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Description of  Nuclear Physics: Physics of Nucleus (Learn Physics Book 34)

Nucleus (1) Rutherford's -scattering experiment established that the mass of atom is concentrated with small positively charged region at the centre which is called 'nucleus'. (2) The stability or instability of a particular nucleus is determined by the competition between the attractive nuclear force among the protons and neutrons and the repulsive electrical interactions among the protons. Unstable nuclei decay, transforming themselves spontaneously into other structure by a variety of decay processes. (3) We could not survive without the 3.90 1026 watt output of one near by fusion reactor, our sun. (4) Nuclei are made up of proton and neutron. The number of protons in a nucleus (called the atomic number or proton number) is represented by the symbol Z. The number of neutrons (neutron number) is represented by N. The total number of neutrons and protons in a nucleus is called it's mass number A so A = Z + N. (5) Neutrons and proton, when described collectively are called nucleons. A single nuclear species having specific values of both Z and N is called a nuclide. (6) Nuclides are represented as where X denotes the chemical symbol of the element. Neutron Neutron is a fundamental particle which is essential constituent of all nuclei except that of hydrogen atom. It was discovered by Chadwick. A free neutron outside the nucleus is unstable and decays into proton and electron. (1) The charge of neutron : It is neutral (2) The mass of neutron : 1.6750 1027 kg (3) It's spin angular momentum : (4) It's magnetic moment : 9.57 1027 J/Tesla (5) It's half life : 12 minutes (6) Penetration power : High (7) Types : Neutrons are of two types slow neutron and fast neutron, both are fully capable of penetrating a nucleus and causing artificial disintegration. Thermal Neutrons Fast neutrons can be converted into slow neutrons by certain materials called moderator's (Paraffin wax, heavy water, graphite) when fast moving neutrons pass through a moderator, they collide with the molecules of the moderator, as a result of this, the energy of moving neutron decreases while that of the molecules of the moderator increases. After sometime they both attains same energy. The neutrons are then in thermal equilibrium with the molecules of the moderator and are called thermal neutrons. Energy of thermal neutron is about 0.025 eV and speed is about 2.2 km/s. Types of Nuclei The nuclei have been classified on the basis of the number of protons (atomic number) or the total number of nucleons (mass number) as follows (1) Isotopes : The atoms of element having same atomic number but different mass number are called isotopes. All isotopes have the same chemical properties. The isotopes of some elements are the following (2) Isobars : The nuclei which have the same mass number (A) but different atomic number (Z) are called isobars. Isobars occupy different positions in periodic table so all isobars have different chemical properties. Some of the examples of isobars are (3) Isotones : The nuclei having equal number of neutrons are called isotones. For them both the atomic number (Z) and mass number (A) are different, but the value of (A Z) is same. Some examples are (4) Mirror nuclei : Nuclei having the same mass number A but with the proton number (Z) and neutron number (A Z) interchanged (or whose atomic number differ by 1) are called mirror nuclei for example.