Deuteron It is believed that atomic nuclei consisting of protons and neutrons, like those of helium-3 nuclei, are formed in collisions between helium-4 nuclei and other nuclei in the interstellar medium. If this were the case, the flux ratio of deuterons to helium-4 should be similar to that of helium-3 to helium-4. However, this is not the case. Alpha Magnetic Spectrometer Astronauts aboard the International Space Station (AMS) are watching.
Aguilar othersThe deuteron flux was measured using the Alpha Magnetic Spectrometer (AMS) on board the International Space Station.
Cosmic rays are high-energy particles with energies ranging from MeV to 10.20 Electronic V.
These properties are studied from measurements of the energy (stiffness) spectrum (number of particles per unit time, solid angle, surface area, and energy as a function of energy), which is characterized by a rapid decrease in the spectrum as the energy increases.
Cosmic rays with energies below PeV are thought to originate in our own Milky Way galaxy.
The elemental composition of these galactic cosmic rays is dominated by hydrogen nuclei, primarily protons, with helium nuclei making up about 10%, and electrons and nuclei heavier than helium making up just 1% each.
The species synthesized in stars, such as protons, electrons, and most atomic nuclei, are called primary cosmic rays.
Light nuclei, synthesized by nuclear fusion in the cores of stars, are more abundant than heavy nuclei because their production becomes energetically unfavorable as mass increases.
The synthesis of atomic nuclei heavier than iron, such as nickel, occurs through explosive phenomena such as supernova explosions that occur at the end of the life of massive stars, so atomic nuclei heavier than iron are extremely rare.
When primary nuclei are ejected from their source in space, they can collide with interstellar material and split into lighter species.
This is the primary production mechanism for atomic nuclei that are energetically unfavorable to produce by stellar nucleosynthesis, such as lithium, beryllium, boron, fluorine, scandium, titanium, and vanadium. These are called secondary cosmic rays.
Compared to primary nuclei of similar mass, secondary nuclei are less abundant and, as stiffness increases, their stiffness spectrum decreases faster than that of primary nuclei.
The energy (or rigidity) dependence of the cosmic ray spectrum arises from a combination of source-directed emission, acceleration, and propagation mechanisms that occur during a cosmic ray's passage through the galaxy.
Cosmic rays are diffusely accelerated by expanding shock waves, propagate diffusely through the interstellar medium, and are scattered by irregularities in the galactic magnetic field, both of which depend on the particle's momentum, and thus on its magnetic stiffness.
Cosmic ray propagation is described by a stiffness-dependent diffusion coefficient that incorporates the properties of turbulence in the galactic magnetic field.
“Hydrogen nuclei are the most abundant species of cosmic ray,” members of the AMS collaboration wrote in the paper.
“They are made up of two stable isotopes: protons and deuterons.”
“Big Bang nucleosynthesis predicts negligible production of deuterium, and over time the abundance of deuterons has decreased from its primordial value, with the ratio of deuterons to protons measured in the interstellar medium being 0.00002.”
“Deuterons are thought to arise primarily from the interaction of helium with interstellar matter, rather than being accelerated in supernova remnants like primary cosmic ray protons and helium-4.”
“Deuterons, along with helium-3, are called secondary cosmic rays.”
For the latest study, AMS physicists examined data from 21 million cosmic deuterons detected by AMS between May 2011 and April 2021.
When investigating how the deuteron flux varies with rigidity, a surprising feature was discovered.
The AMS data show that these ratios differ significantly above a stiffness of 4.5 GV, with the deuteron to helium-4 ratio decreasing more slowly with stiffness than the helium-3 to helium-4 ratio.
Furthermore, and again contrary to expectations, when stiffness exceeds 13 GV, the data show that the flux of deuterons is nearly the same as the flux of protons, the primary cosmic ray.
Simply put, researchers found more deuterons than expected from collisions between main helium-4 nuclei and interstellar matter.
“Measuring deuterons is very challenging due to the large cosmic proton background radiation,” said Dr Samuel Ting, spokesman for the AMS collaboration.
“Our unexpected results show how little we know about cosmic rays.”
“Future upgrades to AMS will increase the acceptance rate by 300 percent, enabling AMS to measure all charged cosmic rays with 1 percent accuracy, providing the experimental basis for the development of accurate cosmic ray theory.”
The team's paper was published in the journal Physics Review Letter.
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M. Aguilar others(AMS Collaboration). 2024. Properties of cosmic deuterons measured with the Alpha Magnetic Spectrometer. Physiotherapy Rev Lett 132(26):261001;doi:10.1103/PhysRevLett.132.261001
