[Paper Review] Isotopic composition of cosmic-ray sources

I. V. Moskalenko, A. W. Strong+1 more|ArXiv.org|Aug 15, 2007

Nuclear Physics and Applications3 citations

TL;DR

This study uses the GALPROP cosmic ray propagation code and ACE data to derive isotopic source abundances for cosmic rays under two propagation models—diffusive reacceleration and plain diffusion. It finds that source abundances differ significantly between models, with many isotopes (e.g., 16O, 14N, 20Ne) underabundant relative to solar composition, while others (e.g., 22Ne, 23Na, 27Al) show strong agreement, and radioactive isotopes like 41Ca and 53Mn are predicted to be significantly present, offering constraints on CR acceleration timescales.

ABSTRACT

We use the GALPROP code and the Advanced Composition Explorer (ACE) data to derive the cosmic ray (CR) isotopic composition at the sources. The composition is derived for two propagation models, diffusive reacceleration and plain diffusion. We show that the compositions derived assuming these two propagation models are different. We also compare the isotopic composition at the sources with the latest solar composition.

Motivation & Objective

• To derive isotopic source abundances for cosmic rays using a physically based, spatially resolved propagation model instead of simplified approximations.
• To assess the impact of different propagation models—diffusive reacceleration and plain diffusion—on derived source compositions.
• To compare derived source isotopic abundances with the latest solar system abundances to evaluate consistency and identify discrepancies.
• To investigate the role of isotopic production cross-section uncertainties and their effect on source abundance inference.
• To explore the implications of radioactive isotopes (e.g., 41Ca, 53Mn) in sources for constraining cosmic ray acceleration timescales.

Proposed method

• The GALPROP code is used to simulate full nuclear reaction networks, including primary, secondary, and tertiary production, from input source abundances.
• Isotopic cross-sections are computed using T16 Los Alamos compilations, CEM2k, and LAQGSM codes, with key channels fitted to experimental data.
• An iterative correction procedure adjusts source abundances to match ACE observations at 200 MeV/nucleon, achieving ~5% agreement in elemental abundances.
• The propagation model includes solar modulation via the force-field approximation with Φ = 450 MV, corresponding to the solar activity level during ACE observations.
• Two propagation models are applied: one with diffusive reacceleration and one without, allowing direct comparison of source abundance results.
• K-capture and electron stripping processes are included as separate species due to their distinct lifetimes in the propagation chain.

Experimental results

Research questions

• RQ1 How do isotopic source abundances derived using a full 3D spatial and energy-dependent propagation model compare to solar system abundances?
• RQ2 To what extent do the assumptions of diffusive reacceleration versus plain diffusion alter the derived source isotopic compositions?
• RQ3 Which isotopes show significant discrepancies between source abundances and solar abundances, and what do these imply about their origin and production mechanisms?
• RQ4 What is the predicted abundance of radioactive isotopes like 41Ca and 53Mn in cosmic ray sources, and how can they constrain the acceleration timescale?
• RQ5 How do uncertainties in isotopic production cross-sections affect the reliability of derived source abundances?

Key findings

• The source abundances derived under the diffusive reacceleration and plain diffusion models differ significantly, indicating that propagation model choice critically affects inferred source compositions.
• Isotopes such as 16O, 14N, and 20Ne are found to be significantly underabundant in cosmic ray sources compared to solar system abundances, with the 16O result being robust due to well-known fragmentation cross-sections.
• A strong agreement is observed between derived source abundances and solar abundances for isotopes like 22Ne, 23Na, 27Al, and 52Cr, suggesting primary or mixed origin mechanisms.
• Radioactive isotopes 41Ca and 53Mn are predicted to be significantly present in sources, with half-lives of 1.03×10⁵ yr and 3.74×10⁶ yr respectively, offering a direct probe of cosmic ray acceleration timescales.
• Isotopes such as 13C, 17O, 21Ne, and 33–36S are predominantly secondary in origin, consistent with prior findings but now confirmed in a full propagation framework.
• Discrepancies in predicted abundances between the two models for 15N, 18O, 21Ne, 33S, and 55Mn provide a potential diagnostic tool to distinguish between propagation models.