[Paper Review] Relative abundances of cosmic ray nuclei B-C-N-O in the energy region from 10 GeV/n to 300 GeV/n. Results from ATIC-2 (the science flight of ATIC)

А. Д. Панов, N. V. Sokolskaya+20 more|arXiv (Cornell University)|Jul 30, 2007

Dark Matter and Cosmic Phenomena1 references18 citations

TL;DR

This paper presents high-precision measurements of cosmic ray B/C, C/O, and N/O flux ratios from 10 to 300 GeV/n using improved charge resolution in the ATIC-2 balloon-borne experiment. By combining the silicon matrix with the upper scintillator hodoscope layer, the study reduces background and enhances charge resolution, yielding B/C ratios that decrease with energy and are consistent with cosmic ray propagation models, though with significant uncertainties limiting model discrimination.

ABSTRACT

The ATIC balloon-borne experiment measures the energy spectra of elements from H to Fe in primary cosmic rays from about 100 GeV to 100 TeV. ATIC is comprised of a fully active bismuth germanate calorimeter, a carbon target with embedded scintillator hodoscopes, and a silicon matrix that is used as the main charge detector. The silicon matrix produces good charge resolution for protons and helium but only partial resolution for heavier nuclei. In the present paper, the charge resolution of ATIC was improved and backgrounds were reduced in the region from Be to Si by using the upper layer of the scintillator hodoscope as an additional charge detector. The flux ratios of nuclei B/C, C/O, N/O in the energy region from about 10 GeV/nucleon to 300 GeV/nucleon obtained from this high-resolution, high-quality charge spectra are presented, and compared with existing theoretical predictions.

Motivation & Objective

• To improve charge resolution for light cosmic ray nuclei (B, C, N, O) in the ATIC-2 experiment by utilizing the upper scintillator hodoscope layer as an additional charge detector.
• To reduce background contamination in the charge spectra for boron and carbon, particularly in the 5–8 charge range where silicon matrix resolution is poor.
• To measure the relative flux ratios B/C, C/O, and N/O in the energy range 10–300 GeV/n with high precision to test cosmic ray propagation models.
• To correct for atmospheric interactions and instrument response effects to derive primary fluxes above the atmosphere.
• To compare the measured ratios with theoretical predictions in leaky box models to assess the validity of different propagation mechanisms.

Proposed method

• The charge of primary particles is reconstructed using trajectory information from the BGO calorimeter and the silicon matrix, with additional charge measurement from 42 parallel scintillator strips in the upper hodoscope layer.
• Events are selected based on consistency between the silicon matrix charge (Q_Si) and the scintillator strip charge (Q_Sci), with a cut |Q_Si - Q_Sci| ≤ 0.25 and spatial proximity (≤5 cm from reconstructed track).
• The final charge is calculated as the average: Q = (Q_Si + Q_Sci)/2, improving resolution and reducing background at the cost of ~4× lower statistics.
• Backgrounds from protons and helium (p, He) interacting in the instrument are subtracted using FLUKA simulations, with corrections up to 36% for boron in the highest energy bins.
• A charge response matrix K^q_q' is constructed to account for instrumental broadening and misreconstruction, enabling unfolding of true fluxes from observed charge spectra.
• Atmospheric corrections are applied using FLUKA-simulated fragmentation coefficients L^q_q', solving a triangular, diagonally-dominant system to recover primary fluxes above the atmosphere.
• Energy-dependent flux ratios are derived via interpolation at geometric mean energy points and corrected for atmospheric absorption, with full error propagation using Monte Carlo methods.

Experimental results

Research questions

• RQ1 How does the inclusion of the upper scintillator hodoscope layer improve charge resolution and background suppression for light cosmic ray nuclei in ATIC-2?
• RQ2 What are the precise energy-dependent B/C, C/O, and N/O flux ratios in the 10–300 GeV/n range, and how do they compare with existing data and theoretical models?
• RQ3 To what extent do atmospheric interactions and instrument response affect the measured flux ratios, and how can these effects be corrected to recover primary cosmic ray spectra?
• RQ4 Can the measured B/C ratio at high energies distinguish between different cosmic ray propagation models, such as leaky box models with varying escape lengths?
• RQ5 How do the experimental uncertainties in the flux ratios affect the ability to discriminate between competing theoretical models of cosmic ray propagation?

Key findings

• The B/C ratio decreases from 0.180 ± 0.011 at 19.9 GeV/n to 0.064 ± 0.063 at 307 GeV/n, indicating a softening of the spectrum at high energies.
• The N/O ratio decreases from 0.219 ± 0.010 at 19.9 GeV/n to 0.144 ± 0.068 at 307 GeV/n, consistent with secondary production in the interstellar medium.
• The C/O ratio remains nearly constant at ~1.0, with values ranging from 1.020 ± 0.026 at 19.9 GeV/n to 1.022 ± 0.227 at 307 GeV/n, suggesting minimal energy dependence in this energy range.
• Atmospheric corrections reduce the B/C ratio by 13%–33% across energies, with the largest corrections at higher energies due to increased fragmentation.
• The measured B/C and N/O ratios are slightly higher than those from HEAO-3-C2 at similar energies, but extend to higher energies (up to 307 GeV/n), improving statistical reach.
• Despite improved resolution and background suppression, the experimental uncertainties remain too large to distinguish between competing propagation models, such as those with different escape length prescriptions.