RAMAN SPECTROSCOPIC STUDY OF THE EFFECT OF RADIATION DAMAGE ON RADON EMANATION FROM ZIRCON

Many studies have been devoted to radon emanation from bulk materials such as soil, rock, ore, and fly ash while fewer have focused on specific minerals and the internal mechanisms that control radon emanation. We focus on how radiation damage affects zircon’s internal structure. We use Raman spectroscopy to obtain a more quantitative view of the degree of metamictization as zircon grains are annealed, and couple that with measurements of radon emanation after the same heating steps. Previous work has suggested that zircons with low U content show a decrease in radon emanation as annealing of radiation damage increases. However, the previous results did not provide a convincing correlation of radon emanation rates with annealing. A potential explanation for the scatter in previous results is heterogeneity in distribution of U, Th, and their daughters within the parent large crystal that was crushed to produce the samples. We will employ a robust cleaning and grain selection method using magnetic separation and careful hand selection of homogenous grains to decrease heterogeneity. Initial measurements from the starting bulk material (crushed single large zircon crystal) and hand selected grains show a minor decrease in U content and significant decrease in Th content of hand selected grains vs. the initial material, suggesting our ‘cleaning’ process removes material with high Th content. We expect the more homogenous hand-selected aliquots to show a clearer trend between radiation damage and radon emanation if such damage is a controlling factor. We will use Raman spectroscopy to determine the temperatures at which annealing is occurring. A better understanding of how radon escapes zircon has potential implications for U/Th-Pb geochronology with insight into the causes of discordance.