Cosmic rays

From early experiments with electricity to detectors in space: Find out about the history of research into cosmic rays 

03 12, 1993
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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. 

15 10, 1991
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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

01 07, 1989
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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

22 02, 1962
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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. 

21 02, 1953
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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

20 12, 1947
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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 particlesG. D. Rochester & C. C. Butler, Nature 160 (1947) 855-857

18 07, 1938
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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.

16 05, 2011
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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. 

30 03, 1937
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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

01 09, 1933
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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.