Constituent particles in a nucleus
| The constituent particles of a nucleus are protons and neutrons, known as nucleons. The number of protons in a nucleus is just equal to its atomic number Z, and the total number of nucleons A is the integer closest to its atomic weight; hence the number of neutron (N) is A-Z. As for example, the nucleus of 79Au197, a gold atom contains 79 protons and 118 neutrons. Nucleons are strongly bound inside the nucleus via meson exchange. Meson obeys Bose-Einstein’s statistics* and therefore it is a boson, whereas proton and neutron obey Fermi-Dirac statistics# and hence are fermions. Nucleons are again composed of sub-nucleon structures known as quarks and antiquarks, which interact via gluon exchange. To get a better idea let us have a look at Fig. m1.1. in this figure protons are represented by red sphere whereas neutrons are by blue sphere. A general nucleon is shown in the centre, indicating quark and gluon interaction. All these particles are confined within a dimension of ~ 10 -15 m. Now imagine how strong the force inside the nucleus that is overcoming strong electrostatic repulsion between protons and making extremely high density at the centre of the atom. | ||
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FIGURE m1.1: Constituent particles of a nucleus | ||
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| # The particles having half integral spin (e.g. electron, proton, neutron etc.) obey Fermi-Dirac (F. D.) statistics. | ||
| For understanding of B. E. and F. D. statistics go through some course on statistical mechanics or basic quantum mechanics. —————————————————————————————————————— | ||
| Nuclear Mass | ||
Nucleus contains almost entire mass of an atom. The mass of a proton is ~ 1.6725 x 10 -27 kg or 938.26 MeV and that of a neutron is 1.6748 x 10 -27 kg, i.e. 939.55 MeV. The atomic masses denoted as atomic mass unit (a.m.u.) correspond to 1/12th of the mass of 12C atom. Its value is 1.660566 x 10 -27 kg = 931.5016 ?/img2.gif){kind=small} 0.0026 MeV/c2. * Electrons, the other constituents of atoms are much lighter (1/1840 of a proton mass) and hence negligible contribution to atomic mass. | ||
| *As per Eienstein’s Relativity theory, mass and energy are interconvertible by an equation E = mc2. Therefore, amu can be converted to MeV | ||