Unexpected excess of cosmic rays found by AMS detector aboard ISS

An analysis of data from the Alpha Magnetic Spectrometer (AMS) aboard the International Space Station has revealed an unexpected surplus of cosmic rays made of deuterons — atomic nuclei made up of a proton and a neutron.

• The study, based on data collected between May 2011 and April 2021, shows surprising discrepancies in deuteron flux ratios, challenging existing theories about cosmic ray formation.
• The coming upgrade of AMS is expected to provide an experimental basis for the development of accurate cosmic-ray theory.

An analysis of data collected by the Alpha Magnetic Spectrometer (AMS) — a space-based detector aboard the International Space Station (ISS), has revealed a surprising surplus of cosmic rays made of deuterons, as reported in a recent paper published in Physical Review Letters. This discovery adds to a series of unexpected findings from AMS, which has detected more than 238 billion cosmic ray particles since it began operations in 2011.

Cosmic rays, which are high-energy particles that travel through space, are typically classified into two categories: primary and secondary. Primary cosmic rays originate from cosmic events like supernova explosions, while secondary cosmic rays are produced through interactions between primary cosmic rays and the interstellar medium.

In this study, the AMS collaboration analyzed data from 21 million cosmic deuterons detected over a ten-year period, from May 2011 to April 2021. Deuterons are atomic nuclei composed of one proton and one neutron.

By examining how the flux of deuterons—essentially, the number of deuterons per unit area per unit time—varies with rigidity, which is the ratio of particle momentum to electrical charge, the AMS team uncovered several unexpected features.

Traditionally, deuterons are thought to be produced in collisions between primary helium-4 nuclei and other nuclei within the interstellar medium, in a similar manner to the formation of helium-3 nuclei. Based on this model, the ratio of deuteron flux to helium-4 flux should resemble the helium-3-to-helium-4 flux ratio. However, the AMS data indicates otherwise.

Above a rigidity of 4.5 gigavolts (GV), the deuteron-to-helium-4 flux ratio does not follow the expected pattern, instead showing a less steep decline than the helium-3-to-helium-4 flux ratio. Furthermore, at rigidities above 13 GV, the deuteron flux aligns closely with that of protons, which are primary cosmic rays, a result that defies current theoretical expectations.

This discovery implies that AMS has observed more deuterons than would be expected from conventional models of cosmic ray formation, particularly in collisions involving helium-4 nuclei and the interstellar medium.

“Our unexpected results continue to show how little we know about cosmic rays,” AMS spokesperson Samuel Ting said.

With the coming upgrade of AMS to increase its acceptance by 300%, the detector will be able to measure all the charged cosmic rays with one percent accuracy and provide an experimental basis for the development of an accurate cosmic-ray theory, Ting said.

References:

¹ Properties of Cosmic Deuterons Measured by the Alpha Magnetic Spectrometer – M. Aguilar et al. – AMS Collaboration – Phys. Rev. Lett. 132, 261001 – Published June 25, 2024 – DOI: 10.1103/PhysRevLett.132.261001

² Cosmic count exceeds expectation – CERN – August 13, 2024