[Paper Review] Elemental energy spectra of cosmic rays measured by CREAM-II

P. Maestro, H. S. Ahn+31 more|arXiv (Cornell University)|Mar 30, 2010

Dark Matter and Cosmic Phenomena3 citations

TL;DR

CREAM-II measured elemental cosmic ray energy spectra from carbon to iron up to ~100 TeV/n using a balloon-borne instrument with redundant particle identification and a sampling calorimeter. The spectra for all primary nuclei follow a nearly identical power-law energy dependence with a spectral index of $\gamma = 2.66 \pm 0.04$, indicating a common acceleration mechanism, while nitrogen data show a spectral hardening above 100 GeV/n, supporting a primary component at high energies.

ABSTRACT

We present new measurements of the energy spectra of cosmic-ray (CR) nuclei from the second flight of the balloon-borne experiment CREAM (Cosmic Ray Energetics And Mass). The instrument (CREAM-II) was comprised of detectors based on different techniques (Cherenkov light, specific ionization in scintillators and silicon sensors) to provide a redundant charge identification and a thin ionization calorimeter capable of measuring the energy of cosmic rays up to several hundreds of TeV. The data analysis is described and the individual energy spectra of C, O, Ne, Mg, Si and Fe are reported up to ~ 10^14 eV. The spectral shape looks nearly the same for all the primary elements and can be expressed as a power law in energy E^{-2.66+/-0.04}. The nitrogen absolute intensity in the energy range 100-800 GeV/n is also measured.

Motivation & Objective

• To measure the elemental energy spectra of cosmic-ray nuclei from C to Fe with high precision in the 100 GeV/n to 100 TeV/n range.
• To test astrophysical models of cosmic ray acceleration and propagation by directly measuring composition and energy spectra.
• To investigate the origin of nitrogen in cosmic rays by measuring its energy spectrum and comparing with secondary production models.
• To validate the consistency of energy and charge reconstruction across multiple detector systems in a balloon-borne experiment.

Proposed method

• CREAM-II used a multi-detector system including a timing-charge detector (TCD), Cherenkov detector (CD), pixelated silicon charge detector (SCD), and a sampling imaging calorimeter (CAL) for redundant particle identification and energy measurement.
• The CAL, a 20-radiation-length tungsten-scintillator stack, measured total shower energy with high granularity and provided trajectory reconstruction via shower core imaging.
• Particle charge was determined using the SCD and TCD, with charge resolution better than 0.35e, enabling separation of elements from H to Ni.
• Energy spectra were reconstructed using trajectory reconstruction, shower core imaging, and corrections for top-of-instrument (TOI) and top-of-atmosphere (TOA) effects.
• Systematic uncertainties were evaluated from reconstruction algorithms (10% below 3 TeV, 5% above), TOI corrections (2% for primary nuclei), and atmospheric secondary corrections (15% for nitrogen due to O spallation).
• Data were analyzed over a 24-day stable period (Dec 19 – Jan 12), yielding 57 GB of data with high-fidelity energy and charge resolution.

Experimental results

Research questions

• RQ1 Do the energy spectra of primary cosmic-ray nuclei from C to Fe follow a common power-law behavior across the 100 GeV/n to 100 TeV/n range?
• RQ2 Is the spectral index of heavy nuclei consistent across different elements, suggesting a common acceleration mechanism?
• RQ3 Does the nitrogen energy spectrum exhibit a hardening at high energies, as expected if the primary component dominates at high rigidity?
• RQ4 How do systematic uncertainties from reconstruction and atmospheric corrections affect the absolute intensity measurements of primary and secondary nuclei?
• RQ5 Is the N/O abundance ratio consistent with previous measurements, particularly from CREAM-I, at high energies?

Key findings

• The energy spectra of C, O, Ne, Mg, Si, and Fe all follow a power-law dependence of $E^{-2.66 \pm 0.04}$, indicating a common acceleration mechanism for primary heavy nuclei.
• The weighted average spectral index $\bar{\gamma} = 2.66 \pm 0.04$ is consistent with the value of $2.65 \pm 0.05$ derived from combined CRN and TRACER data.
• Nitrogen data show a significant flattening of the spectrum above 100 GeV/n, supporting the presence of a primary nitrogen component at high energies.
• The N/O abundance ratio at ~800 GeV/n is measured as $0.080 \pm 0.025$ (stat.) $\pm 0.025$ (sys.), in good agreement with the CREAM-I result.
• The absolute intensity measurements span six decades in energy and are presented without normalization to prior data, enhancing their direct comparability.
• Systematic uncertainties are well-controlled, with 2% for primary nuclei and 15% for nitrogen due to spallation contamination from oxygen.