Daniel Rufer

Potential of Autoradiography to Detect Spatially Resolved Radiation Patterns in the Context of Trapped Charge Dating

Potential of Autoradiography to Detect Spatially Resolved Radiation Patterns in the Context of Trapped Charge Dating Recent developments in optically stimulated luminescence (OSL) dating allow the determination of signals in increasingly smaller sample amounts. This has led to microdosimetry having a larger impact on equivalent dose (DE) distributions and therefore, detection and assessment of spatial distribution of radionuclides has become more important. This study demonstrates the application of autoradiography using imaging plates to determine spatially resolved radiation inhomogeneities in different types of samples. Qualitative evaluations of radiation inhomogeneity are carried out on unconsolidated sediments as well as on hard rock samples. While indicating some limitations of applicability, the results demonstrate that the method is an efficient tool to detect and document spatial variations in a samples radiation field. It therefore provides a possibility to rapidly screen samples to check whether microdosimetry might affect the DE data. Furthermore, an approach to calibrate autoradiographic images for quantitative use is suggested. Using pressed powder pellets of reference materials, a series of calibration images were exposed, from which a functional relation between specific sample activity and greyscale value in the autoradiographic image has been deduced. Testing the calibration on a set of 16 geological samples, of which their radionuclide content is known, shows a good correlation between specific activities calculated from the nuclide content and specific activities deduced from the autoradiographic images. These findings illustrate the potential of autoradiography with imaging plates to detect spatial distributions of radionuclides and to tackle certain aspects of the problem of microdosimetry in modern trapped charge dating.

Direct dating of Quaternary phreatic maar eruptions by luminescence methods

The suitability of quartz optically stimulated luminescence (OSL) and feldspar infrared stimulated luminescence (IRSL) for the direct dating of phreatic eruptions was tested on examples from the Eifel Volcanic Field, Germany. The mean IRSL age of 11.6 ± 0.5 ka for Ulmener Maar Tephra is in excellent agreement with the independent age control (11 ± 0.1 ka), but the mean OSL age of 14.2 ± 0.6 ka overestimates the known age by 3 ka. For Meerfelder Maar Tephra, consistent IRSL (mean 74.9 ± 5.0 ka) and OSL ages (74.9 ± 5.5 ka) have been observed. The consistent results from Meerfelder Maar imply that the overestimation observed for Ulmener Maar quartz OSL might not be relative to the eruption age, but rather represents a small absolute offset. In samples taken from deposits of the eruption of Laacher See Volcano, no resetting of the OSL signal and highly scattered IRSL estimates were found. This implies that phreatomagmatic eruptions are less well suited for this dating approach compared to pure phreatic maar eruptions, where the effect of high-pressure shock waves probably dominates the process of resetting the luminescence signal.

Proposing new approaches for dating young volcanic eruptions by luminescence methods

The application of luminescence dating to young volcanic sediments has been first investigated over three decades ago, but it was only with the technical innovations of the last decade that such analyses became viable. While current analytical procedures show promise for dating late Quaternary volcanic events, most efforts have been aimed at unconsolidated volcanic tephra. Investigations into direct dating of lava flows or of non-heated volcanoclastics like phreatic explosion layers, however, remain scarce. These volcanic deposits are of common occurrence and represent important chrono- and volcanostratigraphic markers. Their age determination is therefore of great importance in volcanologic, tectonic, geomorphological and climate studies. In this article, we propose the use of phreatic explosion deposits and xenolithic inclusions in lava flows as target materials for luminescence dating applications. The main focus is on the crucial criterion whether it is probable that such materials experience complete luminescence signal resetting during the volcanic event to be dated. This is argued based on the findings from existing literature, model calculations and laboratory tests.