Physics

20 Years of Cosmic Rays at Pierre Auger

December 2, 2019• Physics 12, 137

Tons of of researchers from all over the world head to Argentina to have a good time the groundbreaking anniversary of the world’s largest cosmic-ray observatory.

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Pierre Auger

At left, scientists with the Auger Collaboration take part within the Malargüe Day parade. At proper, researchers dressed up as a fluorescence detector (blue and white field), a Cherenkov detector (pink), and an air bathe (black T-shirt).

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Pierre Auger

At left, scientists with the Auger Collaboration take part within the Malargüe Day parade. At proper, researchers dressed up as a fluorescence detector (blue and white field), a Cherenkov detector (pink), and an air bathe (black T-shirt).×

Masquerading as a Cherenkov detector would possibly invite quizzical appears to be like at a typical occasion. However within the good firm of cosmic-ray physicists in a parade, it raises cheers.

Every November, greater than 100 scientists from all over the world come to the small city of Malargüe, Argentina, house of the Pierre Auger cosmic-ray observatory. The purpose is to debate outcomes and future plans, however the scientists additionally be a part of within the Malargüe Day parade on November 16th, which celebrates the city’s official founding in 1950.

“This can be a very touching occasion as a result of the scientists within the collaboration take part. And once they stroll by, the city claps,” says Ingo Allekotte of the Bariloche Atomic Heart in Argentina and a challenge supervisor for Pierre Auger.

The occasion this yr was all of the extra particular as a result of it marked the 20th anniversary of Auger’s floor breaking. The most important cosmic-ray observatory on this planet, it was additionally the primary main experiment to be primarily based in Argentina.

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Pierre Auger

Pierre Auger has all the time been a global challenge. At the moment, 17 international locations are official members of the collaboration, and their flags are proven on the walkway to the Auger customer middle.

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Pierre Auger

Pierre Auger has all the time been a global challenge. At the moment, 17 international locations are official members of the collaboration, and their flags are proven on the walkway to the Auger customer middle.×

The brainchild of physicists Jim Cronin and Alan Watson, Auger is a novel detector for cosmic rays—an lively mix of principally protons and heavier nuclei that hail from sources inside and past our galaxy. The rain of cosmic rays on our environment span in vitality from 106 eV to past 1020 eV—about ten million instances extra energetic than the particles produced at CERN. By finding out the composition, depth, and sky location of the cosmic-ray flux at totally different energies, researchers collect clues in regards to the astrophysical sources that accelerated the particles and the interactions that they had en path to Earth.

Auger is tailor-made to search for the high-energy portion of the cosmic-ray spectrum— particles of 1017eV and above. These particles are uncommon—just one 1018 eV particle per week strikes a sq. kilometer of the Earth’s higher environment, which suggests they’d be virtually unimaginable to detect with a single detector in area. Auger and different so-called ground-based experiments solid a wider internet by searching for the “bathe” of secondary, lower-energy particles produced when a cosmic-ray particle hits an atom or molecule within the environment. Having the widest internet on this planet, Auger is among the few observatories that may detect ultrahigh-energy cosmic rays (UHECRs)—particles with energies above 1019 eV, whose supply is an ongoing thriller.

To collect the total profile of the bathe, the observatory combines two detection schemes: 1660 water tanks (Cherenkov detectors) that sign the passage of fast-moving charged particles and 4 elevated fluorescence detectors that seize radiation from atmospheric nitrogen. The footprint of the detectors is bigger than 30 instances the scale of Paris, and the necessity for huge, flat terrain with clear skies was one of many causes that Malargüe was chosen as a website.

Thomas Gaisser, a cosmic-ray physicist on the College of Delaware, remembers being amazed on the measurement of the observatory when he visited. “Driving again to the airport, for the primary 30 miles, you retain seeing these detectors.”

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ASPERA/G.Toma/A.Saftoiu

An illustration of an air bathe occasion and the array of Cherenkov detectors at Auger.

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ASPERA/G.Toma/A.Saftoiu

An illustration of an air bathe occasion and the array of Cherenkov detectors at Auger.×

Physicists’ understanding of UHECRs has developed since Auger’s floor breaking. Earlier than the collaboration started taking knowledge, measurements painted a conflicting image of UHECRs. The expectation had been that the UHECR flux, being principally protons, would fall off quickly at a sure vitality due to the protons’ interplay with the cosmic microwave background. However whereas the Excessive Decision Fly’s Eye detector within the US noticed this so-called GZK cutoff, the AGASA experiment in Japan noticed a extra gradual roll-off, probably suggesting a novel supply of UHECRs, just like the decay of unknown heavy particles.

Auger and different experiments later confirmed that the cutoff was certainly sharp. In 2017, Auger additionally decided that the sources of UHECRs are virtually actually extragalactic. However precisely what these violent sources are and why they produce the noticed spectrum of particles continues to be a thriller. Researchers have been stunned, for instance, that the composition of the UHECR flux tends in direction of heavier nuclei at greater energies, a pattern meaning the straightforward image explaining the cutoff isn’t the case, says Gaisser.

Understanding the sources of UHECRs is among the drivers for an ongoing improve of the observatory, known as AugerPrime. Amongst a number of enhancements, the collaboration is putting in underground muon detectors, radio-wave antennas and new scintillators on prime of the water tanks, and sooner electronics. When mixed, these enhancements will assist researchers determine the first particle that spawned a bathe occasion, bettering on the tough statistical image they’ve now. One purpose is to isolate proton occasions: Protons are deflected much less by magnetic fields than heavier particles, offering a clearer thought of the route from which they arrived—and due to this fact a greater sense of what sources are liable for UHECR.

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Pierre Auger

The Pierre Auger customer middle attracts about 9000 guests per yr.

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Pierre Auger

The Pierre Auger customer middle attracts about 9000 guests per yr.×

Past its discoveries, Auger is a crucial fixture in Argentina, which continues to be in some ways thought-about a creating nation. The observatory was all the time supposed to be a global collaboration, and Allekotte says it boosts connections between Argentinian researchers and the remainder of the world. Auger has additionally helped the nation entice different scientific initiatives, akin to an antenna for deep-space viewing that was put in in Malargüe by the European House Company.

From the outset, Auger’s researchers have made efforts to be a part of their neighborhood. The observatory hosts a biennial science truthful for highschool college students throughout the nation, and this yr’s themes included the moon touchdown and the periodic desk. Years in the past, native youngsters named every of the detectors. “These names are proudly proven on the detectors, and we often seek advice from our detectors by title as an alternative of by quantity,” says Charles Timmermans, a physicist on the Nationwide Institute for Subatomic Physics (Nikhef) and Radboud College within the Netherlands, who leads Auger’s outreach efforts. Auger additionally affords internships to college students, and a few have resulted in everlasting employment.

“Being a part of the neighborhood means contributing to the neighborhood,” says Timmermans.

Auger has additionally contributed to the tourism business in Malargüe, a city of about 27,000. In 2008, the collaboration helped discovered the Malargüe planetarium, a well-liked vacation spot. And along with the scientists who come to Auger and fill the city’s inns, the observatory itself attracts about 9000 guests per yr. “In a small city like Malargüe,” says Allekotte, “[this] makes a distinction.”

–Jessica Thomas

Jessica Thomas is the Editor of Physics.

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