Nuclear Fission
It is a process of splitting of a heavy nucleus into 2 nuclei of comparable masses with release of huge amount of energy
In 1938, Hahn and Strassmans discovered that when 92U235 is bombarded with thermal neutrons (neutrons having energy 0.025 eV), it splits of into 56Ba141 & 36Kr92 with emission of three neutrons and about 200MeV of energy per fission. This reaction can be represented as
92U235 + 0n1 ———> 56Ba141+ 36Kr92 + 30n1 + Q
The neutrons obtained after fission are secondary neutron & Q is the amount release in the fission. In this reaction
Mass defect (/\m) = mass of reactance – mass of products
/\m = 0.2153 amu
According to Einstein’s mass energy equivalence
E = /\mc2 = 0.2153 * 931 MeV = 200 MeV
The released energy appears in the form K.E. of fission fragments & the secondary neutrons and in the forms of gamma-rays. Each of the 3 neutrons produced carries an energy of about 2 MeV and as such being very fast without causing further fission of 92U235 nuclei. In order to utilize them for further fission of 92U235 , they need to be slowed down somehow.
Nuclear Chain Reaction:-
Nuclear fission of 92U235 by thermal neutrons can be expressed as
92U235 + 0n1 ———> 56Ba141+ 36Kr92 + 30n1 + 200MeV
Each secondary neutron carries an energy of about 2MeV and as such being very fast escape without causing further fission of 92U235 nuclei.
If these 3 neutrons are slowed down somehow to change them into thermal neutrons then further fission of three 92U235 nuclei can be carried out. Thus will produce nine neutrons and so on. Due to multiplication of neutrons in this manner, the entire mass of 92U235 may undergo for fission in very short interval of time and huge amount of energy will be released.
Such a fission reaction which once initiated can’t be shut off i.e. the rate of reaction of which increases continuously without check is called uncontrolled nuclear chain reaction.
It is the underlying principle of working of an atom bomb.
PRESENTED BY:- Miss.Lopamudra Behera(Lect. in Physics)
