Nuclear Reaction definition
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. The 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 nuclear reactions in 1918. He bombarded α-particles on nitrogen. He observed that as a result of this reaction, oxygen is obtained and a photon is emitted. That is
This reaction indicated that when α-particle enters the nucleus, then an excitation is produced in it. And as a result of it and a proton is 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 nor 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 of 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 an 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 of 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 gives 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 possible or not. There is an interesting aspect in a nuclear reaction that it can take place in the opposite direction also. We know that is obtained by the reaction with an α-particle of appropriate energy. If we accelerate protons, with the help of a machine like a cyclotron, increase their velocity and then bombard these high-velocity protons on n , Rutherford’s nuclear reaction 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 the 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 neutrons 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 a neutron is equal to the mass of the proton. It may be remembered that when an object of certain mass collides with another object 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 the velocity of the colliding object. The discovery of neutrons has brought in a revolution in nuclear reactions as the neutrons carry no charge so they can easily enter the nucleus.
The arrangement of Chadwick’s experiment for the discovery of the neutron.