A joint Fermilab/SLAC publication

Beating the odds in the study of ultra-high-energy cosmic rays


Scientists pose triumphantly with a newly installed EASIER-61 detector. Image: LPNHE

About two times per month, an ultra-high-energy particle from beyond this galaxy crashes through Earth’s atmosphere above the Pierre Auger Cosmic Ray Observatory in Argentina.

It’s a mystery where these cosmic rays come from and what they’re made of. But a new technique, currently in the works, could drastically improve scientists’ chances of finding out.

“The number one challenge in this field is statistics,” said astrophysicist Angela Olinto of the University of Chicago.

Ultra-high-energy cosmic rays are rare, and scientists’ best way of detecting them works only 10 percent of the time. This is because they use telescopes to search for ultraviolet light that cosmic rays release when they run into nitrogen in the atmosphere. The light is visible only on dark, moonless nights.

Physicists also use ground detectors – tanks of ultra-pure water or scintillator – to detect showers of particles from cosmic rays. The tanks work day and night, but they do not collect enough information about how cosmic ray showers progress in the air.

Three teams of scientists, one based in France and two in the United States, are installing new sets of detectors at Pierre Auger observatory designed to collect those key details 100 percent of the time. According to theory, ultra-high-energy cosmic rays should release microwaves as well, and those can be seen in the day.

“We would be increasing the power of our instrument in some ways by a factor of 10,” said Fermilab physicist Paul Mantsch.

Scientists from several French institutions, including the Laboratoire de Physique Nucléaire et de Hautes Energies and the Laboratoire de Physique et de Cosmologie in Paris, recently installed Extensive Air Showers Identification with Electron Radiometers detectors, called EASIER-61. A group from the University of Chicago has placed a prototype of their Microwave Detection of Air Showers, or MIDAS, telescope at Pierre Auger. Ohio State University and University of Hawaii scientists created the Air-shower Microwave Bremsstrahlung Radiometer, or AMBER, to detect microwaves.

“One year of data collected at the Pierre Auger observatory will be enough to validate the microwave detection technique, “ said the leader of the MIDAS detector, University of Chicago physicist Paolo Privitera.

“In the best-case scenario, about four microwave telescopes could cover the same area as the 1,600 water-tank detectors at Pierre Auger,” Olinto said.

Scientists estimate that they will need information from about 1,000 ultra-high-energy cosmic ray events in order to make any definitive statements on their origin. At the current rate, that will take about 30 years.

“Getting a new technique in the game would be very useful,” Olinto said, “especially before we all have to retire.”

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