Alpha Decay

Alpha decay is a type of radioactive decay where an unstable atomic nucleus emits an alpha particle and transforms into a different nucleus. This process reduces the atomic number by two and the mass number by four, resulting in the formation of a new element. In this article, we will learn about the process of alpha decay in detail, including its equation, examples and applications.

What is Alpha Decay?

Alpha decay is a type of radioactive decay in which an unstable atomic nucleus emits an alpha particle and transforms into a different nucleus. An alpha particle consists of two protons and two neutrons. This is the same as a helium-4 nucleus. When an atom undergoes alpha decay, it releases this alpha particle. As a result, it decreases its atomic mass by four units and atomic number by two, resulting in a new element.

Rutherford observed that radioactive substances could emit three different types of radiation. He named them alpha, beta, and gamma rays, based on their ability to penetrate materials. He identified alpha particles as helium nuclei, as further experimentation showed that they were identical to the helium atoms.

Alpha Decay Equation

The alpha decay equation represents the transformation of an unstable parent nucleus into a more stable daughter nucleus through the emission of an alpha particle. An alpha particle is composed of two protons and two neutrons. This is identical to the nucleus of a helium-4 atom.

The general formula for alpha decay can be expressed as:

^A_ZX → ^A-4_Z-2Y + ⁴₂He

where,

• ^A_ZX represents the parent atom in which Z is the atomic number (number of protons) and A is the mass number (total number of protons and neutrons).
• ^A-4_Z-2Y represents the daughter atom. The atomic number decreases by 2 and the mass number decreases by 4 because the alpha particle (which contains 2 protons and 2 neutrons) is emitted.
• ⁴₂He is the alpha particle, equivalent to a helium-4 nucleus.

Alpha Decay Examples

The alpha decay of uranium is represented by the following equation

²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He

In this equation, uranium-238 decays into thorium-234 by emitting an alpha particle. This process decreases the uranium’s atomic number by 2 and its mass number by 4, resulting in a thorium atom with fewer protons and neutrons.

Other examples of alpha decay are :

• ²²⁶​₈₈Ra → ²²²₈₆​Rn + ⁴​₂He
• ²¹⁰​Po → ²⁰⁶₈₂​Pb + ⁴​₂He
• ²⁴¹​Am → ²³⁷​₉₃Np + ⁴​₂He
• ²³⁹Pu → ²³⁵​₉₂U + ⁴​₂He
• ²³²​Th → ²²⁸​₈₈Ra + ⁴​₂He
• ²⁴⁴​Cm → ²⁴⁰​₉₄Pu + ⁴​₂He
• ²²²​Rn → ²¹⁸​₈₄Po + ⁴​₂He

What happens in Alpha Decay?

In alpha decay, an unstable atomic nucleus releases an alpha particle, which consists of two protons and two neutrons. Here is what happens during alpha decay:

• Inside the nucleus of an unstable atom, two protons and two neutrons come together to form an alpha particle. This particle formation is driven by the need to achieve greater nuclear stability.
• The alpha particle is emitted from the nucleus due to quantum tunneling. Despite the strong nuclear forces holding it within the nucleus, the particle manages to escape through the potential barrier surrounding the nucleus, a phenomenon explained by quantum mechanics.
• As the alpha particle contains two protons and two neutrons, the loss of this particle results in a decrease in the atomic number by two and the mass number by four. This shifts the element to another position on the periodic table, typically two places to the left, forming a new element with different physical and chemical properties.
• The emission of the alpha particle is accompanied by the release of energy. This energy is released because the mass of the remaining daughter nucleus and the alpha particle combined is less than the original mass of the parent nucleus (following Einstein’s equation E=mc²). This mass difference is released as kinetic energy, part of which is carried away by the alpha particle.
• The product of alpha decay is a new, more stable nucleus. This nucleus may still be radioactive. It can undergo further radioactive decay, either by emitting more alpha particles, beta particles, or other forms of decay.

Gamow Theory of Alpha Decay

Gamow Theory of alpha decay was developed by George Gamow in 1928. It is a quantum mechanical explanation for the emission of alpha particles from radioactive nuclei.

Gamow’s Theory states that,

Alpha decay occurs through quantum tunneling, where an alpha particle within a nucleus manages to escape the potential barrier created by nuclear forces, despite classically not having enough energy to do so. This tunneling effect allows the alpha particle to ‘break out’ of the nucleus. This leads to the transmutation of the original element into a new one with a reduced atomic number and mass number.

This theory helps in quantifying the decay probability. It establishes a relationship between the decay constant and the energy of the emitted alpha particles, as described by the Geiger-Nuttall law. According to this law, the isotopes with shorter half-lives emit more energetic alpha particles.

Applications of Alpha Decay

Here are some of the most important uses of the alpha decay :

• Alpha particles ionize air, improving smoke detection efficiency. This technology is important for early fire warning systems.
• Alpha particles are used to target and destroy cancer cells effectively. Alpha radiation’s high ionizing power allows precise treatment.
• Heat from alpha decay can be converted into electricity. This can power equipment in remote or harsh environments, like space.
• Alpha decay is useful in studying nuclear reactions and structures. It enhances understanding of atomic interactions and energy.
• The age of archaeological and geological specimens can be estimated using alpha decay. This process depends on the alpha decay rates of certain isotopes.
• Alpha spectroscopy measures airborne pollutants. It helps in monitoring and controlling environmental air quality.
• Alpha particles neutralize static charges in production environments. This helps in reducing dust and particulate contamination in sensitive processes.

FAQs on Alpha Decay

What is alpha decay in physics?

Alpha decay is a type of radioactive decay where an unstable atomic nucleus emits an alpha particle (consisting of two protons and two neutrons, the same as a helium-4 nucleus) and transforms into a different, lighter nucleus with a lower atomic number by two and mass number by four.

How does alpha decay occur?

Alpha decay occurs through quantum tunneling, where an alpha particle inside a nucleus overcomes the nuclear force barrier, despite not having enough energy to do so classically, and is thus emitted from the nucleus.

What is an example of alpha decay?

A classic example of alpha decay is the transformation of Uranium-238 into Thorium-234:

²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He

What role does Gamow’s theory play in alpha decay?

Gamow’s theory explains alpha decay using quantum mechanics, particularly quantum tunneling. It quantifies the probability of an alpha particle escaping the nucleus despite the potential barrier, using the relationship between the particle’s energy, the barrier’s properties, and the decay constant.

Can alpha particles be dangerous?

Alpha particles are generally not dangerous when outside the body due to their low penetration power. They can be stopped by a sheet of paper or human skin. However, they pose serious health risks if alpha-emitting substances are ingested or inhaled, as they can damage biological tissues at close range.