Unit 21: Nuclear Chemistry and Applications of Radioactivity

Natural and Artificial Radioactivity

Natural Radioactivity

Natural radioactivity refers to the spontaneous emission of particles or electromagnetic radiation from unstable atomic nuclei that exist in nature. Important naturally occurring radioactive isotopes include uranium-238 (U-238), thorium-232 (Th-232), and radium-226 (Ra-226). These isotopes decay through series of alpha, beta, and gamma emissions, contributing to background radiation.

Artificial Radioactivity

Artificial radioactivity is produced when stable nuclei are bombarded with particles such as neutrons, protons, or alpha particles, causing them to become radioactive. Examples include carbon-14 (C-14) used in dating, phosphorus-32 (P-32) in biochemical tracing, and iodine-131 (I-131) in medical diagnostics and therapy.

Types of Radiation

• Alpha (α) radiation: Consists of helium nuclei (He^{2+}), relatively heavy and low‑penetrating; stopped by a sheet of paper.
• Beta (β) radiation: Emission of electrons (β⁻) or positrons (β⁺); moderate penetration, stopped by a few millimetres of aluminium.
• Gamma (γ) radiation: High‑energy electromagnetic photons; highly penetrating, requiring lead or concrete shielding.

Units of Radioactivity

Radioactivity is quantified by the number of nuclear disintegrations per unit time.

The half‑life (t₁/₂) is the time required for half of the radioactive nuclei in a sample to decay. It is related to the decay constant (λ) by:

t₁/₂ = ln 2 / λ

Activity (A) at any time is given by:

A = λN

where N is the number of undecayed nuclei.

Nuclear Reactions

A nuclear reaction involves changes in the composition of an atomic nucleus, often resulting in the transmutation of one element into another.

Nuclear Transmutation

Transmutation occurs when a nucleus captures a particle or emits radiation, changing its proton count. A classic example is Rutherford’s artificial transmutation:

¹⁴N + α → ¹⁷O + p

Here a nitrogen‑14 nucleus absorbs an alpha particle (α) and ejects a proton (p), producing oxygen‑17.

Nuclear Fission

Nuclear fission is the splitting of a heavy nucleus into two lighter nuclei, accompanied by the release of a large amount of energy and often several neutrons.

Example Reaction

²³⁵U + n → ¹⁴¹Ba + ⁹²Kr + 3n + Energy

The emitted neutrons can induce further fission events, leading to a chain reaction.

Controlled vs. Uncontrolled Fission

• Controlled fission: Neutrons are moderated (e.g., by water or graphite) to sustain a steady reaction rate; used in nuclear power plants to generate electricity.
• Uncontrolled fission: No moderation; the reaction proceeds exponentially, releasing enormous energy in a fraction of a second; the principle behind nuclear weapons (atomic bombs).

Nuclear Fusion

Nuclear fusion combines light nuclei to form a heavier nucleus, releasing energy due to the mass defect (Einstein’s E = Δmc²).

Example Reaction (Solar Fusion)

4 ¹H → ⁴He + 2 e⁺ + 2 νₑ + Energy

Four protons (hydrogen nuclei) fuse into a helium‑4 nucleus, emitting two positrons, two neutrinos, and substantial energy.

Requirements and Applications

• Extremely high temperatures (millions of kelvins) are needed to overcome electrostatic repulsion between nuclei.
• In nature, fusion powers the Sun and stars.
• On Earth, fusion is harnessed in hydrogen bombs, where a fission bomb provides the conditions for fusion of isotopes like deuterium (²H) and tritium (³H).

Nuclear Power and Weapons

Nuclear Power

Nuclear reactors utilize controlled fission of uranium‑235 or plutonium‑239. Heat generated from fission is transferred to a coolant, producing steam that drives turbines connected to electrical generators.

Nuclear Weapons

• Atomic bomb: Relies on uncontrolled fission of U-235 or Pu-239.
• Hydrogen bomb: Uses a fission primary to ignite a fusion secondary (deuterium‑tritium), yielding far greater explosive power.

Safety Concerns

• Radioactive waste requires long‑term isolation (e.g., deep geological repositories).
• Accidents such as Chernobyl (1986) and Fukushima (2011) highlighted the need for robust containment, emergency cooling, and rigorous safety protocols.

Industrial Uses of Radioactivity

• Tracers: Radioactive isotopes (e.g., C-14, P-32) track chemical reactions, study metabolic pathways, and assist in oil‑exploration logging.
• Radiography: Gamma sources (e.g., Ir-192, Co-60) produce images to detect cracks or voids in metal welds, pipelines, and aerospace components.
• Sterilization: Gamma radiation sterilizes medical equipment, pharmaceuticals, and food by destroying microorganisms without leaving residues.
• Thickness Control: Beta or gamma gauges measure the attenuation of radiation through materials (paper, metal sheets) to maintain uniform thickness in manufacturing processes.

Medical Uses of Radioactivity

• Diagnosis:
  • X‑rays (produced by electron deceleration) for imaging bones and teeth.
  • Positron Emission Tomography (PET) uses positron‑emitting isotopes like F-18 to visualize metabolic activity.
  • Thyroid scans employ I-131 or I-123 to assess thyroid function.
• Treatment:
  • Cobalt‑60 (Co-60) teletherapy delivers gamma rays to destroy tumours.
  • Phosphorus‑32 (P-32) treats certain blood disorders such as polycythemia vera.
  • Iodine‑131 (I-131) ablates overactive thyroid tissue or thyroid cancer.
• Sterilization: Gamma irradiation of surgical instruments, syringes, and grafts ensures sterility without heat damage.

Radiocarbon Dating

Radiocarbon dating exploits the radioactive isotope carbon‑14 (C-14) to determine the age of organic materials up to about 50 000 years.

Principle

While alive, organisms maintain a constant ratio of C-14 to stable C-12 through exchange with the atmosphere. After death, intake ceases and C-14 decays with a half‑life of 5730 years:

t = (t₁/₂ / ln 2) × ln(N₀/N)

where N₀ is the initial C-14 amount and N the remaining amount.

Applications

• Dating archaeological artifacts (wood, bone, charcoal).
• Studying past climate changes via ice cores and sediment layers.
• Verifying the authenticity of artworks and manuscripts.

Harmful Effects of Nuclear Radiations

Ionizing radiation possesses sufficient energy to remove electrons from atoms, creating ions that can damage biological molecules.

Acute Effects

• Radiation sickness: nausea, vomiting, fatigue, and at high doses, cardiovascular and neurological failure.
• Skin burns (erythema) and tissue necrosis.
• Potentially lethal within hours to days if the whole‑body dose exceeds ~4 Sv.

Long‑Term Effects

• Increased risk of cancers (leukemia, thyroid, breast, lung).
• Genetic mutations that may be passed to offspring.
• Teratogenic effects leading to birth defects when exposure occurs during pregnancy.

防护 Measures (Radiation Protection)

• Shielding: Use of high‑density materials (lead, concrete) to attenuate gamma and X‑rays; plastic or glass for beta.
• Distance: Radiation intensity follows the inverse‑square law; doubling distance reduces dose to one‑quarter.
• Time Limitation: Minimizing exposure time reduces cumulative dose.
• Personal protective equipment (lead aprons, thyroid shields) for medical and industrial workers.

Summary

This chapter has covered the full spectrum of nuclear chemistry: from the basic phenomena of natural and artificial radioactivity, through the quantitative units and decay laws, to the transformative processes of nuclear fission and fusion. We examined how these principles are harnessed in nuclear power generation, weapons, industrial gauges, medical diagnostics and therapy, radiocarbon dating, and the vital safety practices needed to mitigate harmful radiation effects. Understanding these concepts is essential for appreciating both the benefits and the responsibilities associated with nuclear science.