Alpha Decay

Alpha decay is a type of radioactive decay which brings about transmutation of elements. It was accidentally discovered by the French physicist Henri Becquerel in 1896 when he was investigating the phenomenon of phosphorescence in some materials.

What is an Alpha Particle?
Alpha particles are positively charged nuclei of helium, with two protons and two neutrons bound together. Alpha particles are denoted by the Greek symbol α or the chemical formula of Helium nuclei which is ⁴₂He. They are spin zero particles, which makes them bosons. If you are not familiar with the phenomenon of radioactivity or if you want to know which radioactive elements decay through this mode, please refer to the article list of radioactive elements.

What is Alpha Decay?
This type of decay involves the spontaneous emission of an alpha particle from a radioactive nucleus which is accompanied by a decrease in the atomic mass number of the emitting nucleus by 4 and the atomic number by 2. That is because the alpha particle is made up of four nuclides, out of which two are protons and two neutrons. Thus the nucleus gets transformed into another element due to alpha particle emission.

The next question that may pop up in your mind is why does alpha decay occur? Why does a radioactive nucleus spontaneously pop out an alpha particle? The answer to that lies in understanding the nature of the forces in play inside the atomic nucleus and its composition.

The nucleus consists of protons and neutrons bound together by the strong nuclear force which is a lot stronger than the electromagnetic force. As all protons are positively charged, they repel each other and the nucleus would never be stable, if it wasn't for the strong nuclear force binding them together. However, in higher elements like Uranium, the number of protons is quite high and so is the repulsive electromagnetic force. Such nuclei are unstable and prone to transmute through radioactive decay.

For the alpha particle to be emitted, it needs to overcome the strong nuclear force and classically it is not possible as it does not have the energy to do that. However, it can escape the nuclear potential well through the phenomenon of 'Quantum Tunneling'. Quantum physics predicts that there is a small probability that the alpha particle can overcome the nuclear potential wall, by tunneling through it and it does so. A fundamental quantum theory of alpha decay was provided by George Gamow in 1928.

The emitted alpha particles travel at an average speed of 15,000 km/sec and have kinetic energies in the range of 5 MeV. The energy of emitted alpha particles are always constant for a particular alpha decay process.

Equation
Here is an example that illustrates this radioactive decay phenomenon. If you will observe, every transmuted daughter element has an atomic mass number less by 4 and atomic number by 2.

²³⁸₉₂U -> ²³⁴₉₀Th + ⁴₂He

Here an isotope of Uranium (²³⁸U) gets transformed into Thorium (²³⁴Th).