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BASIC CONCEPTS Summary-1 The net nuclear charge in a nuclear species is equal to + Ze, where Z is the atomic number and e is the magnitude of the electronic charge. The fundamental positively charged particle in the nucleus is the proton The mass number of a nuclear species, indicated by the symbol A, is the integer nearest to the ratio between the nuclear mass and the fundamental mass unit, defined so that the proton has a mass of nearly one unit. the mass of the electrons is negligible compared with the proton mass mP ~ 2000me For nearly all nuclei, A is greater than Z, in most cases by a factor of two or more. Thus there must be other massive components in the nucleus. The presence of electrons within the nucleus could not be proven due to following reasons: i) If electrons existed inside the nucleus, then it should be bound inside with a force stronger than coulomb force. ii) If electrons existed inside the nucleus, due to uncertainty principle electrons should have an energy of 20 MeV but the electrons emitted during -decay have energies less than 1 MeV. iii) the proton and electron each have angular momentum (spin) of (1/2). Quantum mechanically spin of the protons and electrons do not add together to give observed spin of nuclei. three spins of 1/2 in deuterium combine to a total spin of either 3/2 or ½ . Yet the Summary-2 Neutron is proposed as building block of nucleus. The neutron is eclectically neutral and has a mass about equal to the proton mass (actually about 0.1% larger). Thus a nucleus with Z protons and A - Z neutrons has the proper total mass and charge we define a specific nuclear species, or nuclide, by symbol where X is the chemical symbol and N is the neutron number, A - Z. Neutrons and protons are the two members of the family of nucleons. Nuclides with the same proton number but different neutron numbers are called isotopes; for example 35Cl and 37Cl. Nuclides with the same N but different z· these are called isotones. Nuclides with the same mass number A are known as isobars. Nuclear properties are mass, radius, relative abundance (for stable nuclides), decay modes and half-lives (for radioactive nuclides), reaction modes and cross sections, spin, magnetic dipole and electric quadrupole moments, and excited states. In nuclear physics lengths are of the order of 10 -15 m, which is one femtometer (fm), also called one fermi. Nuclear sizes range from about 1 fm for a single nucleon to about 7 fm for the heaviest nuclei. The time scale of nuclear phenomena has an enormous range from 10-20 s to 10-9 s (5He or 8Be, break apart in times of the order of 10-20 s), decays of nuclei occur within lifetimes of the order of 10-9 s to 10-12 s , but many and decays occur with much shorter or longer lifetimes ( from minutes