Alpha Magnetic Spectrometer measurements unveil properties of cosmic helium

The Alpha Magnetic Spectrometer (AMS) collaboration, a large group of researchers from CERN and other institutes worldwide, has recently presented a series of precision measurements of the properties of cosmic Helium isotopes ³He and ⁴He. These measurements were collected by the AMS, a spectrometer located on the International Space Station (ISS).

"Helium is one of the most abundant elements in cosmic rays," Alberto Oliva, one of the researchers who carried out the study, told Phys.org. "It is made by two isotopes, the helium-4 and helium-3."

Helium-4, or ⁴He, is a Helium isotope that was primarily produced in the first three minutes after the Big Bang and in stellar nucleosynthesis, which is the creation of chemical elements as a result of nuclear fusion reactions within stars. Astrophysical accelerators, such as supernova explosion shockwaves, accelerate this isotope to high energy.

Helium-3 or ³He, on the other hand, is generally produced by the interaction of accelerated ⁴He isotopes with the materials of our galaxy. Identifying the differences between the energy dependence of ³He and ⁴He isotopes allows researchers to derive general properties of the sources, as well as the acceleration and propagation of cosmic rays in the Milky Way.

"Helium nuclei can also travel longer distances with respect to heavier nuclei, since helium is more compact and interacts less with the surrounding material," Oliva said. "With helium, we can thus explore properties of cosmic rays in a larger galactic volume, with respect to what has been traditionally done with heavier nuclei such as boron and oxygen."

Measuring helium isotopes in the AMS requires the use of a silicon tracker, which determines the momentum of the incoming cosmic ray by measuring the deflection in the AMS's magnetic field, in combination with the measurement of velocity of the Time-Of-Flight system (TOF) at low energy or the Ring Imaging CHerenkov counter (RICH) at high energy. All these detectors can also measure the atomic number and separate helium from other nuclei.

"The TOF measures the velocity of a particle by the time needed to the particle to pass through two scintillation planes separated by about one meter, while RICH measures the velocity of a particle by the detection of the light ring produced by high energy particles traversing the RICH radiator with a velocity faster than the speed of light in that medium, i.e. by the Cherenkov effect," Oliva explained.

By measuring momentum and velocity independently, the AMS spectrometer can accurately separate ³He and ⁴He isotopes, determining their spectra. In their study, Oliva and his colleagues presented the measurements collected by the AMS spectrometer, which highlight specific properties of the two Helium isotopes.

The researchers observed that ³He and ⁴He fluxes exhibit nearly identical variations with time and that the relative magnitude of these variations decreases with rising rigidity. The AMS spectrometer collected the first measurements of the rigidity dependence of the ³He/⁴He flux ratio and these measurements revealed that the ratio has a long-term time dependence, yet it becomes time independent above 4GV.

"The measurement we perform extends the knowledge of ³He and ⁴He to higher energies (a factor of two higher than previous experiments) and for the first time we were able to observe that the ratio in the rigidity of ³He and ⁴He follows a simple power-law, an observation that can allow discrimination between different cosmic ray propagation models," Oliva said. "We were also able to see that the solar activity is able to affect the ³He and ⁴He spectra in a different way, a result that has never seen before."

The measurements presented by Oliva and the rest of the AMS collaboration provide new interesting insight about the properties of ³He and ⁴He isotopes, which could have important implications for future astrophysics research investigating cosmic ray production and propagation. Notably, these measurements revealed that the ³He/⁴He flux ratio rigidity dependence can be described by a single power law, which is in agreement with B/O and B/C spectral indices at high energies.

"The study of isotopes in cosmic rays (e.g. proton, deuteron, lithium-6 and lithium-7, beryllium-7, beryllium-9 and beryllium-10) exploiting at maximum the combined power of Tracker, TOF and RICH will be able to tell many more things about the production and propagation of cosmic rays, as well as effects coming from the solar modulation," Oliva said. "Maybe we will find something that we are not expecting at all, as it happens when you go where no other measurements have reached before."