NEET Physics Notes Optics-Radioactivity-Nuclear Fission
Nuclear Fission
Nuclear fission is the process of splitting of a heavy nucleus 
into two lighter nuclei of comparable masses along with the release of a large amount of energy (=200 MeV) after bombardment by slow neutrons.
A characteristic nuclear fission reaction equation for 925 U is
on(slow)
Neutrons
In the fission of uranium, the percentage of mass converted into energy is about 0.1%.
Controlled Chain Reaction and Nuclear Reactor
In the fission of one nucleus of 
neutrons are released. These released neutrons may further, trigger more fissions causing more neutrons being formed, which in turn may cause more fission. Thus, a self sustained nuclear chain reaction is formed. To maintain the nuclear chain reaction at a steady (sustained) level the extra neutrons produced, are absorbed by suitable neutron absorbents like cadmium or boron.Neutrons formed as a result of fission have an energy of about 2 MeV, whereas for causing further fission, we need slow thermal neutrons having an energy of about 0.3 eV. For this purpose, suitable material called a moderator is used, which slow down the neutrons. Water, heavy water and graphite are commonly used as moderators.A nuclear reactor is a device in which nuclear fission can be carried out through a sustained and a controlled chain reaction and can be employed for producing electrical power, for producing different isotopes and for various other uses.
Reproduction factor
Reproduction factor (k) of a nuclear chain reaction is defined as
- If k = 1, then the chain reaction will be steady and the reactor is said to be critical.
- If k >1, then the chain reaction is accelerated and it may cause explosion in the reactor. Such a reactor is called super-critical.
- If k<1, then chain reaction gradually slows down and comes to a halt. Such a reactor is called sub-critical.
- The reactors giving fresh nuclear fuel which often exceeds the nuclear fuel used in known as breeder reactor.
Nuclear Fusion
Nuclear fusion is the process, in which two or more light nuclei combine to form a single large nucleus.
The mass of the single nucleus, so formed is less than the sum of the masses of parent nuclei and this difference in mass, results in the release of tremendously large amount of energy.
The fusion reaction going on in the central core of sun is a multistep process but the net reaction is
When two positively charged particles (protons or deuterons) combine to form a larger nucleus, the process is hindered by the. Coulombian repulsion between them.
To overcome the Coulombian repulsion, the charged particles are to be given an energy of atleast 400 keV. For this, proton/deuterons must be heated to a temperature of about 3 x 109 K.
Nuclear fusion reaction is therefore, known as thermo nuclear fusion reaction.
Mass Defect and Binding Energy
The difference in mass of a nucleus and its constituent nucleons is called the mass defect of that nucleus. Thus, Mass defect
is the mass of a given nucleus.
Packing fraction of an atom is the difference between mass of nucleus and its mass number per nucleon. Thus,Packing fraction
The energy equivalent of the mass defect of a nucleus is called its binding energy.
Thus, binding energy 
If masses are expressed in atomic mass units, then
Binding energy per nucleon 
is the average energy needed to separate a nucleus into its individual nucleons.Thus
As the rest mass energy of each of electron and positron, is
Therefore, an energy of atleast 1.02 MeV is needed for pair production.
Mass Excess
Let A be the mass number of a nucleus. Let md (atomic mass units) be the mass of the neutral atom and an is the mass of the nuclide in amu, then excess mass
The figure show binding energy per nucleon versus mass number. The nuclides showing binding energy per nucleon greater than 7.5 MeV/nucleon are stable
