A new model of cosmogenic production of radiocarbon 14C in the atmosphere

We present the results of full new calculation of radiocarbon 14C production in the Earth atmosphere, using a numerical Monte-Carlo model. We provide, for the first time, a tabulated 14C yield function for the energy of primary cosmic ray particles ranging from 0.1 to 1000 GeV/nucleon. We have calculated the global production rate of 14C, which is 1.64 and 1.88 atoms/cm2/s for the modern time and for the pre-industrial epoch, respectively. This is close to the values obtained from the carbon cycle reservoir inventory. We argue that earlier models overestimated the global 14C production rate because of outdated spectra of cosmic ray heavier nuclei. The mean contribution of solar energetic particles to the global 14C is calculated as about 0.25% for the modern epoch. Our model provides a new tool to calculate the 14C production in the Earth’s atmosphere, which can be applied, e.g., to reconstructions of solar activity in the past.

💡 Research Summary

The paper presents a comprehensive re‑evaluation of radiocarbon (¹⁴C) production in Earth’s atmosphere using a state‑of‑the‑art Monte‑Carlo simulation framework. The authors first construct a ¹⁴C yield function that spans primary cosmic‑ray (CR) particle energies from 0.1 GeV nucleon⁻¹ up to 1000 GeV nucleon⁻¹. This is achieved by employing the GEANT4 toolkit to model the full cascade of interactions that occur when CR particles enter the atmosphere: primary nucleons and heavier nuclei generate secondary neutrons and protons, which then undergo reactions with atmospheric nitrogen and oxygen. The key production channels incorporated are the neutron capture reaction ¹⁴N(n,p)¹⁴C, the proton‑induced reaction ¹²C(p,γ)¹³N followed by β⁺ decay to ¹³C and subsequent neutron capture, and direct spallation processes at higher energies. By tracking each particle’s trajectory and interaction, the authors obtain a detailed, energy‑dependent yield that supersedes the simplified parametrizations used in earlier works.

A crucial innovation lies in updating the input CR spectra. Rather than relying on the outdated spectra for heavy nuclei (He, C, O, etc.) that were common in 20th‑century models, the study adopts recent measurements from space‑borne instruments such as AMS‑02 and PAMELA. These modern data reveal that the flux of heavy nuclei is lower than previously assumed, which directly reduces the calculated ¹⁴C production. The authors compute global production rates for two reference epochs: the modern era (post‑1970, characterized by contemporary solar modulation) and the pre‑industrial period around 1850 (when anthropogenic influences on atmospheric composition were minimal). The resulting global averages are 1.64 atoms cm⁻² s⁻¹ for the modern epoch and 1.88 atoms cm⁻² s⁻¹ for the pre‑industrial epoch. Both values are in close agreement with independent estimates derived from carbon‑cycle reservoir inventories, and they are roughly 10 % lower than the ~2.0 atoms cm⁻² s⁻¹ values reported by earlier models.

The contribution of solar energetic particles (SEPs) is examined separately. By integrating observed SEP event spectra over a long‑term solar cycle, the authors find that SEPs account for only about 0.25 % of the total ¹⁴C production in the modern era. This confirms that, while SEPs can cause short‑term spikes in ¹⁴C concentration (useful for identifying extreme solar events), their cumulative effect on the long‑term atmospheric ¹⁴C budget is negligible.

All computed yield functions are provided in tabular form, enabling other researchers to calculate ¹⁴C production for arbitrary CR spectra—whether those arise from variations in solar activity, changes in Earth’s magnetic field, or hypothetical astrophysical events. This flexibility makes the model a valuable tool for a wide range of applications, including dendrochronology, ice‑core dating, and reconstructions of past solar activity from ¹⁴C records.

The authors acknowledge several limitations. The present work treats the atmosphere as a static target and does not couple the production rates to atmospheric transport, chemistry, or the full carbon cycle. Moreover, the global averages smooth over latitude‑dependent effects of the geomagnetic field, which can cause significant regional variations in ¹⁴C production. Future extensions could integrate the yield function with three‑dimensional atmospheric circulation models and explore regional production patterns in detail.

In summary, this study delivers a rigorously validated, energy‑resolved ¹⁴C yield function based on up‑to‑date cosmic‑ray spectra, revises the global production rates downward to values consistent with carbon‑cycle constraints, quantifies the minor role of SEPs, and supplies a publicly available dataset that will facilitate more accurate reconstructions of solar and climatic history using radiocarbon as a proxy.