Advancement in research on Helium nuclei enhances our knowledge of origin and movement of cosmic rays.

Much of our understanding of the universe and its mysterious phenomena depends on theoretical explanations. In order to deepen understanding of distant objects and energetic phenomena, astronomers investigate cosmic rays, which are charged, high-energy particles made up of protons, electrons, atomic nuclei, and other subatomic particles.

Such studies have revealed that cosmic rays contain all the elements known to us in the periodic table, indicating that these elements originate from stars and high-energy events such as supernovae. In addition, due to their charged nature, the path of cosmic rays through space is affected by the magnetic fields of interstellar phenomena and objects.

Thus, detailed observations of cosmic rays can not only shed light on the origins of these particles, but also decipher the existence of high-energy objects and phenomena such as supernova remnants, pulsars, and even dark matter. In an effort to better monitor high-energy radiation, Japan, Italy and the United States of America cooperatively established the CALorimetric Electron Telescope (CALET) on the International Space Station in 2015.

In 2018, observations of cosmic ray proton spectroscopy from 50 GeV to 10 TeV revealed that the flux of proton particles at high energies was much higher than expected. These results deviated from traditional cosmic ray acceleration and scattering models that assume a “single power law distribution”, where the number of particles decreases as energy increases.

Thus, in a study published in 2022, the CALET team, including researchers from Waseda University, found cosmic ray protons in the energy range from 50 GeV to 60 TeV to follow the “double broken energy law”. This law assumes that the number of high-energy particles increases initially up to 10 TeV (known as spectral annealing) and then decreases with increasing energy (known as spectral annealing).

Extending these observations further, the team has now found similar trends of spectral stiffness and softness in the spectrum of helium cosmic rays captured across a wide range of energy, from 40 GeV to 250 TeV.

The study published in the journal Physical review letterswas led by Associate Professor Kazuyoshi Kobayashi of Waseda University, Japan, along with contributions from Professor Emeritus Shoji Tori, Principal Investigator of the CALET project, also of Waseda University, and Research Assistant Paolo Broggi of the University of Siena in Italy.

“CALET has successfully observed the energy spectral composition of cosmic ray helium, especially the spectral hardening starting at about 1.3 TeV, and the softening tendency starting at about 30 TeV,” Kobayashi says.

These observations are based on data collected by CALET aboard the International Space Station (ISS) between 2015 and 2022. They represent the largest energy range to date for cosmic helium nuclei particles, and these observations provide further evidence of particle flux deviation from the single force law model. The researchers noted that the deviation from the expected power-law distribution was more than eight standard deviations away from the mean, indicating a very small chance that this deviation occurred by chance.

Notably, the initial spectral solidification observed in these data indicates that there may be unique sources or mechanisms responsible for accelerating and diffusing helium nuclei to high energies. The discovery of these spectral features is also supported by recent observations from the Dark Particle Explorer, and questions our current understanding of the origin and nature of cosmic rays.

“These results will greatly contribute to understanding the acceleration of cosmic rays in the supernova remnant and the propagation mechanism,” says Torrey.

These findings undoubtedly enhance our understanding of the universe. Even as we prepare for manned missions to the Moon and Mars, the energy distribution of cosmic ray particles can also provide insight into the radiation environment in space and its effects on astronauts.