Alpha Decay

For centuries, physicists have named the three types of radiation emitted by nuclei by denoting them with Greek letters, namely alpha, beta and gamma. Keeping this in mind, Alpha decay is a type of radioactive degeneration in which some unstable large-sized nuclei, usually with a mass number greater than 210, change into another element by shooting out a particle consisting of two protons and two neutrons, known as the alpha particle. Alpha decay spontaneously emits excessive energy by emitting the alpha particle. This alpha particle, which is denoted by the Greek letter α, is structurally similar to a helium atom.

Be it alpha, beta or gamma decay; the nucleus attempts to find stability by emission of radiation by adjusting its proton/neutron ratio. However, different kinds of forces work on the nucleus for each decay. Talking about alpha decay, we know that the alpha particle (consisting of 2 protons and two neutrons) has a mass of 4 units and two positive electric charges. The emission of alpha particles from the nuclei gives rise to daughter nuclei, having 1) a positive nuclear charge, 2) atomic number, which is two units less than their parents’ and mass, which is four units less.

It was Ernest Rutherford, a British Nuclear physicist, who distinguished alpha decay from other forms of radiation. He observed the deflection of radiation through the magnetic field and found that it did not deflect in a way you would expect a positive particle to deflect since α particles have a +2e charge. 

What is the Alpha Decay Equation?

Alpha decay equation can be used to determine the identity of the daughter isotope. 

The alpha decay equation is represented by:

AZX A -4Z -2 Y + 42 He 

In the above alpha decay equation,

AZX is the starting or the parent nucleus

A is the total number of the sum of proton and neutron or nucleons

Z is the total number of protons

A -4Z -2Y  is the ending or the daughter nucleus.

 42 He is the alpha particle that is released. 

In 1928, Gamow solved the theory of alpha decay via tunnelling. According to the Gamow theory of alpha decay, the quantum mechanical basis for alpha particle emission from the potential nuclear barrier is described. A link is detailed between the energy emitted by an alpha particle and the decay constant of a radioactive isotope. Geiger and Nuttall found a significant relationship between the energy of the emitted alpha particles and the half-life of alpha decay. The essence of Geiger-Nuttall’s law is that long-lived alpha particles have lesser Kinetic Energy, while short-lived Alpha particles have greater K.E.

Details on Alpha Particles:

The alpha particle has a positive charge and a larger mass. Due to this, the particle can’t go very deep into the solid or very far into the air. The energy and speed of the alpha particle, which is emitted from a parent nucleus, is the specific property of the parent nucleus. This property will determine the distance or the range the alpha particle will set to travel. As stated earlier, though the alpha particles have an ejection speed that is about 1/10th of the speed of light, we know that the particles are not very penetrating. 

As the alpha particle released due to alpha decay is not very penetrative, it is not used extensively in external medical radiation therapy. According to its penetrative depths, gamma radiation is the most penetrative, while alpha radiation is the least. Although not very penetrative, ingestion of any substance exposed to the intense radiation of alpha particles can prove hazardous to health. If the alpha emitter is ingested, swallowed or absorbed into the bloodstream, it can cause biological damage and increase cancer risk. 

Though hazardous to human health, if used cautiously, radioactive elements that undergo alpha decay have applications in the industry. They are primarily used in smoke detectors, where inside the smoke detectors, alpha particles are released, ionizing the air inside the detector. Smoke absorbs the alpha radiation, and so if smoke is present, it will alter the ionization, and the alarm goes off. It is also used in a process known as Alpha Particle X-ray Spectroscopy (APXS), used to determine the elementary composition of rocks and soil.

Conclusion

Thus, to summarise, a large-sized nucleus undergoes alpha decay as the nature of nuclear force, which is attractive force is dominating as shorter distance, is dominant over the coulombic force. But as the size increases, the distance between nucleons increases, and thus the coulombic repulsion dominates, and the nucleus structure becomes unstable. The only way for this large-sized nucleus to become stable is to lose some nucleons, and that is what happens in the alpha decay process.