New AMS results on heavy secondary fluorine nuclei show that cosmic rays don't all propagate the same way
The Alpha Magnetic Spectrometer (AMS), steadily acquiring cosmic-ray data on the International Space Station (ISS) since May 2011, has measured with percent-level accuracy the spectrum of fluorine nuclei in cosmic rays, providing for the first time the characterization of heavy secondary cosmic-ray spectrum, that is their intensity as function of energy (Figure 1).
Figure 1 : The AMS cosmic-ray fluorine flux as function of kinetic energy per nucleon, EK, multiplied by EK-2.7, together with earlier measurements.
Cosmic rays consist of particles and nuclei continuously impinging the Earth from outer space. Hydrogen and helium nuclei make the most of cosmic rays, heavier nuclei amount to as little as 1% of the total cosmic-ray flux. The bulk of cosmic rays are particles and nuclei directly produced by the sources, such as protons and most of the nuclei, that are called primary cosmic rays. Oxygen and silicon nuclei are mostly pure primary cosmic rays.
Some nuclei, such as boron and fluorine, are mainly produced by collisions of primary cosmic rays with the interstellar medium while they propagate through the interstellar space, therefore they are called secondary cosmic rays and are less abundant than their primary progenitors. Namely, boron nuclei (Z=5) are mainly secondary cosmic rays produced by collisions of carbon (Z=6) and oxygen (Z=8) nuclei; fluorine (Z=9) nuclei are also mainly secondary cosmic rays produced by collisions of neon (Z=10), magnesium (Z=12) and silicon (Z=14) nuclei (Figure 2).
The spectra of secondary cosmic rays and the secondary-to primary ratios, such as boron-to-oxygen or fluorine-to-silicon, are key observables for the understanding of cosmic-ray propagation in the interstellar medium.
Before AMS, only the spectra of the most abundant secondary nuclei were measured in a limited energy range and with large uncertainties. In particular, measurements of heavy secondary cosmic-ray fluorine nuclei were only performed up to kinetic energy per nucleon EK ~100 GeV/n and with errors exceeding 100% at 50 GeV/n. AMS is the first experiment to measure the spectrum of heavy secondary cosmic-ray fluorine nuclei (Z=9) up to the TeV/n and with accuracy of 5.9% at EK~ 50 GeV/n (Figure 1).
Figure 2: Primary cosmic rays, such as protons, helium, carbon, oxygen, silicon and iron nuclei, are produced in the core of stars and expelled in the interstellar medium by supernovae explosions occurring at the end of the life of massive stars. During their journey through the galaxy, primaries may collide with the interstellar medium and produce secondary cosmic rays. Lithium, beryllium, boron and fluorine nuclei in cosmic rays are secondaries produced mainly by collisions of heavier primaries with the interstellar medium.
Boron nuclei (Z=5) are the most abundant secondary cosmic-ray nuclei (AMS Coll. Phys. Reports, 894, 1-116 , 2021), therefore the secondary-to-primary boron-to-carbon ratio is traditionally used to model the propagation of all cosmic rays.
The latest AMS result on heavy secondary cosmic rays shows for the first time that the heavy secondary-to-primary fluorine-to-silicon ratio has distinctly different energy dependence than the light secondary-to-primary boron-to-carbon (or boron-to-oxygen) ratio (Figure 3), therefore the propagation properties of heavy nuclei are different from those of light nuclei requiring a revision of cosmic-ray propagation models.
Figure 3: The AMS fluorine-to-silicon to boron-to-oxygen ratio as function of rigidity (momentum/charge). The fit to kRdshows that the heavy secondary-to-primary nuclei ratio fluorine-to-silicon is different from the light secondary-to-primary nuclei ratio boron-to-oxygen by more than 7s .
These new results are the outcome of data analysis work performed by the AMS group at the Department of Nuclear and Particle Physics of the University of Geneva, led by Dr Mercedes Paniccia, in collaboration with the Shandong Institute of Advanced Technology (SDIAT) in China. Other independent analyses have been performed by the AMS groups at MIT (US), CIEMAT (Spain) and INFN-Bologna (Italy) (Figure 4).
Figure 4: Comparison of the fluorine spectrum results obtained by the three independent AMS groups.
These new AMS measurement have been published in Physics Review Letters (Phys. Rev. Lett. 126, 081102, 2021); the paper has been highlighted by the editors of Physics Review Letters as Editors' Suggestion.
The AMS experiment will continue to take data for the entire ISS lifetime, up to 2030.