A new model of cosmogenic production of radiocarbon 14C in the atmosphere
Highlights
► For the first time, 14C yield function is presented for the full energy range of primary cosmic rays. ► The global production rate of 14C is now close to the results of the carbon cycle reservoir inventory. ► The mean contribution of solar energetic particles to the global 14C is 0.25% for the modern epoch. ► This provides a new tool to calculate the 14C production in the atmosphere.
Introduction
Radiocarbon 14C is a long-living (half-life about 5730 yr) radioactive nuclide produced mostly by cosmic rays in the Earth's atmosphere. Soon after production, it gets oxidized to 14CO2 and in the gaseous form takes part in the complex global carbon cycle (Bolin et al., 1979). Radiocarbon is important not only because it is used for dating in many applications (e.g., Dorman, 2004, Kromer, 2009), but also because it forms a primary method of paleo-reconstructions of solar activity on the millennial time scales (e.g., Stuiver and Quay, 1980, Stuiver and Braziunas, 1989, Bard et al., 1997, Muscheler et al., 2007). An essential part of the solar activity reconstruction from radiocarbon data is computation of 14C production by cosmic rays in the Earth's atmosphere. First such computations were performed in the 1960–1970s (e.g., Lingenfelter, 1963, Lingenfelter and Ramaty, 1970, Light et al., 1973, O'Brien, 1979) and were based on simplified numerical or semi-empirical methods. Later, full Monte-Carlo simulations of the cosmic-ray induced atmospheric cascade had been performed (Masarik and Beer, 1999, Masarik and Beer, 2009). Most of the earlier models, including O'Brien (1979) and Masarik and Beer (1999) deal with a prescribed functional shape of the galactic cosmic ray spectrum, which makes it impossible to be applied to other types of cosmic ray spectra, e.g., solar energetic particles, supernova explosions, etc. A flexible approach includes calculation of the yield function (the number of cosmogenic nuclei produced in the atmosphere by the primary cosmic rays of the given type with the fixed energy and unit intensity outside the atmosphere), which can be convoluted with any given energy spectrum of the primary cosmic rays (e.g., Webber and Higbie, 2003, Webber et al., 2007, Usoskin and Kovaltsov, 2008, Kovaltsov and Usoskin, 2010). This approach can be directly applied to, e.g., a problem of the signatures of extreme solar energetic particle events in the cosmogenic nuclide data, which is actively discussed (e.g., Usoskin et al., 2006, Hudson, 2010, LaViolette, 2011). Some earlier models (Lingenfelter, 1963, Castagnoli and Lal, 1980) provide the 14C yield function however it is limited in energy. Moreover, different models give results, which differ by up to 50% from each other, leading to large uncertainty in the global 14C production rate. Therefore, the present status is that models providing the yield function are 30–50 yr old and have large uncertainties.
In addition, there is a systematic discrepancy between the results of theoretical models for the 14C production and the global average 14C production rate obtained from direct measurements of the specific 14CO2 activity in the atmosphere and from the carbon cycle reservoir inventory. While earlier production models predict that the global average pre-industrial production rate should be 1.9–2.5 atoms/cm2/s, estimates from the carbon cycle inventory give systematically lower values ranging between 1.6 and 1.8 atoms/cm2/s (Lingenfelter, 1963, Lal and Suess, 1968, Damon and Sternberg, 1989, O'Brien et al., 1991, Goslar, 2001, Dorman, 2004). This discrepancy is known since long (Lingenfelter, 1963) but is yet unresolved (Goslar, 2001).
In this work we redo all the detailed Monte-Carlo computations of the cosmic-ray induced atmospheric cascade and the production of 14C in the atmosphere to resolve the problems mentioned above. In Section 2 we describe the numerical model and calculation of the radiocarbon production. In Section 3 we compare the obtained results with earlier models. In Section 4 we apply the model to calculate the 14C production by galactic cosmic rays and solar energetic particle events for the last solar cycle. Conclusions are presented in Section 5.
Section snippets
Calculation of the 14C production
Energetic primary cosmic ray particles, when entering the atmosphere, collide with nuclei of the atmospheric gases initiating a complicated nucleonic cascade (also called shower). Here we are interested primarily in secondary neutrons whose distribution in the atmosphere varies with altitude, latitude, atmospheric state and solar activity. Neutrons are produced in the atmosphere through multiple reactions including high-energy direct reactions, low-energy compound nucleus reactions and
Comparison with earlier models
In Fig. 1 we compare our present results with the yield functions calculated earlier (see the figure caption for references). Our results are consistent with most of the earlier calculations (LR70 and DV91) within 10–20%. The CL80 yield function is not independently calculated but modified from LR70. While it is formally given for protons it effectively includes also via scaling, thus being systematically higher than the other yield functions. Note that all the earlier computations
14C production by solar energetic particles
We also calculated production of radiocarbon by solar energetic particles (SEPs), because presently there is a wide range of the results (e.g., Lingenfelter and Ramaty, 1970, Usoskin et al., 2006, Hudson, 2010, LaViolette, 2011). Here we compute the expected production of 14C by the major known SEP events since 1951, using our calculated yield function (Table 1) and SEP event-integrated spectra as reconstructed by Tylka and Dietrich (2009). The corresponding production rate is shown by big open
Conclusions
- •
We have performed full new calculation, based on a detailed Monte-Carlo simulation of the atmospheric cascade by a GEANT-4 tool PLANETOCOSMICS, of the 14C yield function. This is the first new calculation of the yield function since 1960–1970s, using modern techniques and methods, and the yield function is, for the first time ever, directly computed up to the energy of 1000 GeV/nucleon (earlier models were limited to a few tens GeV/nucleon and extrapolated to higher energies). Our newly computed
Acknowledgments
This work uses results obtained in research funded from the European Union's Seventh Framework Programme (FP7/2007-2013) under grant agreement no 262773 (SEPServer). The High-Energy Division of Institute for Nuclear Research and Nuclear Energy—Bulgarian Academy of Sciences is acknowledged for the given computational time. GAK was partly supported by the Program No. 22 of presidium RAS. University of Oulu and the Academy of Finland are acknowledged for partial support.
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