Cosmic rays

Geiger-Müller counters and the coincidence technique

28 June 1929

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. 

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Cosmic rays

Carl Anderson discovers the positron

9 September 1932

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.

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Cosmic rays

Bruno Rossi: Cosmic rays are positive charged particles

1 September 1933

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.

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Cosmic rays

Carl Anderson and Seth Neddermeyer discover the muon

30 March 1937

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

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Cosmic rays

Pierre Auger and colleagues demonstrate extensive air showers

18 July 1938

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.

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Cosmic rays

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

20 December 1947

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

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Cosmic rays

Air Cherenkov discovery Galbraith & Jelley

21 February 1953

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

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Cosmic rays

John Linsley detects the first 10^20 eV cosmic ray

22 February 1962

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. 

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Cosmic rays

Detection of high-energy gamma rays from Crab Nebula

1 July 1989

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

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Cosmic rays

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

15 October 1991

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

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Cosmic rays

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