Writer: Brain Guo

Radiation is the emission and transmission of energy as electromagnetic radiation or as subatomic particles that carry energy through space or a medium. While many introductions to the concept of radiation in the context of thermal physics treat radiation as a form of energy, a more rigorous understanding of the process utilizes the concepts of quantum mechanics and electromagnetism to explain the phenomenon. Unlike the other forms of heat transfer, radiation is able to occur even in the presence of a vacuum. Radiation utilizes either massless particles (called gauge bosons) that travel at the speed of light, or subatomic particles that have mass and travel at velocities near the speed of light to emit the radiation.

Electromagnetic radiation is composed of electric and magnetic waves that travel perpendicular to each other. Such radiation was explained by both Max Planck and Albert Einstein to contain both wave and particle properties; it exhibits what is known as wave-particle duality. The energy of electromagnetic radiation is found via the Planck-Einstein relation, which states that energy is equal to both Planck’s constant times the frequency of the electromagnetic wave, or the speed of light times Planck’s constant divided by the wavelength of the radiation. The speed of light is approximately 3.00 times 10 to the 8th meters per second.

Radiation is divided into two main categories: non-ionizing and ionizing radiation. The dividing factor between these two types is the first ionization energy of atomic matter, which ranges between 10 and 33 electron volts. Radiation with energy values below the ionization energy of matter will not be able to remove an electron from an atom. Instead, it will interact with the atoms by increasing the rotational, vibrational, or electron energy levels of the atom. Radiation with energy values above the ionization energy of matter will be able to remove an electron from the atom, leading to the formation of an ion.

Beyond electromagnetic radiation is a type of radiation that consists of particulate matter. These particles are emitted by the unstable nuclei of radioactively decaying atoms. The nuclei of those atoms are unstable due to an imbalance between the strong nuclear force between the protons and neutrons of the nuclei, and the electrostatic forces between the positively-charged protons of those nuclei. As a result of these nuclei having higher energy states than stable nuclei of the same element, the nuclei undergo spontaneous transformations to reach their stable energy states: a process known as radioactive decay.

Radioactive decay is a stochastic process, and follows what is known as first-order kinetics. Radioactive decay can be represented mathematically as an integration of the differential equation that describes radioactive decay: the number of radioactive nuclei remaining after time t decreases exponentially over time. The half-life of a radioactive element is the amount of time it takes for half of the initial amount of that element to decay: it can be calculated as the natural log of 2 divided by the decay constant. Radioactive decay often involves the emission of massive amounts of potential energy from those nuclei. The potential energy barrier for radioactive decay is so massive that the particles that are emitted do not have enough energy to overcome this barrier. However, due to the wave-like properties of the radioactive elements, there is a chance for the particles to tunnel straight through the potential energy barrier, which is known as quantum tunneling.

Radioactive decay can occur in three main forms: alpha, beta, and gamma decay. Alpha decay occurs within the nuclei of elements with atomic numbers above 82. The elements release a helium-4 nucleus (containing two protons and two neutrons) to reduce the atomic number of the element. The released particles have high energy levels per unit length (linear energy transfer); they are removed from the nuclei rapidly. Alpha decay is stopped by a sheet of paper or the outermost layer of skin. However, if those alpha particles enter the body, they can create significant damage to the internal tissues.

Beta decay occurs as a result of the weak nuclear force; those weak interactions allow the nuclei to reach a more favorable ratio of neutrons to protons. In beta-minus decay, a neutron in the nucleus becomes a proton. In the process, an electron and an electron antineutrino are emitted. In beta-plus decay, a proton in the nuclei becomes a neutron. In the process, a positron and a neutrino are emitted. These released particles have less energy per unit length of travel than alpha particles. They can travel further, but can be blocked by thin sheets of metal.

Gamma radiation is emitted by nuclei that have undergone alpha or beta decay, but are still in an excited state. The nuclei release high-energy photons to return to their ground state without altering the atomic and mass number of the element. These photons have no mass or electrical charge. Thus, gamma radiation is highly penetrative; it can pass through most materials, and requires shielding with lead or concrete.

Radiation interacts with matter in a variety of ways. For instance, charged particles interact with the electrons of atoms in the material through Coulombic forces. These interactions can result in the ionization of the atoms, or the emission of bremsstrahlung radiation. Photons interact with matter differently, depending upon the energy of that radiation. At low energies, photons can be entirely absorbed by the atoms of the material, and the absorbed photon will remove an inner-shell electron from those atoms (known as the photoelectric effect). In the intermediate range of photon energy, photons interact with the outer-shell electrons of the material through collisions between the two particles (known as Compton scattering). At high energies, photons interact with the electric fields of the nuclei of the material to create an electron and positron pair (known as pair production).

These interaction mechanisms are utilized in a variety of fields. For example, X-rays and CT scans use the different attenuation rates of the various tissues within the human body of the scanned individual. Radioisotope tracers are introduced into the body to perform PET scans. Ionizing radiation is used to destroy the DNA of cancerous cells. Radioactive isotopes are used in industrial applications to inspect welds in metals, or to sterilize medical equipment or food products. Finally, the decay of carbon-14 is used by archaeologists for radiocarbon dating.

Radiation is also a main source of energy production for the planet. Nuclear power plants use nuclear fission reactions to produce energy, typically using uranium-235 fuel. When the nuclei of uranium-235 absorb a neutron, they often undergo fission into two smaller nuclei, releasing a large amount of energy, and releasing more neutrons as a result of the fission reaction. This energy is used to heat water to create steam, which turns a turbine to generate electricity. The use of nuclear fuel does not contribute to the release of greenhouse gases into the atmosphere. However, it does contribute to the release of radioactive elements into the environment.

Despite the benefits that are created by radiation, excessive exposure to ionizing radiation can lead to detrimental changes in the chemistry of the body’s cells. Thus, protection from excessive radiation exposure involves the use of three main variables: time, distance, and shielding. Limiting the length of time that an individual is exposed to radiation will limit their radiation dose. Increasing the distance between the radiation source and the individual reduces the radiation dose that the human body absorbs. Finally, shielding the individuals with high-atomic number elements (like lead) protects them from the effects of radiation. These three variables can be incorporated into engineering and design concepts to both benefit from the quantum properties of radiation, while also minimizing their potential risks to the body.