Radioactivity Revision Notes

Composition of an Atom

An atom consists of a central, positively charged nucleus. This nucleus contains protons (positively charged) and neutrons (no charge). Surrounding the nucleus are negatively charged electrons.

Key Characteristics of Atomic Structure

The nucleus is extremely dense and contains almost all of the atom's mass. The nuclear radius is vastly smaller than the overall radius of the atom, meaning atoms are mostly empty space.

Charge and Mass Distribution

The positive charge of the nucleus is due to its protons. The negative charge of the electrons balances the nuclear charge in a neutral atom. Electrons have negligible mass compared to protons and neutrons.

Size of Atoms

A typical atom has a diameter of around 1 × 10⁻¹⁰ metres (or 0.1 nanometres). This small scale sets the reference for comparing other microscopic structures.

Size of Small Molecules

Small molecules, such as oxygen (O₂), nitrogen (N₂), and water (H₂O), also have diameters of around 1 × 10⁻¹⁰ metres. They may be a few times larger than an atom, but their order of magnitude remains the same.

Atomic and Mass Numbers

The atomic number (Z) is the number of protons in an atom's nucleus. It defines the element. The mass number (A) is the total number of protons and neutrons (nucleons) in the nucleus. The number of neutrons can be found by subtracting the atomic number from the mass number (A - Z).

Isotopes

Isotopes are different forms of the same element. They have the same number of protons (same atomic number, Z) but a different number of neutrons, which leads to a different mass number (A). For example, Carbon-12 and Carbon-14 are isotopes. Notation uses the mass number as a superscript and atomic number as a subscript to the element symbol, such as ¹³C₆.

Nuclear Notation

The symbol for an isotope is written as ⁱ⁳X₆, where 'X' is the element symbol, 'A' is the mass number (protons + neutrons), and 'Z' is the atomic number (protons). For example, ¹⁰Ne₁₀ indicates Neon with 10 protons and a total of 20 nucleons.

Terrestrial Sources of Background Radiation

Background radiation from Earth, or terrestrial radiation, primarily originates from radioactive isotopes found in the Earth's crust. Key examples include uranium, thorium, and potassium-40. These elements are present in varying concentrations in rocks, soil, and water. A significant contributor is radon gas, which is a decay product of uranium and thorium. Radon can accumulate in enclosed spaces like homes, particularly in areas with underlying granite, which is rich in these radioactive elements. Other materials used in construction, such as bricks, concrete, and plaster, also contain trace amounts of these naturally occurring radioactive substances, adding to the terrestrial background radiation.

Cosmic Sources of Background Radiation

Radiation from space, known as cosmic radiation, is composed of high-energy particles from the Sun and distant stars. These primary cosmic rays interact with atoms and molecules in Earth's atmosphere, leading to the creation of secondary radiation, including muons, electrons, and photons. The Earth's atmosphere acts as a shield, reducing the intensity of this radiation at sea level. However, the dose of cosmic radiation increases significantly with altitude (e.g., during air travel) because there is less atmospheric shielding. The Earth's magnetic field also plays a role, deflecting some of these charged particles, with stronger deflection near the equator compared to the poles, meaning cosmic radiation levels vary with latitude.

Overall Contributions to Background Radiation

Natural sources account for about 85% of an average person's annual radiation dose. Terrestrial and cosmic radiation are the main natural components. The remaining percentage comes from human-made sources like medical X-rays, nuclear power, and other industrial applications. The exact proportion of terrestrial versus cosmic radiation varies geographically; for instance, mountainous regions experience higher cosmic radiation, while areas with high uranium concentrations in the ground experience higher terrestrial radiation.