Radioactive decay | Types and Examples

Elements with unstable nuclei undergo radioactive decay in different types or modes. The type of radioactive decay depends upon the particular nuclide involved. Radioactive decay can be characterized by alpha decay (helium nucleus), beta decay (electron or positron emission), gamma decay (emission of electromagnetic radiation), electron capture, and spontaneous nuclear fission of the heavy nuclei.

Discovery of Radioactivity

In 1896, French engineer Henri Becquerel accidentally discovered the evidence of radioactivity for uranium minerals. He was working on how uranium salts are affected by light when he found that photographic plates were blackened when uranium salt emitted radiations. 

Two years later, in 1898, Polish and French physicist and chemist Marie Curie discovered similar properties for thorium. She found differences between the radioactivity of uranium and thorium for which she concluded that these elements must contain unknown radioactive elements. In the same year, Marie Curie and her husband Pierre Curie, discovered two radioactive elements, polonium, and radium.

What is radioactive decay?

Some elements with unstable nuclei undergo spontaneous nuclear transformations. When a nuclide of an element splits into its daughter nuclei, it emits radiation. Hence, the decay of a parent nucleus into daughter nuclei along with the emission of radiation is known as radioactive decay. 

The elements having unstable nuclei undergo radioactive decay to produce more stable nuclei. The stability of nuclides depends upon the proton to neutron ratio.

Number of proton / Number of neutrons = p / n

Stability of Nuclei

The proton to neutron ratio is responsible for the stability of nuclei. Neutrons act as nuclear glue which sticks the protons together in the nucleus. At low atomic masses, 1:1  proton to neutron is stable. As we approach higher atomic masses, a higher number of neutrons is required as more protons have more repulsions and they need more neutrons as nuclear glue. Hence, the stability of nuclei is determined by the neutron to proton ratio.

The elements having a stable nucleus lie in the stability band which is a graph between the number of protons and the number of neutrons. The band of stability provides the necessary information about the radioactive nature of a chemical specie.

Magic numbers are numbers (ratio) of protons or neutrons which have stable atomic nucleus configuration. These are 2, 8, 20, 28, 50, 82, and 126.

Law and Energy of Radioactive Decay

If a sufficient number of radioactive atoms are observed for a sufficient amount of time, the law of radioactive decay is given as:

-dN.dt = 𝜆N

where,

• N = Number of atoms of certain radioactive nuclide
• 𝜆  = Decay constant
• –dNdt = Disintegration rate

The law of radioactive decay describes the kinetics of reactions. For example,

A → B + x + ΔE

In the above equation, A denotes the mother nuclide of the radioactive atom, B is the daughter nuclide, x is emitted (decay) particle, and ΔE is the energy released in the process. 

Radioactive decay is only possible if ΔE > 0. ΔE is determined by a comparison of masses.

ΔE = Δmc² 

ΔE = [m_A – (m_B – m_x)] c²

Types of Radioactive Decay

Types of radioactive decay include:

1. α particles decay
2. β particles decay
3. γ radiation emission
4. Electron capture
5. Positron emission
6. Spontaneous fission
7. Proton decay
8. Internal conversion (IC)
9. Isomeric transition (IT)

Alpha (α) particles emission (α-decay)

Alpha (α) decay is a radioactive decay in which a nucleus emits nuclei of a helium atom (α-particle) to produce a new element. Usually, elements with a high proton to neutron ratio undergo alpha decay radioactivity.

In this process, the atomic (proton) number of the new element is decreased by two units and the atomic mass is decreased by four units.

For example:

₉₂U²³⁸ → ₉₀Th²³⁴ + ₂He⁴  (α-particle)

Alpha decay happens in a nucleus that has a large proton to neutron ratio. Alpha particles decrease the proton to neutron ratio to stabilize the heavy nuclei. 

Properties of α-particles

• Alpha particles are relatively slow and heavy.
• They have the lowest penetrating power.
• These particles have higher ionization power.

Beta (β) particles emission (β-decay)

Nuclei containing excess neutrons emit beta particles. A neutron breaks up into a proton and an electron along with other subatomic particles like antineutrino, etc. This process is termed beta (β^–) decay because the electron produced by the breakage of a neutron is released from the nucleus in the form of beta radiation.

₀n¹  →  ₁p¹ + _-1e⁰ 

In beta decay, the atomic number of an element is increased by one unit, without any change in its atomic mass.

For example,

₆C¹⁴  →  ₇N¹⁴ + _-1e⁰

Gamma (γ) radiation emission

During radioactive decay, when a nucleus changes from an excited state to a ground state or an excited state of lower energy, some photons are emitted known as gamma (γ) rays. Gamma rays have higher penetrating power than all others and usually penetrate through very hard and thick materials.

The gamma rays are produced in nuclear reactions along with alpha and beta particles. Nuclear fission and fusion reactions also produced gamma rays. 

Properties of Gamma (γ) rays

• Gamma rays are electromagnetic radiations of short wavelengths and very high frequency.
• They travel at the speed of light.
• Penetration power of these radiations is greater than alpha or beta particles.
• Their ionization power is less than alpha and beta particles.
• They produce fluorescence on a photographic plate.

Electron capture

Electron capture involves the absorption of inner shell electron(s) by the nucleus. It is done to increase the number of neutrons in a nucleus by a combination of electrons and protons. Therefore, elements present below the belt of stability usually undergo electron capture in order to increase neutron number (N).

Equation:

₁p¹ + _-1e⁰ →  ₀n¹

Positron (β⁺) Emission

In positron emission, a proton is converted into a neutron, with the emission of a positron and a neutrino. A positron is an anti-particle of an electron that has the same properties but is opposite in charge to an electron.

The general equation for positron emission is given as:

₁p¹  →  ₁n⁰ + _-1e^{0 +} 

For example,

₈O¹⁵  →  ₇N¹⁵ + ₊₁e⁰

Properties of Beta (β⁺) particles

• These are fast-moving electrons or positrons emitted from nuclei of elements.
• Beta particles are much lighter than alpha particles.
• The penetration power of these radiations is greater than alpha particles but less than gamma rays.
• Their ionization power is much less than that of alpha particles.

Key Takeaway(s)

Concepts Berg

What is a radioactive source?

A radioactive source emits the ionizing radiation from the radionuclide in either way of radiation types i.e. alpha, beta, gamma, etc. 

What are the three main types of radioactive decays:

The three main types of radioactive decays are:

1. Alpha-decay
2. Beta-decay
3. Gamma emission

What type of radioactive decay would you expect for ²³⁸U and why?

U-238 decays by alpha emission and turns into thorium-234, due to a high proton to neutron ratio. Therefore, U-238 tends to decrease its atomic mass and atomic number.

Which type of radioactive decay is the most dangerous?

The gamma rays emission is the most dangerous radioactive decay. Gamma rays penetrate through matter like metals, etc, and are usually blocked by a few inches of lead or a few feet of concrete. This penetration capability of these radiations makes them dangerous.

Which types of radioactive decay will cause the nucleus to change into a different element?

There are two types of radioactive decay in which a different element is formed:

1. Alpha-decay
2. Beta-decay 

Which type of radioactive decay produces the most neutrinos?

A positron emission decay produces a neutrino. It is shown by the symbol “ν”.

What type of radioactive decay increases the atomic number of a radioactive atom?

Beta-decay increases the atomic number of a radioactive atom when a neutron changes into a proton with the emission of an electron.

Do objects radioactively decay forever?

The radioactive decay of elements is due to the instability of nuclei. An element does not decay further after it gains stability.

Why are tritium and lead radioactive?

Tritium(an isotope of hydrogen) has two neutrons in the nucleus while regular hydrogen doesn’t have any. So, due to these neutrons, the nuclei of tritium are radioactive. On the other hand, Lead emits gamma rays as it has a high electron density.

Do all three forms of radioactive decay accompany each other?

The three types of radioactive decay may follow one another, so we say that these forms accompany each other. For example, U-238 changes into Th-234 by alpha decay which further emits beta rays to change into U-234. 

What happens to a radiolabeled molecule after decay?

After radioactive decay, an atom changes into another form.

References

• Nuclear and Radiochemistry: Fundamentals and Applications by Karl Heinrich Lieser (Eduard-Zintl-Institut, Darmstadt, Germany)
• Radiochemistry and nuclear chemistry 2nd edition by G. Choppin, J. Rydberg, J. O. Liljenzin
• Basics of Radiation (orise.orau.gov)