Krypton-85 (⁸⁵Kr) is 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 ...

krypton Krypton (from 'the hidden one') is a chemical element; it has symbol (chemistry), symbol Kr and atomic number 36. It is a colorless, odorless noble gas that occurs in trace element, trace amounts in the Earth's atmosphere, atmosphere and is of ...

. Krypton-85 has a

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 10.756 years and a maximum

decay energy The decay energy is the energy change of a nucleus having undergone a radioactive decay. Radioactive decay is the process in which an unstable atomic nucleus loses energy by emitting ionizing particles and radiation. This decay, or loss of energ ...

of 687 keV. It decays into stable

rubidium Rubidium is a chemical element; it has Symbol (chemistry), symbol Rb and atomic number 37. It is a very soft, whitish-grey solid in the alkali metal group, similar to potassium and caesium. Rubidium is the first alkali metal in the group to have ...

-85. Its most common decay (99.57%) is by

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 � ...

emission with a maximum energy of 687 keV and an average energy of 251 keV. The second most common decay (0.43%) is by beta particle emission (maximum energy of 173 keV) followed by

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 ...

emission (energy of 514 keV). Other decay modes have very small probabilities and emit less energetic gamma rays. Krypton-85 is mostly

synthetic Synthetic may refer to: Science * Synthetic biology * Synthetic chemical or compound, produced by the process of chemical synthesis * Synthetic elements, chemical elements that are not naturally found on Earth and therefore have to be created in ...

, though it is produced naturally in trace quantities by cosmic ray spallation. In terms of radiotoxicity, 440 Bq of ⁸⁵Kr is equivalent to 1 Bq of

radon-222 Radon-222 (222Rn, Rn-222, historically radium emanation or radon) is the most stable isotope of radon, with a half-life of approximately 3.8215(2) days. It is transient in the decay chain of primordial uranium-238 and is the immediate decay pro ...

, without considering the rest of the radon decay chain.

Presence in Earth's atmosphere

Natural production

Krypton-85 is produced in small quantities by the interaction of

cosmic ray Cosmic rays or astroparticles are high-energy particles or clusters of particles (primarily represented by protons or atomic nuclei) that move through space at nearly the speed of light. They originate from the Sun, from outside of the ...

s with stable krypton-84 in the atmosphere. Natural sources maintain an equilibrium inventory of about 0.09 PBq in the atmosphere.

Anthropogenic production

As of 2009, the total amount in the atmosphere is estimated at 5500 PBq due to anthropogenic sources. At the end of the year 2000, it was estimated to be 4800 PBq, and in 1973, an estimated 1961 PBq (53 megacuries). The most important of these human sources is

nuclear fuel reprocessing Nuclear may refer to: Physics Relating to the nucleus of the atom: * Nuclear engineering * Nuclear physics * Nuclear power * Nuclear reactor * Nuclear weapon * Nuclear medicine *Radiation therapy *Nuclear warfare Mathematics * Nuclear space * ...

, as krypton-85 is one of the seven common

medium-lived fission product Long-lived fission products (LLFPs) are radioactive materials with a long half-life (more than 200,000 years) produced by nuclear fission of uranium and plutonium. Because of their persistent Ionizing radiation, radiotoxicity, it is necessary to is ...

s. Nuclear fission produces about three atoms of krypton-85 for every 1000 fissions (i.e., it has a fission yield of 0.3%). Most or all of this krypton-85 is retained in the

spent nuclear fuel Spent nuclear fuel, occasionally called used nuclear fuel, is nuclear fuel that has been irradiated in a nuclear reactor (usually at a nuclear power plant). It is no longer useful in sustaining a nuclear reaction in an ordinary thermal reactor and ...

rods; spent fuel on discharge from a reactor contains between 0.13–1.8 PBq/Mg of krypton-85. Some of this spent fuel is reprocessed. Current nuclear reprocessing releases the gaseous ⁸⁵Kr into the atmosphere when the spent fuel is dissolved. It would be possible in principle to capture and store this krypton gas as

nuclear waste Radioactive waste is a type of hazardous waste that contains radioactive material. It is a result of many activities, including nuclear medicine, nuclear research, nuclear power generation, nuclear decommissioning, rare-earth mining, and nuclear ...

or for use. The cumulative global amount of krypton-85 released from reprocessing activity has been estimated as 10,600 PBq as of 2000. The global inventory noted above is smaller than this amount due to radioactive decay; a smaller fraction is dissolved into the deep oceans. Other man-made sources are small contributors to the total. Atmospheric nuclear weapons tests released an estimated 111–185 PBq. The 1979 accident at the Three Mile Island nuclear power plant released about . The

Chernobyl accident On 26 April 1986, the no. 4 reactor of the Chernobyl Nuclear Power Plant, located near Pripyat, Ukrainian Soviet Socialist Republic, Ukrainian SSR, Soviet Union (now Ukraine), exploded. With dozens of direct casualties, it is one of only ...

released about 35 PBq, and the Fukushima Daiichi accident released an estimated 44–84 PBq. The average atmospheric concentration of krypton-85 was approximately 0.6 Bq/m³ in 1976, and has increased to approximately 1.3 Bq/m³ as of 2005. These are approximate global average values; concentrations are higher locally around nuclear reprocessing facilities, and are generally higher in the northern hemisphere than in the southern hemisphere. For wide-area atmospheric monitoring, krypton-85 is the best indicator for clandestine plutonium separations. Krypton-85 releases increase the electrical conductivity of atmospheric air. Meteorological effects are expected to be stronger closer to the source of the emissions.

Uses in industry

Krypton-85 is used in arc discharge lamps commonly used in the entertainment industry for large HMI film lights as well as

high-intensity discharge lamp High-intensity discharge lamps (HID lamps) are a type of electrical gas-discharge lamp which produces light by means of an electric arc between tungsten electrodes housed inside a translucent or transparent fused quartz or fused alumina arc ...

s. The presence of krypton-85 in discharge tube of the lamps can make the lamps easy to ignite. Early experimental krypton-85 lighting developments included a railroad signal light designed in 1957 and an illuminated highway sign erected in

Arizona Arizona is a U.S. state, state in the Southwestern United States, Southwestern region of the United States, sharing the Four Corners region of the western United States with Colorado, New Mexico, and Utah. It also borders Nevada to the nort ...

in 1969. A 60 μCi (2.22 MBq) capsule of krypton-85 was used by the random number server HotBits (an allusion to the radioactive element being a

quantum mechanical Quantum mechanics is the fundamental physical theory that describes the behavior of matter and of light; its unusual characteristics typically occur at and below the scale of atoms. Reprinted, Addison-Wesley, 1989, It is the foundation of a ...

source of entropy), but was replaced with a 5 μCi (185 kBq) Cs-137 source in 1998. Krypton-85 is also used to inspect aircraft components for small defects. Krypton-85 is allowed to penetrate small cracks, and then its presence is detected by autoradiography. The method is called "krypton gas penetrant imaging". The gas penetrates smaller openings than the liquids used in dye penetrant inspection and fluorescent penetrant inspection. Krypton-85 was used in cold-cathode voltage regulator electron tubes, such as the type 5651. Krypton-85 is also used for Industrial Process Control mainly for thickness and density measurements as an alternative to Sr-90 or Cs-137. Krypton-85 is also used as a charge neutralizer in aerosol sampling systems.

References

{{Reflist Fission products Krypton-085