Radioactivity Revision Notes

Relative Mass of Subatomic Particles

The relative mass of a proton is approximated as 1. The relative mass of a neutron is also approximated as 1, making them significantly heavier than electrons. An electron has a relative mass of approximately 1/1836, which is often considered negligible in comparison to protons and neutrons. A positron has the same relative mass as an electron, approximately 1/1836.

Relative Electric Charge of Subatomic Particles

The relative electric charge of a proton is +1. A neutron carries no electric charge, so its relative charge is 0. An electron has a relative electric charge of -1, which is equal in magnitude but opposite in sign to that of a proton. A positron, being the antiparticle of an electron, has an identical relative mass but an opposite relative electric charge of +1.

Basic Atomic Structure

An atom consists of a nucleus (containing protons and neutrons) and electrons orbiting the nucleus. Protons have a positive charge, neutrons have no charge, and electrons have a negative charge.

Electrical Neutrality of Atoms

For an atom to be electrically neutral, the total positive charge must equal the total negative charge. This is achieved when the number of protons (positive charges) exactly matches the number of electrons (negative charges).

Charge Balance

Each proton carries a +1 elementary charge, and each electron carries a -1 elementary charge. Therefore, an equal quantity of protons and electrons ensures that these charges cancel out, leading to a net charge of zero for the atom.

Atomic Structure Basics

An atom consists of a tiny, dense central nucleus surrounded by electrons. The nucleus contains positively charged protons and neutral neutrons (except for hydrogen-1, which has no neutrons). Electrons are negatively charged and are much lighter than protons and neutrons.

Electron Orbits and Energy Levels

Electrons orbit the nucleus at distinct, set distances. These distances correspond to specific energy levels or shells. Electrons cannot exist between these defined orbits. Each orbit has a characteristic energy, and electrons in orbits further from the nucleus generally have higher energy. This quantized nature of electron energy levels is a key aspect of models of atomic structure.

Energy Transitions

Electrons can move between different energy levels by gaining or losing energy. Gaining energy allows an electron to jump to a higher-energy orbit (excitation), while losing energy causes it to fall to a lower-energy orbit (de-excitation). When electrons fall to lower energy levels, they often emit electromagnetic radiation, such as light, with a specific energy related to the difference between the energy levels.

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.