Cosmic Ray shower.

New Surprising Feature Found In The Energy Spectrum Of Ultra-high Energy Cosmic Rays

A cosmic ray shower.
Credit – Simon Swordy. Source.

The detection

University of Delaware researchers are part of a collaboration studying cosmic rays. In addition to Cherenkov detector tanks filled with water, the Pierre Auger Observatory in Argentina has fluorescence detectors.

The cosmic rays interact with atmospheric nitrogen, causing it to emit ultraviolet light which can be detected by a detector. This process is known as fluorescence. The researchers analyzed 215,030 cosmic ray events with energies above 2.5 quintillion electron volts taken over the past decade by the Pierre Auger Observatory in Argentina which is the largest observatory in the world studying cosmic rays.

A new feature found in the energy distribution

A new feature called the ‘kink’ was discovered in the energy spectrum of the cosmic rays at about 13 quintillion electron volts. This research was published in Physical Review Letters and Physics Review D. The measurements in this study provide clues to the origin of the cosmic rays and their source.

The observatory has more than 1,600 detectors called water-Cherenkov stations spread across the high plains of the Pampa Amarilla (Yellow Prairie), overlooked by 27 fluorescence telescopes. Collectively, these instruments measure the energy that an ultrahigh-energy cosmic ray particle gives up in the atmosphere and hence provide an estimation of its mass.

What is the composition of these ultra-high-energy cosmic rays?

These ultra-high-energy cosmic ray particles were previously considered to be composed of mostly protons, but this research provided evidence that some of these particles may be heavier than oxygen or helium like silicon and iron, for example.

Active galactic nuclei which are supermassive black holes at the center of galaxies are speculated to be the source of ultra-high-energy cosmic rays. Present research being done by the same research team focuses on increasing the accuracy of energy measurements and measuring the spectrum of lower-energy cosmic rays.


1. A. Aab et al. Features of the Energy Spectrum of Cosmic Rays above 2.5×1018 eV Using the Pierre Auger Observatory, Physical Review Letters (2020). DOI: 10.1103/PhysRevLett.125.121106

2. A. Aab et al. Measurement of the cosmic-ray energy spectrum above 2.5×1018 eV using the Pierre Auger Observatory, Physical Review D (2020). DOI: 10.1103/PhysRevD.102.062005

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