Cosmic rays

The AGASA Cosmic-Ray event

3 December 1993

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. 

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Cosmic rays

Baikal – first underwater neutrino telescope

25 February 1997

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. 

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Cosmic rays

AMANDA sees first neutrinos in ice

15 July 2003

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. 

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Cosmic rays

Pierre Auger Observatory completes its 100th detector

21 October 2003

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. 

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Cosmic rays

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

9 November 2007

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. 

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Cosmic rays

IceCube construction completed

18 December 2010

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. 

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Cosmic rays

Launch of the Alphamagnetic Spectrometer (AMS)

16 May 2011

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. 

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Cosmic rays

IceCube presents the 1 PeV event

9 August 2011

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.  

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Cosmic rays

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

21 May 2014

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. 

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Cosmic rays

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