Cosmic rays are generally categorized as either primary, originating from cosmic events like supernova explosions, or secondary, produced when primary cosmic rays interact with the interstellar medium.
In this latest research, the AMS team scrutinized data from 21 million cosmic deuterons gathered between May 2011 and April 2021. They focused on how the number, or "flux," of deuterons varies with rigidity - a measure of particle momentum over electrical charge. The study revealed some surprising features.
Deuterons are traditionally thought to form similarly to helium-3 nuclei, arising from collisions between primary helium-4 nuclei and other nuclei within the interstellar medium. Under this assumption, the flux ratio of deuterons to helium-4 should closely resemble that of helium-3 to helium-4. However, AMS data diverges from this expectation. Above a rigidity of 4.5 gigavolts (GV), the deuteron-to-helium-4 flux ratio declines less sharply than the helium-3-to-helium-4 ratio. Additionally, beyond 13 GV, the deuteron flux nearly matches that of protons, which are primary cosmic rays.
In essence, AMS has detected a higher number of deuterons than previously predicted based on primary helium-4 nuclei collisions with the interstellar medium.
"Measurement of deuterons is quite difficult because of the large cosmic proton background," said AMS spokesperson Samuel Ting. "Our unexpected results continue to show how little we know about cosmic rays. With the coming upgrade of AMS to increase its acceptance by 300%, AMS will be able to measure all the charged cosmic rays to one percent accuracy and provide an experimental basis for the development of an accurate cosmic-ray theory."
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