Radioluminescence is the phenomenon by which light is produced in a material by bombardment with

ionizing radiation Ionizing (ionising) radiation, including Radioactive decay, nuclear radiation, consists of subatomic particles or electromagnetic waves that have enough energy per individual photon or particle to ionization, ionize atoms or molecules by detaching ...

such as

alpha particle Alpha particles, also called alpha rays or alpha radiation, consist of two protons and two neutrons bound together into a particle identical to a helium-4 nucleus. They are generally produced in the process of alpha decay but may also be produce ...

beta particle A beta particle, also called beta ray or beta radiation (symbol β), is a high-energy, high-speed electron or positron emitted by the radioactive decay of an atomic nucleus, known as beta decay. There are two forms of beta decay, β− decay and � ...

s, or

gamma ray A gamma ray, also known as gamma radiation (symbol ), is a penetrating form of electromagnetic radiation arising from high energy interactions like the radioactive decay of atomic nuclei or astronomical events like solar flares. It consists o ...

s. Radioluminescence is used as a low level light source for night illumination of instruments or signage. Radioluminescent paint is occasionally used for clock hands and instrument dials, enabling them to be read in the dark. Radioluminescence is also sometimes seen around high-power radiation sources, such as

nuclear reactor A nuclear reactor is a device used to initiate and control a Nuclear fission, fission nuclear chain reaction. They are used for Nuclear power, commercial electricity, nuclear marine propulsion, marine propulsion, Weapons-grade plutonium, weapons ...

s and radioisotopes.

Mechanism

Radioluminescence occurs when an incoming particle of

collides with an atom or molecule, exciting an orbital electron to a higher energy level. The particle usually comes from the

radioactive decay Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is conside ...

of an atom of a

radioisotope A radionuclide (radioactive nuclide, radioisotope or radioactive isotope) is a nuclide that has excess numbers of either neutrons or protons, giving it excess nuclear energy, and making it unstable. This excess energy can be used in one of three ...

, an

isotope Isotopes are distinct nuclear species (or ''nuclides'') of the same chemical element. They have the same atomic number (number of protons in their Atomic nucleus, nuclei) and position in the periodic table (and hence belong to the same chemica ...

of an element which is radioactive. The electron then returns to its ground energy level by emitting the extra energy as a

photon A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless particles that can ...

of light. A chemical that releases light of a particular color when struck by ionizing radiation is called a

phosphor A phosphor is a substance that exhibits the phenomenon of luminescence; it emits light when exposed to some type of radiant energy. The term is used both for fluorescent or phosphorescent substances which glow on exposure to ultraviolet or ...

. Radioluminescent light sources usually consist of a radioactive substance mixed with, or in proximity to, a phosphor.

Applications

Since radioactivity was discovered around the beginning of the 20th century, the main application of radioluminescence has been in radioluminescent paint, used on

watch A watch is a timepiece carried or worn by a person. It is designed to maintain a consistent movement despite the motions caused by the person's activities. A wristwatch is worn around the wrist, attached by a watch strap or another type of ...

and

compass A compass is a device that shows the cardinal directions used for navigation and geographic orientation. It commonly consists of a magnetized needle or other element, such as a compass card or compass rose, which can pivot to align itself with No ...

dials, gunsights, aircraft flight instrument faces, and other instruments, allowing them to be seen in darkness. Radioluminescent paint consists of a mixture of a chemical containing a

with a radioluminescent chemical (

). The continuous

of the isotope's atoms releases radiation particles which strike the molecules of the phosphor, causing them to emit light. The constant bombardment by radioactive particles causes the chemical breakdown of many types of phosphor, so radioluminescent paints lose some of their luminosity during their working life. Radioluminescent materials may also be used in the construction of an

optoelectric nuclear battery An optoelectric nuclear battery (also radiophotovoltaic device, radioluminescent nuclear battery or radioisotope photovoltaic generator) is a type of nuclear battery in which nuclear energy is converted into light, which is then used to generate ...

, a type of radioisotope generator in which nuclear energy is converted into light. The radioluminescence of nitrogen in air can be used to detect alpha radiation in nuclear contamination sites.

Radium

The first use of radioluminescence was in luminous paint containing

radium Radium is a chemical element; it has chemical symbol, symbol Ra and atomic number 88. It is the sixth element in alkaline earth metal, group 2 of the periodic table, also known as the alkaline earth metals. Pure radium is silvery-white, ...

, a natural

. Beginning in 1908, luminous paint containing a mixture of

and

copper Copper is a chemical element; it has symbol Cu (from Latin ) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkish-orang ...

- doped

zinc sulfide Zinc sulfide (or zinc sulphide) is an inorganic compound with the chemical formula of ZnS. This is the main form of zinc found in nature, where it mainly occurs as the mineral sphalerite. Although this mineral is usually black because of various i ...

was used to paint watch faces and instrument dials, giving a greenish glow. Phosphors containing

-doped zinc sulfide (ZnS:Cu) yield blue-green light; copper and

manganese Manganese is a chemical element; it has Symbol (chemistry), symbol Mn and atomic number 25. It is a hard, brittle, silvery metal, often found in minerals in combination with iron. Manganese was first isolated in the 1770s. It is a transition m ...

-doped zinc sulfide (), yielding yellow-orange light are also used. Radium-based luminescent paint is no longer used due to the radiation hazard posed to persons manufacturing the dials. These phosphors are not suitable for use in layers thicker than 25 mg/cm², as the self-absorption of the light then becomes a problem. Zinc sulfide undergoes degradation of its crystal lattice structure, leading to gradual loss of brightness significantly faster than the depletion of radium. ZnS:Ag coated

spinthariscope A spinthariscope () is a device for observing individual Radioactive decay, nuclear disintegrations caused by the interaction of ionizing radiation with a phosphor (see radioluminescence) or scintillator. Invention The spinthariscope was invente ...

screens were used by

Ernest Rutherford Ernest Rutherford, 1st Baron Rutherford of Nelson (30 August 1871 – 19 October 1937) was a New Zealand physicist who was a pioneering researcher in both Atomic physics, atomic and nuclear physics. He has been described as "the father of nu ...

in his experiments discovering the

atomic nucleus The atomic nucleus is the small, dense region consisting of protons and neutrons at the center of an atom, discovered in 1911 by Ernest Rutherford at the Department_of_Physics_and_Astronomy,_University_of_Manchester , University of Manchester ...

. Radium was used in luminous paint until the 1960s, when it was replaced with the other radioisotopes mentioned above due to health concerns. In addition to

alpha Alpha (uppercase , lowercase ) is the first letter of the Greek alphabet. In the system of Greek numerals, it has a value of one. Alpha is derived from the Phoenician letter ''aleph'' , whose name comes from the West Semitic word for ' ...

and

s, radium emits penetrating

s, which can pass through the metal and glass of a watch dial, and skin. A typical older radium wristwatch dial has a radioactivity of 3–10 kBq and could expose its wearer to an annual dose of 24 millisieverts if worn continuously. Another health hazard is its decay product, the radioactive gas

radon Radon is a chemical element; it has symbol Rn and atomic number 86. It is a radioactive noble gas and is colorless and odorless. Of the three naturally occurring radon isotopes, only Rn has a sufficiently long half-life (3.825 days) for it to b ...

, which constitutes a significant risk even at extremely low concentrations when inhaled. Radium's long

half-life Half-life is a mathematical and scientific description of exponential or gradual decay. Half-life, half life or halflife may also refer to: Film * Half-Life (film), ''Half-Life'' (film), a 2008 independent film by Jennifer Phang * ''Half Life: ...

of 1600 years means that surfaces coated with radium paint, such as watch faces and hands, remain a health hazard long after their useful life is over. There are still millions of luminous radium clock, watch, and compass faces and aircraft instrument dials owned by the public. The case of the "

Radium Girls The Radium Girls were female factory workers who contracted radiation poisoning from painting radium dials – watch dials and hands with radioluminescence, self-luminous paint. The incidents occurred at three factories in the United States: one ...

", workers in watch factories in the early 1920s who painted watch faces with radium paint and later contracted fatal cancer through ingesting radium when they pointed their brushes with their lips, increased public awareness of the hazards of radioluminescent materials, and

radioactivity Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is conside ...

in general.

Promethium

In the second half of the 20th century, radium was progressively replaced with paint containing

promethium Promethium is a chemical element; it has Symbol (chemistry), symbol Pm and atomic number 61. All of its isotopes are Radioactive decay, radioactive; it is extremely rare, with only about 500–600 grams naturally occurring in the Earth's crust a ...

-147. Promethium is a low-energy beta-emitter, which, unlike alpha emitters like radium, does not degrade the phosphor lattice, so the luminosity of the material will not degrade so quickly. It also does not emit the penetrating

s which radium does. The half-life of ¹⁴⁷Pm is only 2.62 years, so in a decade the radioactivity of a promethium dial will decline to only 1/16 of its original value, making it safer to dispose of, compared to radium with its half life of 1600 years. This short half-life meant that the luminosity of promethium dials also dropped by half every 2.62 years, giving them a short useful life, which led to promethium's replacement by tritium. Promethium-based paint was used to illuminate

Apollo Lunar Module The Apollo Lunar Module (LM ), originally designated the Lunar Excursion Module (LEM), was the lunar lander spacecraft that was flown between lunar orbit and the Moon's surface during the United States' Apollo program. It was the first crewed sp ...

electrical switch tips and painted on control panels of the

Lunar Roving Vehicle The Lunar Roving Vehicle (LRV) is a Battery electric vehicle, battery-powered four-wheeled Rover (space exploration), rover used on the Moon in the last three missions of the American Apollo program (Apollo 15, 15, Apollo 16, 16, and Apollo 17 ...

Tritium

The latest generation of radioluminescent materials is based on

tritium Tritium () or hydrogen-3 (symbol T or H) is a rare and radioactive isotope of hydrogen with a half-life of ~12.33 years. The tritium nucleus (t, sometimes called a ''triton'') contains one proton and two neutrons, whereas the nucleus of the ...

, a radioactive isotope of

hydrogen Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...

with half-life of 12.32 years that emits very low-energy beta radiation. It is used on

wristwatch A watch is a timepiece carried or worn by a person. It is designed to maintain a consistent movement despite the motions caused by the person's activities. A wristwatch is worn around the wrist, attached by a watch strap or another type of ...

faces, gun sights, and emergency exit signs. The tritium gas is contained in a small glass tube, coated with a

on the inside. Beta particles emitted by the tritium strike the phosphor coating and cause it to

fluoresce Fluorescence is one of two kinds of photoluminescence, the emission of light by a substance that has absorbed light or other electromagnetic radiation. When exposed to ultraviolet radiation, many substances will glow (fluoresce) with color ...

, emitting light, usually yellow-green. Tritium is used because it is believed to pose a negligible threat to human health, in contrast to the previous radioluminescent source, radium, which proved to be a significant radiological hazard. The low-energy 5.7 keV beta particles emitted by tritium cannot pass through the enclosing glass tube. Even if they could, they are not able to penetrate human skin. Tritium is only a health threat if ingested or inhaled. Since tritium is a gas, if a tritium tube breaks, the gas dissipates in the air and is diluted to safe concentrations. Tritium has a half-life of 12.32 years, so the brightness of a tritium light source will decline to half its initial value in that time.

Infrared radiofluorescence

Infrared radiofluorescence (sometimes spelt radio-fluorescence) is a

dating Dating is a stage of Romance (love), romantic relationships in which individuals engage in activity together, often with the intention of evaluating each other's suitability as a partner in a future intimate relationship. It falls into the cate ...

technique involving the

infrared Infrared (IR; sometimes called infrared light) is electromagnetic radiation (EMR) with wavelengths longer than that of visible light but shorter than microwaves. The infrared spectral band begins with the waves that are just longer than those ...

(~ 880 nm) luminescence signal of

orthoclase Orthoclase, or orthoclase feldspar ( endmember formula K Al Si3 O8), is an important tectosilicate mineral which forms igneous rock. The name is from the Ancient Greek for "straight fracture", because its two cleavage planes are at right angles ...

from exposure to

. It can reveal the last time of daylight exposure of sediments, e.g., a layer of sand exposed to light before deposition.

References

{{Authority control Luminescence