass of the particles present in the a rays (called a-particles) have been investigated by suitable experiments. It is found that each a particle carries two units of positive charge and has mass nearly four times that of a hydrogen atom. Thus, the a-particles are like the helium nuclei.
- • The velocity of a rays is found to be nearly 1/10th to 1/20th of that of light, depending upon the nature of source.
- • Alpha (a) rays ionize the gas through which they pass. The ionization takes place due to the knockout of the electrons from the molecules of the gas when the highspeed heavy a-particles hit these molecules.
- • Since a-particles are heavy, they cannot pass through thick sheets of metal. In other words, a rays have a low penetrating power. It has been observed that they can penetrate through air only to a distance of about 7 cm and then they are absorbed in the air. Similarly, they can be stopped by an aluminium foil less than 1/10th of a mm in thickness.
- • Alpha (a) rays affect a photographic plate and produce luminescence when they strike a zinc sulphide screen.