Caltech Noble Gas Lab Research Activities:
Neon chronology

Urucum

Neon, the second-lightest noble gas, has three stable isotopes and no long-lived radioactive isotopes. The stable isotopes, neon-20, neon-21, and neon-22, are present in a ratio of 9048 : 27 : 925 in the atmosphere and in varying compositions in other materials.

Neon is produced by both cosmic ray and nuclear processes in geologic materials. As a result, the neon system can provide a wealth of information about different processes. The proportion of the atmospheric component and these other components depends on the exposure and thermal histories of the material and on its chemical composition.

Cosmogenic neon

Cosmogenic neon comes primarily from spallation reactions caused by the interaction of cosmic radiation with elements heavier than neon. These reactions simply involve an energetic particle colliding with a heavy element and causing it to break into smaller pieces. These pieces are dominated by small particles such as protons, neutrons, and helium-3, but also include larger fragments such as neon isotopes. Cosmogenic neon accumulates in materials near the Earth's surface at a rate dependent on the cosmic radiation flux and on the properties of the material.

We are currently exploring the production and retention of cosmogenic neon in various phases through diffusion experiments, high altitude target experiments, and proton-irradiation experiments. We also have ongoing projects to analyze natural samples and compare the results to other systems and known geologic histories.

Nucleogenic neon

Nucleogenic neon is produced indirectly by radioactive decay. The radium-series, thorium-series, and actinium-series decay chains of uranium-238, thorium-232, and uranium-235, respectively, emit alpha particles while decaying to the stable lead isotopes lead-206, lead-208, and lead-207. Most of these alpha particles come to rest as helium-4. These decay chains are the basis for the well-known (U-Th)/He, (U-Th)/Pb, and U-series disequilibrium dating systems. A small proportion of the alpha particles react with other elements like oxygen and fluorine. Some of these reactions produce neon, predominantly the 18O(α,n)21Ne and 19F(α,n)22Ne reactions (confused by this notation?). The production rate and isotopic composition of nucleogenic neon depend on the composition of the material and the energies of the incident alpha particles, but they tend to be of the order of 10-8 per alpha particle. Neon produced in this way serves as a daughter product in the (U-Th)/Ne dating system.

Production rates of neon-21 and neon-22 are currently calculated using calculated reaction cross section and mineral stopping power data that has not been verified experimentally. We have used the Dynamitron linear accelerator at the University of Albany to implant alpha particles into synthetic targets in order to verify these calculations. We are also performing neon diffusion experiments to investigate retentivity of neon in target phases.

In addition, we have measured the (U-Th)/Ne ages of natural samples of known and unknown ages and thermal histories. In some phases, such as iron oxides, common lead contamination and low radiogenic lead concentrations preclude the use of the (U-Th)/Pb system, while low retentivity preclude the use of the (U-Th)/He system for dating even formation at low temperatures. Neon, however, appears to be retained in these phases, and we have used the (U-Th)/Ne system to date them successfully.