Spectral Measurement of the Iron and Nickel Components of Cosmic Rays at the Highest Energy Ranges

A high precision measurement of the iron and nickel spectra of galactic cosmic rays reaching the highest energy range worldwide has been performed with the CALorimetric Electron Telescope (CALET) detector installed on the Exposed Facility (EF) of the Japanese Experiment Module (JEM) “Kibo” on the International Space Station (ISS). The CALET mission is operated by an international research collaboration comprising the Japan Aerospace Exploration Agency (JAXA) and research institutes from Japan, Italy and the Unites States, with TORII Shoji, Professor Emeritus at Waseda University's Faculty of Science and Engineering, as principal investigator.
The effort was headed by AKAIKE Yosui, Researcher ( Associate Professor) at the Waseda Research Institute for Science and Engineering, together with Caterina Checchia and Francesco Stolzi of Sienna University (Italy).

The calorimeter-type CALET detector, which has conducted a still ongoing continuous observation on the ISS for more than five years, directly measures the spectrum of the iron component of cosmic rays with high precision over a wide energy range from 10 giga electron volt (GeV) to 20 tera electron volt (TeV), and experimentally confirmed that the spectral power law index remains at -2.60 all the way into the TeV region. Furthermore, the nickel component of cosmic rays, in the same way as for iron, was measured　from 8.8 GeV to 240 GeV per nucleon, and the spectral index was determined to be -2.51. These results do not show the spectral hardening generally exhibited by the lighter nuclei. This conclusion could be an important piece of information for the verification of models about the currently vividly discussed galactic cosmic-ray acceleration and propagation mechanisms. Including a comparison with previous results, it was deemed worthy of an announcement to the scientific community. The iron results were published on June 14, 2021, and the nickel results on April 1, 2022, each in the renowned international scientific journal Physical Review Letters.

In the approximately 100 years since their discovery, observation of cosmic rays has brought forth important information for understanding elementary particles and the mysteries of the universe. However, many unclear points remain about where and in what way high-energy cosmic rays are accelerated. The general understanding based on the various observations so far is a "standard" model, according to which galactic cosmic rays (cosmic rays originating from inside the Milky Way, our home galaxy) are accelerated by shock-waves of supernova explosions, and reach earth after a diffusion-like propagation due to the interstellar magnetic field present throughout the galaxy.

This model predicts a monotone, single-index spectrum (power law spectrum) for the shape of the cosmic-ray energy spectra observed at earth. However, in the spectra of protons and helium, as well as light nuclei with atomic number (charge: Z) around or less than 6, such as carbon and oxygen, a discrepancy from the single power law, a spectral hardening, has been reported in recent years by direct observations from balloons, artificial satellites and the ISS. This kind of result cannot be understood based on the “standard” model, indicating the necessity of a paradigm shift in modeling the acceleration and propagation of cosmic rays, giving rise to vigorous research into their interpretation. An essential key in this understanding are the elements iron (Z=26) and nickel (Z=28), which are created in fusion reactions at the final stage of nucleosynthesis in stars. Since there are practically no nuclei heavier than these until just before the star goes supernova, iron and nickel are important components of cosmic rays which can provide direct information about the final stages of star evolution and the acceleration mechanisms.

The energy range of the iron component has so far been measured in separate parts by two types of detectors, magnet spectrometers (PAMELA, AMS-02) and calorimeters (ATIC, CREAM, NUCLEON, etc.), but this measurement by CALET is the first high-precision measurement of the whole energy range by a single detector. For the observation of nickel, essentially no high-precision measurement at the high energy region has been done until now, due to nickel’s low elemental abundance. However, it has now been achieved for the first time.

The main conclusion obtained from these observation results is that the iron and nickel energy spectra have within measurement error a single power law index shape, which gives a negative result concerning a spectral hardening as observed in the light nuclei spectra. For the final conclusion, we still need confirmation by the results of further observation obtained by higher statistics in the high-energy regions. However, these observation results by CALET are expected to suggest explanations to longstanding questions about the models of acceleration and propagation of cosmic rays, and have the potential to give important hints for drawing up a consistent experimental description. Furthermore, the reliable high-energy nuclei spectra obtained with CALET might also become important fundamental data used for other fields in astronomy.

It has become clear in recent years as an eye-opening development, that a complete understanding of the cosmic radiation, including X-rays and gamma-rays, requires understanding of the charged cosmic rays as its source. This is due to the fact that, unlike in the observation of thermal emission dominated by radio, infrared and visible electromagnetic waves with a characteristic blackbody spectrum, the non-thermal radiation background characterized by a power law spectrum is always entangled with the acceleration and propagation of the cosmic rays.

Therefore, it is necessary to directly capture (direct observation) cosmic rays striking earth at great height where the atmosphere is thinned out, with a focus on the observation of galactic cosmic rays. Therefore, using a variety of flying objects and apparatus designs, observations have been performed in Japan and abroad. Their results developed into the "standard model"; that cosmic rays are accelerated by shockwaves in supernova remnants and leak out to intergalactic space after diffusive propagation throughout the galactic magnetic field.

Furthermore, skillful adaption of particle detection technology developed for elementary particle physics experiments gained pace from the start of the millennium, leading to the implementation of balloon-borne experiments circling the south pole and space-borne experiments. A result hinted that for the main components; such as protons, helium, carbon and oxygen, there is a deviation from the single power law, suggesting a spectral hardening. This indicated that new hypotheses should be introduced into the theoretical models of cosmic ray acceleration and propagation mechanisms. Many theoretical models were proposed, setting off a lively discussion. As a spectral hardening of proton, a main component in cosmic rays, was already reported in CALET observations, the attention has shifted to discerning the spectral structure differences of both light nuclei with atomic number (charge: Z) around 6, such as helium, carbon and oxygen, as well as that of heavy nuclei, such as iron and nickel, by high precision observation.

Currently, it is common to conduct direct observation of cosmic rays with two types of detectors, magnetic spectrometers and calorimeters.

A magnet spectrometer is a detector equipped with a magnetic field, measuring the momentum and the sign of charge for particles passing through it based on their curvature radius and direction, respectively. While in principle, although this allows for high precision observation, the observable energy is limited to the sub-teraelectronvolt range. An example of this type of detector is AMS-02, which has been conducting a continuous, still ongoing observation on the ISS since 2011, and has reported high precision observation results on the heavy nuclei components up to iron.

A calorimeter type detector features a large column density of detection material and measures the energy by absorbing the particle shower created by incident high-energy particles. Therefore, it is suitable for measurements of the high-energy regions, and CALET can be considered as a representative example. CALET is the world's first full-scale calorimeter-type detector developed for observation in space. Thanks to its wide energy measurement range and reliable detector calibration, it has measured for the first time as a single detector the full range covered by magnet spectrometers and previous calorimeter-type detectors, and achieved the measurement of the nuclei components reaching into the above-TeV high-energy region inaccessible to AMS-02.

CALET was installed on the ISS in August, 2015, and began observation in October of the same year, with more than five years of observation accumulated up to now and continuing. For the measurement of the nuclei component spectra, long time observation and data accumulation by a detector with high charge selection and energy determination capabilities is necessary. While CALET, being Japan's mainstay cosmic-ray detector, is primarily optimized for the observation of high-energy electrons, it also shows excellent performance in the observation of protons and the nuclei components, with charge and energy determination capabilities that, as shown in Fig. 1, can separate the species from protons (atomic number Z=1) to nickel (Z=28) and cover 6 orders of magnitude, from 1 GeV to 1 PeV (peta electron volt) in energy, respectively.

The Total Absorption Calorimeter (TASC) in the bottom part provides a higher column density than earth's atmosphere and measures the total energy of the shower particles generated by the incident high energy particle. By combining the information obtained from these three detectors, virtually all there is to know about the cosmic ray can be reconstructed. Particularly the thickness of the TASC, the used materials, and the signal readout method are decisive in determining up to which particle energy observation is possible, and CALET is especially superior in this aspect to previous detectors.

Fig. 3 shows an example event of the observation of a TeV-range iron nucleus. Striking from above, it passes through the CHD as iron, but within the IMC it undergoes a nuclear interaction developing into a particle shower, with the shower energy measured by the TASC. Unlike for electrons, for which the incident particle energy is almost completely absorbed, there is significant leakage out of the detector, but the precision of the shower energy is still high since the energy response is constant up to and including the TeV region. This is an important feature not achievable with magnet spectrometers. Furthermore, by using the CHD and the IMC together, the nucleus type of the incident particle can be accurately determined.

Making use of the data accumulated over five years of continuous observation since October 13, 2015, the iron and nickel spectra measured with CALET are shown in Fig. 4 (as red dots), together with data from other observations for comparison. The yellow band represents the statistical error associated with CALET's observation up to now, while the green band shows the total error including systematic errors. The absolute value of CALET's iron flux observation result is significantly lower compared to that of AMS-02, though in spectral shape there is good agreement with the AMS-02 result. Even though CALET's nickel result is so far limited to the range below 200 GeV/n, there certainly is merit in comparing it with iron despite the energy, as it shows that both could be explained by the same acceleration and propagation mechanisms.

Despite the nuclei measurement with calorimeters having unique advantages, it requires a high level of sophistication, and estimating the systematic error is not easy. For CALET, a detailed evaluation of the systematic error has been performed, based on performance verification experiments using particle accelerator beams, as well as simulation calculations. Furthermore, excluding AMS-02, the many other experimental results on the iron spectrum , albeit their error being large, show a tendency of agreement on the absolute flux value. Concerning the difference in absolute value to AMS-02, careful mutual verification with regards to yet unknown systematic errors is necessary.

Moreover, with future observation data accumulation, CALET aims at discovering the acceleration limit, which is proportional to charge, by reaching the 100 TeV/particle region in determining the proton and nuclei spectra. This would be a direct verification of the upper energy limit for shock-wave acceleration in supernova remnants. On the other hand, not finding the acceleration limit, but instead that the power law spectrum extends all the way up to the 100 TeV/particle region, would also be an exceptional and important observation result. It would be a direct indication by charged particle observation that the acceleration limit indeed increases by effects such as magnetic field amplification near the shockwaves.

The observation by CALET gathers a lot of interest from Japan and abroad. Of the items observed, dark matter, one of the greatest mysteries of the universe, has been featured in both print and international TV media. Because of this, this is bringing attention once again to the scientific results of CALET, as well as to "Kibo" on the ISS. The results obtained this time are expected to continue to have a ripple effect.

In the charged cosmic ray spectra observed up to now, the phenomenon of spectral hardening has been confirmed in those of proton, helium, carbon and oxygen. However, to support or falsify the proposed theoretical models on the origin of the spectral hardening, further measurements of heavy nuclei spectra as those of iron and nickel reported herein, with higher precision, are exceptionally important. Furthermore, measurement of the energy dependence of the boron to carbon ratio also plays an important role. Iron and nickel are produced in the final stage of stellar nucleosynthesis and released to interstellar space through the acceleration in supernova shockwaves, and thus are comprised only of a primary cosmic ray component. In contrast, boron is a secondary component, made from interaction of primary cosmic rays with interstellar matter during propagation within the galaxy. Because of this, both measurements are important for a quantitative understanding of the acceleration mechanism and the propagation process inside the galaxy. CALET is also carrying out a measurement of the boron to carbon ratio up to the TeV region, which by combination with previous observation results, is thought to have the potential of contributing to an understanding of the spectral hardening.