Radioactive Decay and Half life :
Radioactive Decay is the process of formation of a nuclide of another element by the emission of α or β - particles from a nuclide of a radio active element. The original radioactive nuclide is called parent nuclide and the new nuclide formed is called daughter nuclide.
(Nuclide is a species of an atom characterised by the composition of nucleus in terms of number of protons and neutrons).
The race of radioactive decay varies from element to element and is independent of the physical and chemical conditions of the element such as shape, pressure, temperature etc.
The law of radioactive decay states that the number of nuclei (ΔN) disintegrating or decaying within a short period of time (Δt) is proportional to the total number of non - disintegrated radioactive nuclei (N).
Thus ΔN ∝ — NΔt or ΔN = —λNΔt
where λ, is the radioactive decay constant or disintegration constant. Minus sign indicates decrease in N with time.
At t = 0, N = No and it can be shown that N = No e^-λt
Half-life period is the time required for disintegration of one hall of the original amount of the radioactive substance.
The rate of radioactive disintegration is conveniently expressed by means of half - life period.
"Half-life period is the time needed for half of the initial nuclei to disintegrate".
Half-life period is a measure of the radioactivity of the element. Shorter the half-life period, greater is the number of atoms undergoing disintegration and hence greater is its radioactivity.
Assuming N = 0.5 No and using the relation
Ta is called average life or mean life of a radioactive substance.
Ta = Sum of the lives of all the atoms / Total number of atoms
Fertile Materials :
Neutrons produced by the fission process are ejected from the nucleus at a very high velocity (~ 1.5 x 10000000 m/s) and thus have a very high kinetic energy and are known as fast neutrons. Materials fissionable by thermal or low speed neutrons are U-233, U-235 and Pu-239. These are fissile materials.
Fertile materials are those which can be transformed into fissile materials. They cannot sustain chain reaction. However, when a fertile material is hit by neutrons and absorbs some of them, it is converted into fissible material. U-238 and Th-232 are for example, fertile materials used for reactor purposes.
Artificial Radioactivity :
Artificial radioactivity is also called induced radioactivity or artificial transmutation.
It is the process in which a stable nuclei is changed into another by artificial methods, i.e., by bombarding the target nuclide with projectiles such as protons, deutrons, α - particles and neutrons. The first artificial transmutation was carried out by Rutherford in 1919, by bombarding nitrogen nuclei with alpha particles.
His source of α - particles was a radioactive piece of ore. He showed experimentally that, when an α - particle collided with a nitrogen atom, a proton bounced out and the heavy atom recoiled a short distance. The α - particle disappeared. It is absorbed in the process, transforming nitrogen into oxygen.
By means of artificial radioactivity, Aluminium can be converted into Phosphorous which again decays into Silicon. Juliot and Curie found that Boron bombarded by α - particles produce the radio active nitrogen. A few other synthetic elements produced by artificial radioactivity are curium, berkelium, fermium, nobelium, plutonium etc.