Radioactive decay

Radioactive decay is the process by which an unstable atomic nucleus loses energy by sending out radiation. Materials with unstable nuclei are called radioactive. The three most common types of radioactive decay are alpha, beta, and gamma decay. These processes happen because of forces inside the nucleus, such as the weak force, electromagnetic force, and nuclear forces.

One interesting thing about radioactive decay is that it is random. We cannot predict exactly when a single atom will decay, but we can measure how quickly large groups of atoms decay. This is described using a half-life, which tells us how long it takes for half of the atoms in a sample to decay.

When an atom undergoes radioactive decay, it changes into a different atom, called a daughter nuclide. This change can create a new chemical element. Some elements, like uranium and thorium, have been decaying for billions of years and are still found in nature today. These long-lasting materials are part of what scientists call primordial radionuclides, and they play a role in the history of our Solar System.

History of discovery

Pierre and Marie Curie in their Paris laboratory, before 1907

Henri Poincaré inspired the discovery of radioactivity through his work on X-rays. In 1896, Henri Becquerel discovered radioactivity while studying materials that glow in the dark. He found that uranium salts could darken a wrapped photographic plate, showing that these materials gave off invisible rays.

Marie Curie named these "Becquerel Rays" and showed they came from the atoms themselves. Scientists later learned that many elements, not just uranium, could emit these rays. Marie and Pierre Curie even discovered two new elements, polonium and radium, during their research. Their work helped start the use of radium in treating diseases like cancer, marking the beginning of modern nuclear medicine.

Early health dangers

Main article: Ionizing radiation

Main article: Radiation protection

Taking an X-ray image with early Crookes tube apparatus in 1896. The Crookes tube is visible in the centre. The standing man is viewing his hand with a fluoroscope screen; this was a common way of setting up the tube. No precautions against radiation exposure are being taken; its hazards were not known at the time.

See also: Sievert and Ionizing radiation

When X-rays were discovered in 1895 by Wilhelm Röntgen, many people didn’t understand their dangers. Scientists and doctors began using X-rays a lot, and some suffered burns and hair loss from too much exposure. Even early warnings about these risks weren’t always followed.

Radioactive substances were also sold as health treatments back then, even though they could be very harmful. It took many years for scientists to understand how radiation could damage our bodies and to create safety rules to protect people. Today, we know that even small amounts of radiation can increase the risk of health problems like cancer, and there are strict guidelines to keep people safe.

Units

Graphic showing relationships between radioactivity and detected ionizing radiation

The International System of Units (SI) uses a unit called the becquerel (Bq) to measure radioactive activity. One Bq means that one atom breaks apart, or decays, every second.

Another older unit is the curie (Ci). Today, one curie equals 37 billion becquerels. While some places still use curies, many countries have moved to using becquerels instead.

Types

Radioactive decay is the process by which an unstable atomic nucleus loses energy by radiation. This happens when the nucleus is not balanced and needs to become more stable. During decay, the nucleus can release energy and small particles.

Scientists discovered that radioactive emissions can be split into three main types by using electric or magnetic fields. These types are called alpha, beta, and gamma decay, named by Ernest Rutherford. Alpha decay happens in heavier elements, beta decay occurs in all elements, and gamma decay often follows alpha or beta decay. Alpha particles are heavy and have a positive charge, beta particles are light and have a negative charge, and gamma rays have no charge and are very penetrating.

Decay modes in NUBASE2020
Mode	Name	Action	Nucleus changes
α	alpha emission	An alpha particle (A = 4, Z = 2) emitted from nucleus	(A − 4, Z − 2)
p	proton emission	A proton ejected from nucleus	(A − 1, Z − 1)
2p	2-proton emission	Two protons ejected from nucleus simultaneously	(A − 2, Z − 2)
n	neutron emission	A neutron ejected from nucleus	(A − 1, Z)
2n	2-neutron emission	Two neutrons ejected from nucleus simultaneously	(A − 2, Z)
ε	electron capture	A nucleus captures an orbiting electron and emits a neutrino; the daughter nucleus is left in an excited unstable state	(A, Z − 1)
e+	positron emission	A nuclear proton converts to a neutron by emitting a positron and an electron neutrino	(A, Z − 1)
β⁺ ε + e⁺	positron emission	In NUBASE2020, β⁺ refers to the combined rate of electron capture (ε) and positron emission (e⁺): β⁺ = ε + e⁺	(A, Z − 1)
β⁻	β⁻ decay	A nucleus emits an electron and an electron antineutrino	(A, Z + 1)
β⁻β⁻ 2β⁻	double β⁻ decay	A nucleus emits two electrons and two antineutrinos	(A, Z + 2)
β⁺β⁺ 2β⁺	double β⁺ decay	A nucleus emits two positrons and two neutrinos	(A, Z − 2)
β⁻n	β⁻-delayed neutron emission	A nucleus decays by β⁻ emission to an excited state, which then emits a neutron	(A − 1, Z + 1)
β⁻2n	β⁻-delayed 2-neutron emission	A nucleus decays by β⁻ emission to an excited state, which then emits two neutrons	(A − 2, Z + 1)
β⁻3n	β⁻-delayed 3-neutron emission	A nucleus decays by β⁻ emission to an excited state, which then emits three neutrons	(A − 3, Z + 1)
β⁺p	β⁺-delayed proton emission	A nucleus decays by β⁺ emission to an excited state, which then emits a proton	(A − 1, Z − 2)
β⁺2p	β⁺-delayed 2-proton emission	A nucleus decays by β⁺ emission to an excited state, which then emits two protons	(A − 2, Z − 3)
β⁺3p	β⁺-delayed 3-proton emission	A nucleus decays by β⁺ emission to an excited state, which then emits three protons	(A − 3, Z − 4)
β⁻α	β⁻-delayed alpha emission	A nucleus decays by β⁻ emission to an excited state, which then emits an α particle	(A − 4, Z − 1)
β⁺α	β⁺-delayed alpha emission	A nucleus decays by β⁺ emission to an excited state, which then emits an a particle	(A − 4, Z − 3)
β⁻d	β⁻-delayed deuteron emission	A nucleus decays by β⁻ emission to an excited state, which then emits a deuteron	(A − 2, Z)
β⁻t	β⁻-delayed triton emission	A nucleus decays by β⁻ emission to an excited state, which then emits a triton	(A − 3, Z)
CD	cluster decay	A nucleus emits a specific type of smaller nucleus (A₁, Z₁) which is larger than an alpha particle (e.g. ¹⁴C, ²⁴Ne)	(A − A₁, Z − Z₁) & (A₁, Z₁)
IT	internal (isomeric) transition	A nucleus in a metastable state drops to a lower energy state by emitting a photon or ejecting an electron	(A, Z)
SF	spontaneous fission	A nucleus disintegrates into two or more smaller nuclei and other particles, all of which may vary with each decay	variable
β⁺SF	β⁺-delayed fission	A nucleus decays by β⁺ emission to an excited state, which then undergoes spontaneous fission	β+ & variable
β⁻SF	β⁻-delayed fission	A nucleus decays by β⁻ emission to an excited state, which then undergoes spontaneous fission	β⁻ & variable

Occurrence and applications

According to the Big Bang theory, the lightest elements like H, He, and Li were formed very early in the universe. Heavier elements, including radioactive ones, were created later in stars and during supernovae. For example, carbon-14 is made in Earth's atmosphere when cosmic rays hit nitrogen.

Radioactive decay is useful in many ways. Scientists use it to track how substances move through systems, like inside a living organism, by adding unstable atoms and detecting where they decay. It also helps estimate the age of rocks and organic materials by measuring the decay of certain isotopes.

Aggregate processes

Radioactive decay is the process by which an unstable atom loses energy by sending out tiny particles or energy. Materials with unstable atoms are called radioactive.

Simulation of many identical atoms undergoing radioactive decay, starting with either 4 atoms (left) or 400 (right). The number at the top indicates how many half-lives have elapsed.

There are three main types of radioactive decay: alpha decay, beta decay, and gamma decay. In alpha decay, an atom sends out a cluster of two protons and two neutrons, called an alpha particle. In beta decay, a neutron changes into a proton and sends out an electron or another small particle. Gamma decay sends out high-energy light called gamma rays.

Scientists describe radioactive decay using special words and numbers. The half-life is the time it takes for half of the unstable atoms in a sample to decay. Another important term is the decay constant, which tells us how quickly atoms decay. These help scientists predict how much of a radioactive material will remain after a certain time.

Nuclear processes

Nuclides can be stable or unstable. Unstable nuclides decay until they become stable. There are 251 known stable nuclides, and about 3000 unstable nuclides have been discovered.

The most common types of natural radioactive decay are alpha-decay, beta-decay, and gamma-decay. In alpha decay, a small particle breaks away from the nucleus. Beta decay changes a neutron into a proton or a proton into a neutron. Gamma decay releases energy from the nucleus when it moves to a lower energy state.

Hazard warning signs

When there is radioactive material around, special symbols are used to warn people of the danger. One common symbol is the trefoil, which looks like a three-leaf flower and alerts people to the presence of radioactive material or ionising radiation.

In 2007, the International Organization for Standardization (ISO) created a new radioactivity hazard symbol for certain very dangerous sources. This symbol is used for materials that could cause serious harm if not handled properly. There are also special signs used when transporting radioactive materials to make sure everyone knows they are dangerous and need careful handling.