31.1 Nuclear Radioactivity

Nuclear radioactivity describes how unstable atomic nuclei release energy and particles to reach a more stable state. This process is central to topics ranging from energy production to medical imaging, and understanding it starts with knowing what makes a nucleus unstable and what kinds of radiation it can emit.

Fundamentals of Nuclear Radiation

Principles of nuclear radiation

A nucleus becomes unstable when the balance between its protons and neutrons falls outside a stable range. To correct this imbalance, the nucleus undergoes radioactive decay, emitting particles or energy until it reaches a more stable configuration.

• The half-life of a radioactive isotope is the time it takes for half of a sample's nuclei to decay. Half-lives vary enormously: uranium-238 has a half-life of about 4.5 billion years, while radon-220 has a half-life of roughly 56 seconds.
• Natural sources of radiation include radioactive isotopes found in rocks and soil (uranium, thorium, radon) and cosmic rays that reach Earth from space.
• Artificial sources include nuclear reactors, particle accelerators, and medical equipment like X-ray machines and radiation therapy devices.

Alpha vs beta vs gamma emissions

These three types of radiation differ in composition, charge, and how easily they penetrate matter.

• Alpha ($\alpha$) particles are clusters of two protons and two neutrons, identical to a helium-4 nucleus. They're relatively massive and carry a +2 charge.
  • Lowest penetrating power of the three: a sheet of paper or the outer layer of skin can stop them.
  • Because they deposit all their energy over a very short distance, they are especially dangerous if a source is inhaled or ingested (radon gas inhalation, for example, is a leading cause of lung cancer among non-smokers).
• Beta ($\beta$) particles are high-energy electrons ($\beta^-$) or positrons ($\beta^+$) ejected from the nucleus when a neutron converts to a proton or vice versa. They are much lighter than alpha particles.
  • Moderate penetrating power: a few millimeters of aluminum or thick plastic will stop them.
  • They can penetrate the skin and damage underlying tissue, potentially causing radiation burns.
• Gamma ($\gamma$) rays are high-energy photons (electromagnetic radiation) with no mass and no charge. They are often emitted alongside alpha or beta decay as the nucleus releases excess energy.
  • Highest penetrating power: it can take several centimeters of lead or thick concrete to significantly reduce their intensity.
  • This is why nuclear power plants use dense shielding materials like lead and reinforced concrete around reactor cores.

Interaction of Radiation with Matter

Ionizing radiation interactions

Ionizing radiation carries enough energy to knock electrons out of atoms, turning those atoms into ions. All three types discussed above are forms of ionizing radiation.

• When ionization occurs in living tissue, it can break chemical bonds in DNA. This damage may lead to mutations, cell death, or, over time, cancer and genetic defects.
• Excitation is a related but less destructive process: radiation transfers energy to an atom's electrons, bumping them to a higher energy level without fully removing them. When those electrons drop back down, they release the extra energy as light or heat. Scintillation detectors exploit this effect to detect radiation by measuring the flashes of light produced.

Factors in radiation range

How far radiation travels through a material depends on three main factors:

1. Material density and atomic number. Denser materials with higher atomic numbers (like lead, $Z = 82$) are much more effective at stopping radiation than low-density materials (like air). This is why lead aprons are used during medical X-rays.

2. Type and energy of the radiation.

   • Alpha particles have the shortest range (a few centimeters in air) because their large mass and charge cause frequent interactions with surrounding atoms.
   • Beta particles travel farther (up to a few meters in air) but are still stopped by relatively thin solid materials.
   • Gamma rays travel the farthest and lose intensity gradually rather than stopping at a sharp boundary.

3. Environmental conditions. Atmospheric pressure, humidity, and airborne particles (dust, smoke, water vapor) can scatter or absorb radiation, slightly altering its effective range in air.

Nuclear processes and effects

• Radioactive decay is a spontaneous process. Different isotopes of the same element share the same number of protons but have different numbers of neutrons, and some of those neutron counts result in instability.
• Nuclear fission is the splitting of a heavy nucleus (such as uranium-235) into lighter nuclei, releasing a large amount of energy along with additional neutrons. Those neutrons can trigger further fission events, creating a chain reaction. This is the principle behind both nuclear power plants and nuclear weapons.
• Radiation dose quantifies how much energy ionizing radiation deposits in a given mass of material. It's measured in units like the gray (Gy) or the sievert (Sv), which accounts for the biological effectiveness of different radiation types. Understanding dose is essential for setting safe exposure limits in medical, industrial, and environmental contexts.