NUCLEAR PHYSICS

Nuclear physics deals with the study of the atomic nucleus. Nuclei are only 10⁻¹⁵ to 10⁻¹⁴ m in diameter. The nucleus of an atom consists of protons and neutrons. The number of protons in a nucleus is referred to as the atomic number and denoted by Z. The total number of protons and neutrons in a nucleus in called the mass number and denoted by A. Nuclear species or nuclides are identified according to the scheme ᴬz X where X is the chemical symbol of the species. Thus, the carbon nucleus which has 6 protons and 6 neutrons will be denoted by ¹²₆C.

Isotopes Nuclides with the same atomic number but different mass numbers are called isotopes. For example, 23592U and 23892U are two isotopes of uranium. The two isotopes have the same number of protons (92) but different number of neutrons (143 and 146).

Nuclear Forces A nucleus consists of positively charged protons and chargeless neutrons. Since like charge repel, there should be repulsion between protons and the nucleus should break apart. This does not happen because the nucleons (common name for protons and neutrons) inside a nucleus are bound by nuclear forces. These attractive nuclear forces exist between protons and protons, protons and neutrons, and neutrons and neutrons. For very short distances between the protons (Such as those inside a nucleus), the nuclear forces are stronger than the electrostatic repulsive forces.

Radioactivity

In a large nucleus, the distance between protons becomes comparatively so large that the nuclear forces cannot overcome the electrostatic repulsive forces and therefore the nucleus becomes unstable. It is found that all nuclei having 83 or more protons are unstable. In order to achieve greater stability, these unstable nuclei disintegrate spontaneous with the emission of alpha (α), beta (β) and gamma (γ) rays. Elements emitting these rays are called radioactive and the phenomenon is known as radioactivity. Alpha rays are positively charged helium nuclei (⁴₂He), beta rays are negatively charged electrons and gamma rays are changeless photons.

When an alpha or a beta particle is emitted from a nucleus, a different element is formed. The nuclei of the elements thus formed may be in an excited state. These nuclei de-excite with the emission of one or more gamma particles. Thus gamma emission does not result in the formation of a different element.