Discuss the alpha decay process with example.

Explanation

When unstable nuclei decay by emitting an `alpha`-particle (`._2^4He` nucleus), it loses two protons and two neutrons. As a result, its atomic number Z decreases by 2, the mass number decreases by 4. We write the alpha decay process symbolically in the following way
`._Z^A X to ._(Z-2)^(A-4)Y + ._2^4He`
Here X is called the parent nucleus and Y is called the daughter nucleus.
Example: Decay of Uranium `._92^238U` to thorium `._90^234Th` with the emission of `._2^4He` nucleus (`alpha`-particle)
`._92^238U to ._90^234Th + ._2^4He`
As already mentioned, the total mass of the daughter nucleus and `._2^4He` nucleus is always less than that of the parent nucleus. The difference in mass `(Deltam=m_x-m_y-m_alpha)` is released as energy called disintegration energy Q and is given by
`Q=(m_x -m_y -m_alpha)c^2`
Note that for spontaneous decay (natural radioactivity) Q > 0. In alpha decay process, the disintegration energy is certainly positive (Q > 0). In fact, the disintegration energy Q is also the net kinetic energy gained in the decay process or if the parent nucleus is at rest, Q is the total kinetic energy of daughter nucleus and the `._2^4He` nucleus. Suppose Q<0, then the decay process cannot occur spontaneously and energy must be supplied to induce the decay.