Cosmic rays
  1. 28/06/1785

    First observations of the spontaneous discharge of an electrometer

    The torsion balance electrometer Coulomb used to…

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  2. 26/02/1896

    Becquerel discovers radioactivity

    The first evidence for radioactivity – images…

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  3. 05/05/1899

    The source: Earth, atmosphere or outer space?

    In studying electrical conduction through air in…

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  4. 01/03/1909

    Theodor Wulf, a new electrometer, and the Eiffel tower

    The original Wulf electroscope (Image: Wikimedia…

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  5. 15/10/1910

    Albert Gockel’s flights

    To measure ionizing radiation away from the earth…

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  6. 02/04/1911

    Pacini and underwater measurements

    Domenico Pacini making a measurement on 20…

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  7. 09/06/1911

    Charles Thomson Rees Wilson sees particle tracks

    Cloud formed on ions due to α-Rays (Image: CTR…

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  8. 07/08/1912

    Victor Hess discovers cosmic rays

    Hess back from his balloon flight on 7 August…

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  9. 28/06/1914

    Kolhörster confirms Hess’s findings

    Increase in ionization with height measured by…

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  10. 13/04/1928

    Robert Millikan coins the term ‘cosmic rays’

    Robert Millikan originally set about to disprove…

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  11. 28/06/1929

    Geiger-Müller counters and the coincidence technique

    Bothe and Kolhorster’s experiment (Image: L.…

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  12. 09/09/1932

    Carl Anderson discovers the positron

    Photograph from Occhialini and Blacket’s paper…

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  13. 01/09/1933

    Bruno Rossi: Cosmic rays are positive charged particles

    Bruno Rossi (Image: Wikimedia Commons) Rossi’s…

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  14. 30/03/1937

    Carl Anderson and Seth Neddermeyer discover the muon

    The muon was discovered as a constituent of…

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  15. 18/07/1938

    Pierre Auger and colleagues demonstrate extensive air showers

    Pierre Auger, who had positioned particle…

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  16. 20/12/1947

    Clifford Butler and George Rochester discover the kaon, first strange particle

    Stereoscopic photographs showing an unusual fork…

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  17. 21/02/1953

    Air Cherenkov discovery Galbraith & Jelley

    The detector used for the first observations of…

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  18. 22/02/1962

    John Linsley detects the first 10^20 eV cosmic ray

    John Linsley detected first cosmic ray with an…

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  19. 01/07/1989

    Detection of high-energy gamma rays from Crab Nebula

    Astrophysicists detected pulsed gamma-ray…

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  20. 15/10/1991

    Fly’s Eye detects record-breaking cosmic ray: 3.2 x 10^20 eV

    The Fly's Eye Mirrors (Image: Courtesy of…

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  21. 03/12/1993

    The AGASA Cosmic-Ray event

    On 3 December 1993, the Akeno Giant Air Shower…

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  22. 25/02/1997

    Baikal – first underwater neutrino telescope

    Satellite image showing spring ice melt underway…

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  23. 15/07/2003

    AMANDA sees first neutrinos in ice

    The photomultiplier tubes within these basketball…

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  24. 21/10/2003

    Pierre Auger Observatory completes its 100th detector

    Upon completing its 100th surface detector, the…

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  25. 09/11/2007

    Pierre Auger Observatory discovers extragalactic origin of highest-energy cosmic rays

    In November 2007, the Auger project published…

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  26. 18/12/2010

    IceCube construction completed

    On 18 December 2010, the expanded version of…

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  27. 16/05/2011

    Launch of the Alphamagnetic Spectrometer (AMS)

    AMS during tests at the University of Geneva,…

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  28. 09/08/2011

    IceCube presents the 1 PeV event

    In 2013 IceCube presented two events at around 1…

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  29. 21/05/2014

    IceCube measures 5σ significance level for the observation of extraterrestrial neutrinos

    Inspired by the 1PeV events, IceCube began a…

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First observations of the spontaneous discharge of an electrometer

The torsion balance electrometer Coulomb used to make his observations (Image: Wikimedia commons)In 1785, the French physicist Charles Augustin de Coulomb made three reports on electricity and magnetism to France’s Royal Academy of Sciences. His third paper described an experiment with a torsion balance, which showed that the device would spontaneously discharge due to the action of the air rather than defective insulation. In 1850, Italian physicist Cano Matteucci and later British physicist William Crookes in 1879 showed that the rate of spontaneous discharge decreased at lower atmospheric pressures. The search for an explanation for the nature of this spontaneous discharge paved the way for the discovery of cosmic rays – high-energy particles from outer space.   Read more: Extract from Mémoires sur l'électricité et le magnétisme (1785-89) by Charles Augustin de Coulomb.

Becquerel discovers radioactivity

The first evidence for radioactivity – images formed by Becquerel’s uranium salts (Image: Wikimedia Commons) French physicist Henri Becquerel discovered radioactivity while working on a series of experiments on phosphorescent materials. On 26 February 1986, he placed uranium salts on top of a photographic plate wrapped in black paper. The salts caused a blackening of the plate despite the paper in between. Becquerel concluded that invisible radiation that could pass through paper was causing the plate to react as if exposed to light. Marie Curie decided to study the new radiation using the sensitive electrometer invented by her husband, Pierre, to measure the conductivity of air that the radiation induced. The discovery of radioactivity cultivated great research interest in Germany and the UK about the origin of the spontaneous electrical discharge observed earlier in the air. The simplest hypothesis was that the discharge was caused by the radioactive materials on Earth, though this was difficult to prove. Researching natural radioactivity eventually lead to the discovery of cosmic rays. 

The source: Earth, atmosphere or outer space?

In studying electrical conduction through air in 1899, Julius Elster and Hans Geitel designed a key experiment where they found that surrounding a gold leaf electroscope with a thick metal box would decrease its spontaneous discharge. From this observation, they concluded that the discharge was due to highly penetrating ionizing agents outside of the container. In a similar experiment at about the same time, Charles Thomson Rees Wilson in Cambridge came to the same conclusion. To test whether the ionizing radiation originated beyond the atmosphere, in 1901 Charles Thomson Rees Wilson took measurements of natural radioactivity using an electroscope inside an old railway tunnel in Scotland. If the radiation were coming from outer space, Wilson could have expected to measure a signification reduction in the tunnel compared to outside on the surface. But he saw no such reduction. Following Wilson’s observations, the scientific community largely dismissed the extra-terrestrial theory. Since some of the radiation was found to be too penetrating and perhaps too abundant to originate from known sources, altitude-dependent studies were carried out to test the idea of an extraterrestrial source – although at first the results were contradictory. 

Theodor Wulf, a new electrometer, and the Eiffel tower

The original Wulf electroscope (Image: Wikimedia Commons) In 1909 Theodor Wulf, a Jesuit priest, designed and built a more sensitive and more transportable electrometer than the gold leaf electroscopes. He measured the ionization of the air in various locations in Germany, Holland and Belgium, concluding that his results were consistent with the hypothesis that the penetrating radiation was caused by radioactive substances in the upper layers of the Earth’s crust. Wulf then started measuring changes in radioactivity with height to understand the origin of the radiation. The hypothesis was simple: if the radioactivity was coming from the Earth, it should decrease with height. Wulf took his electroscope to the top of the Eiffel tower in 1909 and found that the intensity of radiation “decreases at nearly 300 m [altitude to] not even to half of its ground value”. This was too small a decrease to confirm his hypothesis. However, unknown to Wulf, his results were due to the radioactive metal of the Eiffel tower. The search for the source of the mysterious ionizing radiation would continue. 

Albert Gockel’s flights

To measure ionizing radiation away from the earth’s surface, several researchers took to the air in balloon flights in the first decade of the 20th century. One of these pioneers, Albert Gockel, measured the levels of ionizing radiation up to a height of 3000 metres. He concluded that the ionization did not decrease with height and consequently could not have a purely terrestrial origin. He also introduced the term “kosmische Strahlung” – cosmic radiation. Later calculations by Schrödinger showed that the radioactivity came in part from above and in part from the Earth’s crust and that the decrease in the radioactivity from the Earth’s crust could be offset by the growth of radioactivity from extraterrestrial sources up to 3000 m. 

Pacini and underwater measurements

Domenico Pacini making a measurement on 20 October 1910 (Image: Wikimedia Commons) In 1911, Italian physicist Domenico Pacini took readings on a Wulf-style electroscope in various locations and noted a 30% reduction in radioactivity between ionization levels on a ship 300 m off shore from Livorno compared to measurements on land. This result suggested that a significant portion of the penetrating radiation must be independent of emission from the Earth’s crust. He published his paper Penetrating radiation at sea on the 2 April 1911. Pacini also measured the levels of radiation in the deep sea of the Genova gulf. This experiment pioneered the technique of underwater measurement of radiation. He noted that there was 20% less radiation 3 metres below the water compared to on the surface, concluding that the ionizing radiation must come from the atmosphere. Read more: "Domenico Pacini and the origin of cosmic rays" – CERN Courier

Charles Thomson Rees Wilson sees particle tracks

Cloud formed on ions due to α-Rays (Image: CTR Wilson Roy, Proceedings of the Royal Society A, Volume 85, Plate 9) The cloud chamber was fundamental in the history of particle physics and cosmic rays. This device made it possible to record individual charged particles in the secondary particle showers that are initiated when cosmic rays strike particles in the upper atmosphere. Wilson won the 1927 Nobel Prize for his development of the cloud chamber, which he originally undertook to study atmospheric phenomena. In April 1911 he presented his first rough photographs of particle tracks at the Royal Society in London. A cloud chamber is a box containing a supersaturated vapor. As charged particles pass through, they ionize the vapor, which condenses to form droplets on the ions. The tracks of the particles become visible as trails of droplets, which can be photographed. During the first half of the 20th century, experiments that looked at cosmic rays passing through cloud chambers revealed the existence of several fundamental particles, including the positron, the muon and the first strange particles.

Victor Hess discovers cosmic rays

Hess back from his balloon flight on 7 August 1912 (Image: Wikimedia Commons) In 1911 and 1912 Austrian physicist Victor Hess made a series of ascents in a balloon to take measurements of radiation in the atmosphere. He was looking for the source of an ionizing radiation that registered on an electroscope – the prevailing theory was that the radiation came from the rocks of the Earth. In 1911 his balloon reached an altitude of around 1100 metres, but Hess found "no essential change" in the amount of radiation compared with ground level. Then, on 7 April 1912, Hess made an ascent to 5300 metres during a near-total eclipse of the Sun. Since ionization of the atmosphere did not decrease during the eclipse, he reasoned that the source of the radiation could not be the Sun – it had to be coming from further out in space. High in the atmosphere, Hess had discovered a natural source of high-energy particles: cosmic rays. Hess shared the 1936 Nobel prize in physics for his discovery, and cosmic rays have proved useful in physics experiments – including several at CERN – since. Read more: "A discovery of cosmic proportions" – CERN Courier

Kolhörster confirms Hess’s findings

Increase in ionization with height measured by Hess and Kolhörster (Image: Wikimedia Commons) German physicist Werner Kolhörster took balloon measurements up to a height of 9300 m, confirming Hess’s results for greater heights. His results confirmed unambiguously that an unknown radiation with an extreme penetrating power was causing ionization. The intensity of the radiation was relatively constant, with no day-night or weather-dependent variations. In 1913 Kolhörster made three balloon flights, reaching 6200 m on the third flight. In 1914 he reached an altitude of 9300 m where he found the ionization was nine times the value on the ground. Kolhörster’s final flight on 28 June 1914 was the same day as the assassination of Franz Ferdinand and the beginning of the First World War. Research on cosmic rays ceased during the war as scientists became involved in other duties and only resumed in the early 1920’s. 

Robert Millikan coins the term ‘cosmic rays’

Robert Millikan originally set about to disprove Hess and Kolhörster’s discovery. He and Ira Sprague reached a height of 1500 m in a balloon over Texas where they recorded a radiation intensity of approximately one quarter of Hess and Kohörster's measurement. The difference was caused by a geomagnetic difference between Texas and Central Europe but was blamed on turnover in the intensity curve at high altitude. Millikan and Harvey Cameron reported on experiments on high-altitude lakes in 1926. They measured ionization rates at various depths in lakes at altitudes of 1500 m and 3600 m. The underwater rate of the lower lake corresponded to the rate obtained 2 m deeper in the higher lake. The pair concluded that particles shoot through space equally in all directions. This demonstrated that two metres of water absorbed about the same as two kilometers of air, and convinced Millikan that rays do come from above. Millikan was convinced that penetrating radiation entering the atmosphere was electromagnetic and coined the term ‘cosmic rays’ in a paper where he argued that cosmic rays were the ‘birth cries of atoms’ in the galaxy. Read more: "The Origin of the Cosmic Rays" – R.A. Millikan, G.H. Cameron, Physical Review Letters, 32 (1928) 533

Geiger-Müller counters and the coincidence technique

Bothe and Kolhorster’s experiment (Image: L. Bonolis, American Journal of Physics, 79 (2011), 1133. Reproduced under Creative Commons license) In 1929 Hans Geiger and Walter Müller developed a gas filled ionization detector – a tube that registers individual charged particles. This Geiger-Müller counter was ideal for studying high-energy cosmic rays. Two such tubes placed one above the other could register 'coincidences' – when an incoming particles passes through both tubes – and thus define the path of a cosmic ray. Walther Bothe and Werner Kolhörster connected two Geiger counters to electrometers and immediately observed these ‘coincidences’. A gamma ray only fires a Geiger counter if it knocks an electron out of an atom. The observation of coincident signals suggests that a cosmic gamma ray had either produced two electrons or that a single electron had fired both counters. To test if it was an electron that had set off both counters Bothe and Kolhörster put gold 4 cm thick between the counters to absorb the electrons knocked off from the atoms. They found that the rays were not affected and concluded that cosmic rays consisted of electrically charged particles and not gamma rays. Interposing a 4 cm thick gold piece between the tubes only slightly reduced the coincidence rate proving that cosmic rays contain charged particles of much higher energy than the Crompton electrons that would be produced by gamma rays. 

Carl Anderson discovers the positron

Photograph from Occhialini and Blacket’s paper showing tracks of radiation (Image: Blackett, P.M.S., & Occhialini, G.P.S., Royal Society of London Proceedings Series A 139 (1933) 699) In 1932 Carl Anderson, a young professor at the California Institute of Technology in the US, was studying showers of cosmic particles in a cloud chamber and saw a track left by "something positively charged, and with the same mass as an electron". After nearly a year of effort and observation, he decided the tracks were actually antielectrons, each produced alongside an electron from the impact of cosmic rays in the cloud chamber. He called the antielectron a "positron", for its positive charge and published his results in the journal Science, in a paper entitled The apparent existence of easily deflectable positives (1932). The discovery was confirmed soon after by Occhialini and Blacket, who in 1934 published Some photographs of the tracks of penetrating radiation in the journal Proceedings of the Royal Society A. Anderson's observations proved the existence of the antiparticles predicted by Dirac. For discovering the positron, Anderson shared the 1936 Nobel prize in physics with Victor Hess. For years to come, cosmic rays remained the only source of high-energy particles. The next antiparticle physicists were looking for was the antiproton. Much heavier than the positron, the antiproton is the antipartner of the proton. It would not be confirmed experimentally for another 22 years.

Bruno Rossi: Cosmic rays are positive charged particles

Bruno Rossi (Image: Wikimedia Commons) Rossi’s coincidence circuits form the basis of all modern electronic-counter experiments. In 1930, Bruno Rossi used electronic valves to register coincident pulses from the Geiger counters. He arranged the detectors in a triangle so that the cosmic rays could not transverse all three counters. In 1932 he found that 60% of the cosmic rays that pass through the 25 cm piece of lead could also traverse a full metre of lead. This was the first demonstration of the production of showers of secondary particles. Rossi also demonstrated that the cosmic ray flux contains a soft component easily absorbed in a few millimeters of lead and a hard component of charged particles with energies above 1 GeV. This ended Millikan’s theory that the cosmic rays consisted of gamma rays. Rossi demonstrated that the Earth’s magnetic field bends incoming charged particle showers so that if they are more negative, more come from the east than from the west and vice-versa. In 1933, Rossi and others demonstrated an east-west effect that showed that the majority of cosmic rays were positive. Rossi noted coincidences between several counters placed in a horizontal plane, far in excess of chance coincidences. "It would seem that occasionally very extensive groups of particles arrive on the equipment," he noted in one of his papers.

Carl Anderson and Seth Neddermeyer discover the muon

The muon was discovered as a constituent of cosmic-ray particle “showers” in 1936 by the American physicists Carl D. Anderson and Seth Neddermeyer. Because of its mass, it was at first thought to be the particle predicted by the Japanese physicist Yukawa Hideki in 1935 to explain the strong force that binds protons and neutrons together in atomic nuclei. It was subsequently discovered, however, that a muon is correctly assigned as a member of the lepton group of subatomic particles—it never reacts with nuclei or other particles through the strong interaction. A muon is relatively unstable, with a lifetime of only 2.2 microseconds before it decays by the weak force into an electron and two kinds of neutrinos. Because muons are charged, before decaying they lose energy by displacing electrons from atoms (ionization). At high-particle velocities close to the speed of light, ionization dissipates energy in relatively small amounts, so muons in cosmic radiation are extremely penetrating and can travel thousands of metres below the Earth’s surface. Read more: "Note on the nature of cosmic ray particles" – Seth H. Neddermeyer and Carl D. Anderson, Physical Review Letters, 51 (1937) 884

Pierre Auger and colleagues demonstrate extensive air showers

Pierre Auger, who had positioned particle detectors high in the Alps, noticed that two detectors located many metres apart both signaled the arrival of particles at exactly the same time. A systematic investigation of the showers showed coincidences between counters separated horizontally by as far as 75 metres. While the counting rate dropped sharply in going from 10 centimetres to 10 metres, the rate decreased slowly at larger distances. Auger had recorded "extensive air showers," showers of secondary subatomic particles caused by the collision of primary high-energy particles with air molecules. On the basis of his measurements, Auger concluded that he had observed showers with energies of 1015 eV – 10 million times higher than any known before.

Clifford Butler and George Rochester discover the kaon, first strange particle

Stereoscopic photographs showing an unusual fork (a b) in the gas. The direction of the magnetic field is such that a positive particle coming downwards is deviated in an anticlockwise direction (Image: Nature) Butler and Rochester discovered the kaon – the first strange particle – in an experiment using a cloud chamber. They took two photos – one of two cloud chamber photographs – one of them seemed to be a charged particle decaying into a charged particle and something neutral. The estimated mass of the particle was roughly 200 times that of the proton. Their paper, Evidence for the existence of new unstable elementary particles, noted: Among some fifty counter-controlled cloud-chamber photographs of penetrating showers which we have obtained during the past year as part of an investigation of the nature of penetrating particles occurring in cosmic ray showers under lead, there are two photographs containing forked tracks of a very striking character. These photographs have been selected from five thousand photographs taken in an effective time of operation of 1500 hours. On the basis of the analysis given below we believe that one of the forked tracks represents the spontaneous transformation in the gas of the chamber of a new type of uncharged elementary particle into lighter charged particles, and that the other represents similarly the transformation of a new type of charged particle into two light particles, one of which is charged and the other uncharged Read more: "Evidence for the existence of new unstable elementary particles" G. D. Rochester & C. C. Butler, Nature 160 (1947) 855-857

Air Cherenkov discovery Galbraith & Jelley

The detector used for the first observations of atmospheric-Cherenkov radiation: a dustbin with a small parabolic mirror and phototube (Image: G Hallewell) In September 1952 a simple experiment allowed the first observation of Cherenkov light produced by cosmic rays passing through the atmosphere. This experiment birthed a new field of astronomy. In 1952 armed only with a rubbish bin painted black on the inside from the UK Atomic Energy Research Establishment at Harwell, a recycled 25 cm searchlight mirror and a 5 cm phototube, Bill Galbraith and his colleague John Jelley set out to measure flashes of Cherenkov light in the night sky. They observed a count rate of about one pulse per minute, which confirmed Patrick Blackett’s assertion that Cherenkov light from charged cosmic rays traversing the atmosphere should contribute to the overall night sky intensity. In 1953, with improved apparatus at the Pic du Midi, the pair successfully demonstrated that the light signals they recorded had the polarization and spectral distribution characteristic of Cherenkov radiation. These experiments also revealed the correlation of the amplitude of the light signal with shower energy. The first steps towards Cherenkov astronomy had been taken. Read more: "The discovery of air-Cherenkov radiation" – CERN Courier

John Linsley detects the first 10^20 eV cosmic ray

John Linsley detected first cosmic ray with an energy higher than 1020 electronvolts (eV) in New Mexico, US, in 1962. This was the highest energy cosmic ray particle ever detected at the time.  He made this discovery using a ground-based array of detectors. His observations suggested that not all cosmic rays are confined to the galaxy and gave evidence for a flattening of the cosmic ray spectrum at energies above 1018 eV. This discovery clarified the structure of air showers and provided the first evidence of ultra-high energy cosmic ray composition and arrival directions. 

Detection of high-energy gamma rays from Crab Nebula

Astrophysicists detected pulsed gamma-ray emissions from the Crab pulsar with energies that exceed 100 billion electronvolts (GeV). A pulsar is a highly magnetized, rotating neutron star that emits a beam of electromagnetic radiation. The Whipple Observatory 10-metre reflector, operating a 37-pixel camera, was used to observe the Crab Nebula in TeV gamma rays. The paper announcing their finding was published on July 1 1989. The Crab pulsar is a rapidly spinning neutron star that exploded in a supernova in the year 1054 to leave behind the Crab Nebula. The Nebula rotates at about 30 times a second and the pulsar has a co-rotating magnetic field from which it emits beams of radiation. Read more: "Observation of TeV gamma rays from the Crab nebula using the atmospheric Cerenkov imaging technique" – Astrophysical Journal, Part 1, 342 (1989) 379-395

Fly’s Eye detects record-breaking cosmic ray: 3.2 x 10^20 eV

The Fly's Eye Mirrors (Image: Courtesy of University of Utah) On 15 October 1991 the HiRes Fly's Eye cosmic-ray detector in Utah, US, recorded the highest-energy cosmic ray ever detected. Located in the desert in Dugway Proving Grounds 120 kilometres southwest of Salt Lake City, the Fly's Eye detects cosmic rays by observing the light that they cause when they strike the atmosphere. Cosmic rays are mainly (89%) protons – nuclei of hydrogen, the lightest and most common element in the universe – but they also include nuclei of helium (10%) and heavier nuclei (1%), all the way up to uranium. When they arrive at Earth, they collide with the nuclei of atoms in the upper atmosphere, creating more particles, which start a cascade of charged particles that can produce light as they fly through the atmosphere. The charged particles of a cosmic ray air shower travel together at very nearly the speed of light, so the Utah detectors see a fluorescent spot move rapidly along a line through the atmosphere. By measuring how much light comes from each stage of the air shower, one can infer not only the energy of the cosmic ray but also whether it was more likely a simple proton or a heavier nucleus. The Utah researchers measured the energy of the unusual cosmic ray event in 1991 to be 3.2x1020 eV. 

The AGASA Cosmic-Ray event

On 3 December 1993, the Akeno Giant Air Shower Array (AGASA) recorded a cosmic ray with an energy of 2x1020 eV. This was a particularly well-measured event because the cosmic rays fell completely inside the detector array and arrived from a nearly vertical direction. This was the highest energy cosmic ray observed at AGASA and greatly exceeded that of any known source. AGASA consists of 111 particle detectors dispersed about a kilometer apart over a 100 square kilometer area. Each detector is roughly 2.2 square kilometers in size. AGASA was completed in 1991 and has been measuring cosmic rays ever since. 

Baikal – first underwater neutrino telescope

Satellite image showing spring ice melt underway on Lake Baikal (Image: NASA Earth Observatory) NT200, a detector in lake Baikal played a pioneering role in neutrino astronomy. NT200 was constructed between 1993 and 1998. However, in 1994 NT200 detected two neutrino events when only 36 of the final 192 photodetectors were set up. These were the first of several hundred thousand atmospheric neutrinos which NT200 later detected. Many expansions have taken place recently and neutrino research at lake Baikal continues to be an important part of the efforts to better understand the high energy process that occurs in the far-distant astrophysical sources, to determine the origin of cosmic particles of the highest energies ever registered, to search for dark matter, to study properties of elementary particles and to learn a great deal of the new information about the structure and evolution of the universe as a whole. 

AMANDA sees first neutrinos in ice

The photomultiplier tubes within these basketball-sized glass orbs are at the heart of the AMANDA neutrino telescope, a novel telescope being built at the South Pole to detect cosmic neutrinos (Image: Jeff Miller)
 The Antarctic Muon and Neutrino Detector Array (AMANDA) telescope - which lies buried under kilometer of ice - aims to detect high-energy cosmic neutrinos from our own or nearby galaxies. Neutrinos are mysterious particles associated with radioactive phenomena. They have little mass, no electric charge and can travel straight through the earth as they interact only very weakly with other matter. Neutrinos are numerous in the cosmos at large so have a significant influence on the events of the universe. They are present in the universe as leftovers of creation and are emitted by processes that fuel the sun. Neutrinos spill out in huge numbers from colossal stellar explosions. The idea behind the AMANDA telescope is that neutrinos interacting with ice emit a brief flash of blue light which can be detected if the ice is clear enough. Since neutrinos are so weakly interacting, a cubic kilometer of ice is required to detect them. In the Antarctic, the ice at this depth below is as clear as a diamond because the pressure from the snow above squeezes out all the air bubbles. Consequently, the ice is clear enough that the blue light flashes caused by the interaction of neutrinos can travel undimmed for more than 100 metres to be detected by photomultipliers. Photomultipliers convert the faint light to an electric current which travels to the surface to record the interaction. When star explodes as a supernova in the galaxy, bursts of neutrinos will burst through the earth and send flashes of blue light through the Antarctic ice. After nine years of operation, AMANDA was incorporated into the full size detector, IceCube. 

Pierre Auger Observatory completes its 100th detector

Upon completing its 100th surface detector, the Pierre Auger Observatory became the largest cosmic-ray air shower array in the world. The Pierre Auger Observatory is a hybrid detector that uses two independent methods of detecting and studying cosmic rays. The observatory detects high-energy particles through their interaction with water placed in the surface detector tanks. The other method of detection is through tracking the development of air showers through observing the ultraviolet light emitted high in the earth’s atmosphere. 

Pierre Auger Observatory discovers extragalactic origin of highest-energy cosmic rays

In November 2007, the Auger project published results showing that the direction of origin of the 27 highest energy events were strongly correlated with the location of active galactic nuclei (AGN). An active galactic nucleus is a compact region at the centre of a galaxy that has a much higher than normal luminosity over at least some portion, and possibly all, of the electromagnetic spectrum. These results supported the theory that at the centre of each AGN is a large black hole exerting a magnetic field strong enough to accelerate a bare proton to energies of 1020 eV and higher. The Auger observatory is the world’s largest and most accurate observatory for studying cosmic rays. It consists of 1600 surface detectors and 27 fluorescent telescopes and covers an area of 3000 km2. The Pierre Auger collaboration has made many low-energy incidents, which do not require physical analysis, available on the Public Event Display website. 

IceCube construction completed

On 18 December 2010, the expanded version of AMANDA, IceCube was completed. IceCube works in the same way as AMANDA but on a larger scale. AMANDA was incorporated into IceCube after operating for nine years. IceCube took seven years to complete and measures Cherenkov light emitted by charged particles produced in neutrino interactions in a cubic kilometer of transparent ice – the water equivalent of one million swimming pools. IceCube detects 275 million cosmic rays each day and approximately 100,000 neutrinos per year. Approximately a terabyte of unfiltered data is collected daily and about 100 gigabytes are sent over satellite for analysis. 

Launch of the Alphamagnetic Spectrometer (AMS)

AMS during tests at the University of Geneva, Switzerland, in 1999 (Image: Laurent Guiraud) On 16 May 2011, the space shuttle Endeavor delivered the Alphamagnetic Spectrometer (AMS) to the International Space station. This took place as part of the space shuttle mission STS-134. AMS is a particle physics detector which looks for dark matter, antimatter and missing matter from a module attached to the outside of the International space station. AMS also performs precision measurements of cosmic rays. Data are received by NASA in Houston and is then relayed to the AMS Payload Operations Control Centre at CERN for analysis. The detector measures 64 cubic metres and weighs 8.5 tonnes. By the 19 May 2012, after a year of operating, about 17 billion cosmic-ray events had been collected. 

IceCube presents the 1 PeV event

In 2013 IceCube presented two events at around 1 PeV, the first recorded on 9 August 2011, the second on 3 January 2012. Both of these events were part of the search for ultra high energy cosmogenic neutrinos  and were completely unexpected. These were the highest neutrino energies to be observed with an equivalent mass energy of over 1 million protons or about 250 times the energy of one of the protons accelerated at the LHC. The neutrinos detected may have originated from Galactic or extragalactic sources of cosmic rays. The 1 PeV events were spectacular but the energies were too low to be produced by cosmic rays interacting with cosmic microwave background photons.  

IceCube measures 5σ significance level for the observation of extraterrestrial neutrinos

Inspired by the 1PeV events, IceCube began a follow up search with combined two powerful techniques. The first was to distinguish neutrino interactions that originated inside the detector from events which originate outside it. The second technique capitalized on the fact that downgoing atmospheric neutrinos should be accompanied by a cosmic-ray air shower depositing one or more muons inside IceCube whereas cosmic neutrinos should be unaccompanied. Consequently, a very high energy isolated downgoing neutrino is likely to be cosmic. This search found 26 additional events and produced evidence for cosmic neutrinos at the 4σ significance level. The search was continued for an extra year in order to push the significance up to 5σ. One of the new events had an energy of above 2 PeV, making it the most energetic neutrino ever seen. Many explanations have been proposed for the IceCube observations, ranging from the relativistic particle jets emitted by active galactic nuclei to gamma ray busts, to galaxies to magnetars. Overall the solution is clear: More data is needed.