Radioactive decay

Radioactive decay is the way 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 helped start the discovery of radioactivity through his work on X-rays. In 1896, Henri Becquerel found radioactivity when he studied materials that glow in the dark. He saw that uranium salts could change a wrapped photographic plate, showing these materials give off invisible rays.

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

Early health dangers

Main article: Ionizing radiation

Main article: Radiation protection

See also: Sievert and Ionizing radiation

When X-rays were found in 1895 by Wilhelm Röntgen, many people did not know they could be dangerous. Scientists and doctors used X-rays a lot, and some people got sick from too much exposure. Even early warnings were not always followed.

Radioactive substances were sold as health treatments, even though they could be very harmful. It took many years for scientists to learn how radiation could hurt our bodies and to make safety rules. Today, we know that even small amounts of radiation can increase the risk of health problems, and there are strict rules 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. Some places still use curies, but many countries now use becquerels instead.

Types

Radioactive decay is when an unstable atomic nucleus loses energy by sending out radiation. This happens when the nucleus is not balanced and needs to become more stable. During decay, the nucleus can send out energy and tiny particles.

Scientists found 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 can go through many materials.

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 has many useful purposes. Scientists use it to follow how substances move, like inside a living organism, by adding unstable atoms and seeing where they decay. It also helps figure out the age of rocks and organic materials by measuring how certain isotopes decay.

Aggregate processes

Radioactive decay is when 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 change 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. It alerts people that there is radioactive material or ionising radiation nearby.

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