What is Alpha Decay? - Equations and Examples

Any decay in which the atomic nucleus of a certain element emits ⁴₂He and converts into an atom of a completely different element is known as alpha decay or -decay. Due to the emission of a helium atom, this decay results in a drop in mass number and atomic number and is denoted by the Greek letter α. Let’s discuss the topic in-depth along with some important questions.

Keyterms: Radioactivity, Alpha Decay, Beta Decay, Gamma Decay, Q value, Gamow theory, Atoms, Helium, Nucleus, Emission

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What is Alpha Decay?

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The transition of a radioactive nucleus into helium is known as alpha decay. During this transition, the starting element transforms into an entirely another element, changing its mass and atomic number in the process. Alpha decay can occur in any element heavier than lead. Lighter elements, on the other hand, show no signs of radioactive decay.

Alpha decay is a type of radioactive decay in which unstable atomic nuclei emit a helium nucleus (alpha particle) and transition into a more stable element in the process. The alpha particle, which was ejected out, is made up of four nucleons: two neutrons and two protons. The ratio of protons to neutrons in the parent nucleus is reduced by alpha radiation, resulting in a more stable structure. The nucleus of a helium atom is identical to an alpha particle. Ernest Rutherford, who employed alpha particles in his gold foil scattering experiment, conducted the first observations and investigations of alpha decay.

The atomic number of the radioactive sample changes as the alpha particle, which is made up of two protons and a neutron, exits the nucleus. After alpha decay, the element left behind is two atomic numbers and four mass numbers smaller.

For example, Uranium ²³⁸ ₉₂ decays to form Thorium²³⁴₉₀.

²³⁸₉₂U → ^238-4_92-2Th + ⁴₂He → ²³⁴₉₀Th + ⁴₂He

Thus, the transformation of a nucleus in an alpha decay can be written as;

^A_ZX → ^A-4_Z-2Y + ⁴₂α

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Alpha Decay Equation

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Henry Becquerel discovered radioactivity. A Uranium compound was wrapped with black paper and placed in a drawer containing photographic plates. Later examination of the plates revealed that they had been exposed. Thus, the concept of radioactive decay was born as a result of this exposure. Such kinds of radioactivity can be observed.

• Gamma Decay (Photons having high energy are emitted)
• Beta Decay (Emission consists of Electrons)
• Alpha Decay (Emission consists of Helium nucleus)

Consider this alpha decay example to fully comprehend this. Assume that element Z has the mass number a and the atomic number b. This element converts to X during decay. Take a look at the following equation.

^a_bZ → ^a-4_b-2X + ⁴₂He

where:

⁴₂He is the released alpha particle.

^a-4_b-2X is the daughter nucleus, the ending nucleus.

^a_bZ is the parent nucleus, the starting nucleus.

As a result, on both sides of this equation, the mass number and the atomic number balance out.

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Occurrence of Alpha Decay

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Only the strongest elements experience alpha decay. The element's nucleus must be big enough or unstable enough to undergo spontaneous fission-like modifications. In such elements, it is the most common type of degradation. The alpha particles that are emitted from the nucleus have an energy level of roughly 5 MeV and a speed of around 5% of that of light. It's worth noting that alpha particles have a charge of +2 because they don't have any electrons. Because of its charge and mass, an alpha particle reacts violently with its environment, losing all of its energy practically instantly. A few centimeters of air can halt their forward momentum.

This type of radioactive decay reacts most aggressively with the human body due to its heaviness and charge. They have a strong ionizing power, which allows them to cause havoc on tissue. The victims' bodies develop blisters and burns as a result of an alpha radiation overdose.

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Alpha Decay Example

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One of the popular examples of alpha decay is the decay of uranium ²³⁸₉₂U to thorium ²³⁴₉₀Th with the emission of a helium nucleus ⁴₂He. 

²³⁸₉₂Ur → ²³⁴₉₀Th + ⁴₂He

Other examples of alpha decay are:

²³⁷₉₃No → ²³³₉₁Pa + ⁴₂He

¹⁷⁵₇₈Pt → ¹⁷¹₇₆Os + ⁴₂He

¹⁴⁹₆₄Gd → ¹⁴⁵₆₂Sm + ⁴₂He

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Q value of Alpha Decay

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The difference between the total of the rest masses of the initial reactants and the sum of the end product masses is known as the Q-value in physics and chemistry. The Q-value is the difference between the final and beginning mass energy of the decaying products, to put it another way.

For alpha decay equations, this Q-value is,

Q = (m_X – m_Y – m_He) c²

This decay's energy Q is split evenly between the modified nucleus and the Helium nucleus.

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Gamow Theory of Alpha Decay

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The relationship between the energy of an alpha particle emitted and the decay constant for a radioactive isotope is defined by Gamow Theory or Geiger-Nutall law. It was created in 1911 by John Mitchell Nutall and Hans Geiger, hence the name.

When comparing shorter-lived isotopes to longer-lived isotopes, this rule clearly shows that shorter-lived isotopes emit more energy. Decay, on the other hand, is only one sort of radioactive decay. Beta and gamma decay can also occur in a nucleus.

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Application and Importance

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Smoke detectors use radioactive materials that undergo alpha decay. Americium is a common element because it is a primary producer of alpha particles. Alpha particles are released inside the smoke detector. As a result, the air inside the detector becomes ionized. Because smoke in the detector absorbs this alpha radiation, the ionization is altered and the alarm is triggered if smoke is present.

Alpha particles are also utilized in a technique called Alpha Particle X-Ray Spectroscopy (APXS). The elemental composition of rocks and soil is determined using this method. This method was employed by NASA on their trips to Mars, notably the Pathfinder probes, to figure out what elements were present in Martian rocks.

In the medical field, alpha particles are also useful. TAT, or targeted alpha therapy, is a revolutionary cancer treatment that uses alpha decay to attack cancer cells. When lead-212 is ingested, it travels to the tumor location and emits alpha radiation, which kills all of the cells in the area.

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Things to Remember

• Alpha decay is a nuclear process in which an unstable nucleus transforms into a different element by ejecting a particle containing two protons and two neutrons.
• Although alpha decay is not very penetrating, ingesting a chemical that undergoes it is dangerous because the released alpha particles can rapidly damage inside tissues despite their low range. Contact with membranes and live cells causes this harm.
• The alpha decay equation is: ^a_bZ → ^a-4_b-2X + ⁴₂He; where: ⁴₂He is the released alpha particle, ^a-4_b-2X is the daughter nucleus, the ending nucleus, ^a_bZ is the parent nucleus, the starting nucleus.
• For alpha decay equations, this Q-value is, Q = (m_X – m_Y – m_He) c²
• This decay's energy Q is split evenly between the modified nucleus and the Helium nucleus.
• The relationship between the energy of an alpha particle emitted and the decay constant for a radioactive isotope is defined by Gamow Theory or Geiger-Nutall law. 
• Alpha particles are utilized in a technique called Alpha Particle X-Ray Spectroscopy (APXS). 

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