Duduli

It is the process in which a positron and an electron combine and their mass is transformed into the energy of gamma rays ( y- rays) is usually known as annihilation or pair annihilation. This is represented by (e^+) + (e^- ) → 2 y Where, e^+ = positron e^- = electron 

Packing fraction of a nucleus is defined as mass defect per nucleon of the nucleus. Thus,       Packing fraction = mass defect/A                                   = ∆m/A

Following are the limitations of Rutherford atom model (i) According to electromagnetic theory a revolving electron should continuously emit energy and hence the radius of its path should go on decreasing and ultimately,it should fall into the nucleus. However, electrons revolve around the nucleus without falling in it. Hence, Rutherford atom model cannot explain the stability of atom. (ii) Rutherford model cannot explain line spectra of atoms like hydrogen. Because if the Rutherford atom model is true, the electrons can revolve in orbits of all possible radii hence, it should emit continuous energy spectrum.

Following are merits of shell model : (1) magic numbers are explained. (2) The closed shell structure in shell model explains extra stability and binding energy. (3) The shell model explains the angular momentum of nuclei. (4) The shell model explains nuclear magnetic moments. (5) shell model explains nuclear isomerism.

Parity of a nucleus refers to the behaviour of the wave function(¥) under inversion of coordinates. e.g if we replace x by -x, y by -y and z by -z and wave function remains the same, then it has even parity. On other hand, if it appears with negative sign, it has odd parity as shown below       ¥(x,y,z)=¥(-x,-y,-z)   even parity       ¥(x,y,z)= - ¥(-x,-y,-z)   Odd parity 

According to this model, nuclei of all the elements behave like a liquid drop. As observed that binding energy of the nucleus is proportional to the number of nucleons. It is similar to binding of molecules in a liquid drop. A small drop is spherical because of surface tension. A nucleus is also assumed to be spherical in shape. The density of a liquid drop is independent of its value. But it depend on the nature of liquid. In case of nucleus, the nuclear density is independent of nuclear volume and it is same for all type of nuclei.

The gamma decay is the spontaneous emission of electromagnetic  Photon from the nucleus. When the nucleus in the excited state passes through lower emission state or ground state from higher energy state then it emits a high energy Photon known as gamma emission.

Binding energy is defined as the amount of energy required to break the nucleus.    Binding energy B = ∆m × 931Mev

We know that the nucleus consists of protons and neutrons.Due to the positive charge on proton, there Wii be a repulsive electrostatic force between the two protons and the resulting repulsive force between protons will tend to push the nucleus apart.Therefore, for the nucleus to have a permanent existence, there must be some strong attractive forces between protons and neutrons inside the nucleus.These attractive forces cannot be gravitational because they are much smaller than the attractive force demanded.Moreover, these forces cannot be electrical in nature because the strong repulsive forces between protons will lead to distruption of nucleus. Actually these forces are short range attractive forces known as nuclear forces.

Fission reaction may be employed to detect neutrons.A typical detector consists of an ionisation chamber whose walls are coated with fissionable material.The incident neutrons cause Fission in the material.This gives rise to heavy Fission fragments, each having kinetic energy of more than 50MeV.These fission fragments cause ionisation in the gas in ionisation chamber.

Fermions are elementary particles which obey Fermi-Dirac statistics.They have a particle spin equal to half integral.The name fermions is due to Italian physicist Enrico Fermi.These particles follow pauli's exclusion principle.There are two major type of fermions (A) Light weight fermions (leptons) (B) Heavy weight fermions (Baryons)

(i) Cyclotron is used in nuclear reactions.High speed particles produced in cyclotron act as bombarding agent to carry out nuclear reactions to produce new particles. (ii) The accelerated charged particles are used in artificial transmutation of elements. (iii) The cyclotron is used to implant the particles in solids to modify their properties. (iv) Cyclotron is used in hospitals to produce different radioactive nuclei.

Tandem accelerator is a modified form of Van de graaff accelerator.The Van de graaff accelerator is a single stage accelerator while Tandem accelerator is a two stage accelerator.The output energy is double in comparison of Van de graaff accelerator with the same operating voltage.

Van de graaff generator is used to produce a parallel homogeneous beam of charged particles like electrons, protons,α-particles etc. It was developed by van de graaff in 1931 at Princeton university, USA. Principle :- The instrument is based on the following two facts: (i) If a charged conductor is brought into internal contact with a hollow conductor, all of its charge transfers to the hollow conductor, no matter how high is potential of the latter. (ii) The discharge of electricity  occurs readily at pointed objects (corona points).

According to shell model the nucleus consists of protons and neutrons which are placed in certain discrete levels or shells just like the electrons in the discrete shells of an atom.

The phenomenon of spontaneous emission of radiations from the elements much lighter than those occuring in nature by modern techniques of artificial transmutation of elements is known as artificial or induced Radioactivity.

The phenomenon of spontaneous emission of powerful radiations exhibited by the heavier elements found in nature is called natural Radioactivity.

According to tunnel effect, if a particle is impinging (means strike) on a barrier, with energy less than the height of the potential barrier, it will not necessarily be reflected by the barrier but there is always a probability to cross the barrier and continue its forward motion.

The range of α-particles is customarily defined as the distance which these particles travel through air at 76cm of mercury pressure and 15°C temperature before they lose their energy to the extent that they no longer ionise the gas particles. ★The range of α-particles depends upon the following factors (i) Initial velocity of α-particle (ii) Nature of emitting radioactive element (iii) Nature & pressure of the gas or nature of absorber.

The following conservation laws must be obeyed for α-decay ★Conservation of charge and number of nucleons ★Conservation of linear momentum ★Conservation of mass-energy       The alpha decay may be represented as: