Deuterons — atomic nuclei made up of a proton and a neutron — are thought to form in the same way as helium-3 nuclei, in collisions between primary helium-4 nuclei and other nuclei in the interstellar medium. If that is indeed the case, the deuteron-to-helium-4 flux ratio should be similar to the helium-3-to-helium-4 flux ratio. But this is not what the Alpha Magnetic Spectrometer (AMS) on board the International Space Station sees.
Aguilar et al. used the Alpha Magnetic Spectrometer (AMS) on board the International Space Station to measure the deuteron flux.
Cosmic rays are energetic particles of energies ranging from the MeV to 1020 eV.
Their properties are studied from measurements of their energy (rigidity) spectra, that is the number of particles per unit time, solid angle, surface, and energy, as functions of energy. They are characterized by spectra that are rapidly decreasing with increasing energy.
Cosmic rays of energy below the PeV are thought to originate in our Milky Way Galaxy.
The elemental composition of these Galactic cosmic rays is dominated by hydrogen nuclei, mainly protons. Helium nuclei account for about 10%, electrons, and nuclei heavier than helium account for only 1% each.
Species synthesized in stars, such as protons, electrons, and most of the nuclei, are called primary cosmic rays.
Light nuclei which can be synthesized by nuclear fusion in the core of stars are more abundant than heavier nuclei, as their production becomes less energetically favorable as mass increases.
A synthesis of nuclei heavier than iron, such as nickel, proceeds through explosive phenomena, such as supernova explosions occurring at the end of the life of massive stars. This makes nuclei beyond iron very rare.
Primary nuclei, once emitted by their sources in outer space, might collide with the interstellar medium and fragment in lighter species.
This is the main production mechanism of nuclei such as lithium, beryllium, boron, fluorine, scandium, titanium, and vanadium whose production through stellar nucleosynthesis is energetically disfavored. They are called secondary cosmic rays.
Compared to primary nuclei of similar mass, secondary nuclei are less abundant and have rigidity spectra decreasing faster than the spectra of primaries as rigidity increases.
The energy (or rigidity) dependence of cosmic-ray spectra results from the combination of emission from their sources, acceleration, and propagation mechanisms occurring while cosmic rays travel through the galaxy.
Cosmic rays are accelerated by diffusing through expanding shocks and propagate diffusively in the interstellar medium, scattering on the irregularities of the galactic magnetic field. Both these mechanisms depend on the particle’s momentum, or magnetic rigidity.
Cosmic-ray propagation is described in terms of a rigidity-dependent diffusion coefficient, which embeds the properties of the galactic magnetic field turbulence.
“Hydrogen nuclei are the most abundant cosmic ray species,” members of the AMS Collaboration wrote in their paper.
“They consist of two stable isotopes, protons and deuterons.”
“Big Bang nucleosynthesis predicts a very small production of deuterium and, with time, the abundance of deuterons decreases from its primordial value, with the measured deuteron-to-proton ratio in the interstellar medium of 0.00002.”
“Instead of being accelerated in supernova remnants like primary cosmic ray protons and helium-4, deuterons are thought to overwhelmingly originate from interactions of helium with the interstellar medium.”
“Together with helium-3, deuterons are called secondary cosmic rays.”
In their latest study, the AMS physicists investigated data from 21 million cosmic deuterons detected by AMS from May 2011 to April 2021.
Examining how the flux of deuterons varies with rigidity, they found surprising features.
The AMS data show that these ratios are notably different above a rigidity of 4.5 GV, with the deuteron-to-helium-4 ratio falling less steeply with rigidity than the helium-3-to-helium-4 ratio.
In addition, and again defying expectations, above a rigidity of 13 GV the data show that the deuteron flux is nearly identical to that of protons, which are primary cosmic rays.
To put it simply, the researchers found more deuterons than expected from collisions between primary helium-4 nuclei and the interstellar medium.
“Measurement of deuterons is quite difficult because of the large cosmic proton background,” said Dr. Samuel Ting, spokesperson of the AMS Collaboration.
“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.”
The team’s paper was published in the journal Physical Review Letters.
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M. Aguilar et al. (AMS Collaboration). 2024. Properties of Cosmic Deuterons Measured by the Alpha Magnetic Spectrometer. Phys. Rev. Lett 132 (26): 261001; doi: 10.1103/PhysRevLett.132.261001
