Experimental and Simulation Study of In-situ ¹⁴C Production in Wood using Neutron Irradiation

When exposed to neutrons from cosmic rays, atmospheric nitrogen reacts to produce the radioactive isotope 14C. Radiocarbon dating of trees is based on the fact that 14C in wood originates from the atmosphere. However, several studies have investigated the possibility of in-situ 14C production in wood exposed to cosmic radiation at high altitudes. In this study, a neutron irradiation experiment was conducted on Japanese cedar (Cryptomeria japonica) specimens and the increase in 14C concentration was measured. In practice, the increase in 14C could not be measured, as has been reported in previous studies. We also conducted computer simulations using PHITS, assuming the experimental conditions and a standing tree at high altitude. Neither simulation confirmed a sufficient increase in 14C in the wood. In the simulations of a standing tree, an increase in 14C equivalent to 0.1 % of the natural 14C level required approximately 15,000 years of exposure. The increase in 14C was too small to confirm in-situ production in the tree, even if a sudden neutron increase event occurred. This study reaffirmed that the in-situ production of 14C in wood was extremely limited based on nuclear reaction simulations and cosmic ray fluxes prediction model, and empirically confirmed the validity of conventional radiocarbon dating.