We have measured mass spectra for positive ions for low-energy electron impact on thymine using a reflectron time-of-flight mass spectrometer. Using computer controlled data acquisition, mass spectra have been acquired for electron impact energies up to 100 eV in steps of 0.5 eV. Ion yield curves for most of the fragment ions have been determined by fitting groups of adjacent peaks in the mass spectra with sequences of normalized Gaussians. The ion yield curves have been normalized by comparing the sum of the ion yields to the average of calculated total ionization cross sections. Appearance energies have been determined. The nearly equal appearance energies of 83 u and 55 u observed in the present work strongly indicate that near threshold the 55 u ion is formed directly by the breakage of two bonds in the ring, rather than from a successive loss of HNCO and CO from the parent ion. Likewise 54 u is not formed by CO loss from 82 u. The appearance energies are in a number of cases consistent with the loss of one or more hydrogen atoms from a heavier fragment, but 70 u is not formed by hydrogen loss from 71 u.

Abstract:8 Rehydroxylation dating trials were conducted on post-medieval ceramics (bricks) of known age, testing a new 9 component based approach. Age estimates were produced using both a t 1/4 and a more generalised t 1/n model for 10 describing the Stage 2 mass gain, the latter providing a more satisfactory description of the mass gain curves. Despite 11 this, the estimated ages are generally too old for agreement with known ages; the magnitude of the discrepancies 12 suggest issues beyond the particular models used. The effect of uncertainties associated with the effective lifetime 13 temperature (ELT) of the ceramic were explored, aided by the generation of age-temperature curves; while potentially 14 a strong contributor, the large age discrepancies can not be explained by ELT issues.

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bs_bs_banner research shows that adequate and proper sampling of mortar samples is of fundamental importance for a conclusive radiocarbon analysis. Therefore, this article thoroughly reviews the processes and environmental factors that may cause problems for successful radiocarbon dating of mortar samples, and presents best-practice sampling strategies for radiocarbon mortar dating.

The mass gain behavior of archeological bricks was examined following drying (130°C)/reheating (500°C) and aging at a range of temperatures (25°C, 35°C, 45°C). For drying or reheating, samples exhibit a two‐stage mass gain behavior, the second stage, Stage 2, continuing indefinitely and better described by a t1/n model (1/n=1/6‐1/2); a correlation between the 1/n value and the specific surface area/pore volume demonstrates diffusion mechanisms with some pore geometry/morphology dependence. Stage 2 is shown to have an Arrhenius temperature dependence with activation energies of similar orders of magnitude following both drying and reheating. Supported by thermogravimetric‐mass spectrometry (TG‐MS), Stage 2 is demonstrated as likely due to the recombination of chemisorbed water, previously removed, whereas following reheating due to two components, a chemisorbed component associated with drying and a component associated with rehydroxyls removed at higher temperatures during reheating. Differences between activation energies of chemisorption and rehydroxylation components support this. Evidence for a fundamental compositional relationship between these processes is presented by a strong linear relationship between the drying and reheating mass gain rates. Stage 1, following drying or reheating, is shown to be likely associated with physisorption processes alone.