Krypton (from Ancient Greek: κρυπτός, romanized: kryptos 'the hidden one') is a chemical element; it has symbol Kr and atomic number 36.

It is a colorless, odorless noble gas that occurs in trace amounts in the atmosphere and is often used with other rare gases in fluorescent lamps.

From 1960 to 1983, the official definition of the metre was based on the wavelength of one spectral line of krypton-86, because of the high power and relative ease of operation of krypton discharge tubes.

[12] William Ramsay was awarded the 1904 Nobel Prize in Chemistry for discovery of a series of noble gases, including krypton.

[13] In 1960, the International Bureau of Weights and Measures defined the meter as 1,650,763.73 wavelengths of light emitted in the vacuum corresponding to the transition between the 2p10 and 5d5 levels in the isotope krypton-86.

This also made obsolete the 1927 definition of the ångström based on the red cadmium spectral line,[16] replacing it with 1 Å = 10−10 m. The krypton-86 definition lasted until the October 1983 conference, which redefined the meter as the distance that light travels in vacuum during 1/299,792,458 s.[17][18][19] Krypton is characterized by several sharp emission lines (spectral signatures) the strongest being green and yellow.

[24] Traces of 81Kr, a cosmogenic nuclide produced by the cosmic ray irradiation of 80Kr, also occur in nature: this isotope is radioactive with a half-life of 230,000 years.

Krypton is highly volatile and does not stay in solution in near-surface water, but 81Kr has been used for dating old (50,000–800,000 years) groundwater.

They have a face-centered cubic structure where krypton octahedra are surrounded by randomly oriented hydrogen molecules.

[37] The amount of krypton in space is uncertain, because measurement is derived from meteoric activity and solar winds.

Krypton gas is also combined with mercury to make luminous signs that glow with a bright greenish-blue light.

Krypton (along with xenon) is also used to fill incandescent lamps to reduce filament evaporation and allow higher operating temperatures.

[42] The krypton fluoride laser is important in nuclear fusion energy research in confinement experiments.

The laser has high beam uniformity, short wavelength, and the spot size can be varied to track an imploding pellet.

The advantage of krypton is a smaller Molière radius of 4.7 cm, which provides excellent spatial resolution with little overlapping.

[44] Although xenon has potential for use in computed tomography (CT) to assess regional ventilation, its anesthetic properties limit its fraction in the breathing gas to 35%.