While studying radioactivity,we have seen that an α-particle is emitted from radium-226 and radon-222 is obtained. This nuclear is change is represented by the following equation:
Such an equation represents a nuclear reaction. Above mentioned nuclear reaction takes place on its own accord. However, it was Rutherford who, first of all, expressed his opinion that besides natural radioactivity decay processes,other nuclear reactions can also occur. A particle x is bombarded on any nucleus X and his process yield a nucleus Y and a light object y as given below:
X + x → Y +y
Rutherford performed an experiment on the nuclear reaction in 1918. He bombarded α-particles on nitrogen. He observed that as a result of this reaction, oxygen is obtained and a proton is emitted. That is
This reaction indicated that when α-particle enters the nucleus of ,then an excitation is produced in it. And as a result of it and a proton are produced. Since the experiment of Rutherford, innumerable nuclear reactions have been observed. For nuclear reactions to take place the fulfillment of certain conditions is a must.
Before and after any nuclear reaction the number of protons and neutrons must remain the same because protons and neutrons can neither be destroyed or can they be created. We elaborate this point from the example of Rutherford’s nuclear reaction of and here:
A nuclear reaction can take place only when the total energy of the reactants including the rest mass energy is equal to the total energy of the products. For its explanations we again take the example of the nuclear reaction of Rutherford involving and . In this reaction the mass of the reactants is :
Mass of =14.0031 u
Mass of =4.0026 u
Total mass of the reactants = 18.0057 u
In the same way the mass of the products is
Mass of = 16.9991 u
Mass of = 1.0078 u
Total mass of the products after the reaction = 18.0069 μ.
This shows that the total mass after the reaction is greater than the total mass before the reaction by 0.0012 μ. We known that a 1μ mass = 931 MeV energy, therefore a mass difference of 0.0012μ is equivalent to an energy of 931 MeV × 0.0012μ = 1.13 MeV. Hence this reaction is possible only when a additional mass of 0.0012 μ is added to the reactants to the minimum kinetic energy of the α-particle is 1.13 MeV such as obtained from . The energy these α-particle is equal to 7.7 MeV which is greater than 13 MeV. Had these α-particles been obtained from a source that give out α-particles whose energy was less than 13 MeV then this reaction would not have taken place.
From the conditions described above we can tell whether any nuclear reaction is posible or not. There is an interesting aspect in a nuclear reaction that it can take place in the opposite direction also. We known that is obtained by the reaction with an α-particle of appropriate energy. If we accelerate protons, with the help of a machine like cyclotron, and increase their velocity and then bombard these high velocity protons on n , Rutherford’s nuclear reaction of and will proceed in the backward direction as:
By bombarding different elements with α-particles, protons and neutrons, many nuclear reactions have been produced.Now we describe one such nuclear reaction with the help of which James Chadwick discovered neutron in 1932. When was bombarded with α-particles emitting out of ,then as a result of a nuclear reaction and a neutron were obtained. This reaction is shown below with an equation:
As neutron carries no charge, therefore it presented a greater amount of difficulty for its identification. Anyhow when neutron were passed through a block of proffin, fast moving protons were ejected out and these were easily identified. It may be remembered that a large amount of hydrogen is present in proffin and the nuclei of hydrogen atoms are protons. The emission of protons is the consequence of elastic collisions between the neutrons and the protons. This indicates that the mass of neutron is equal to the mass of the proton. It may be remembered that when an object of certain mass collides with another object of equal to mass at rest, then as a result of elastic collisions, the moving object comes to rest and the stationary objects begin to move with velocity of the colliding object. The discovery of neutron has brought in a revolution in the nuclear reactions as the neutrons carry no charge so that can easily enter the nucleus.
The arrangement of Chadwick’s experiment for the discovery of neutron.