Autoradiography is an imaging method which allows visualization of the spatial distribution of radioactivity in solid sample materials. Contrary to commonly known radiographic methods such as X-ray imaging in the medical sciences, autoradiography does not rely on an external source of radiation, but instead uses radiation emitted by radionuclides which are naturally or artificially embedded in the sample itself.
This enables efficient screening of large areas of solid samples, such as rocks or speleothem surfaces, thinsections or even unconsolidated sand by placing the material in direct contact with the imaging sensor plate for a certain timespan, during which the sample’s radioactivity generates a signal within the sensor material. After readout of the sensor plate by a laser-induced luminescence imager (colloquially called a “beta-scanner”), the obtained dosimetry distribution patterns can help elucidate various geological problems.
Our lab is equipped with a BAS-1800 Bio Imaging Analyzer System from Fujifilm Life Sciences Corporations, which has a minimal pixel size of 50μm and covers a dynamic range of approximately 5 orders of magnitude.
Imaging sensors used by the lab are 23x25 cm large BAS-MS Imaging Plates from Fuji Photo Film Co Ltd. These sensor plates are equipped with a protective Mylar layer, making them sufficiently robust to even work with rock samples.
Three identical, radionuclide-depleted lead sarcophagi with 5 cm wall thickness and additional internal shielding by 1 cm wood and 1 mm copper allow simultaneous exposure of three Imaging Plates shielded from external radiation, such as cosmic rays or general natural background radioactivity.
The system is housed in a temperature-controlled basement darkroom equipped with strongly subdued red light sources, enabling work on light-sensitive sample material.
Examples of work done at the Autoradiography Lab:
In a study performed in the crystalline rocks of the Grimsel Underground Rock Laboratory, migration pathways in water-conducting features and within the matrix rock were investigated using fluid tracers containing radionuclides (Möri et al., 2006, Möri 2009). To obtain information about spatial localization and distribution of fluid flow, a series of rock slabs were investigated by autoradiography, which revealed the localization of radionuclides along the flow paths.
Visual light photograph and autoradiography of a granodiorite from the Grimsel Underground Laboratory (Möri, 2009). The dark features in the autoradiograph are radionuclide-rich minerals such as zircons or monazites (discrete spots) and flowpaths of the radionuclide tracer fluid (elongate, continuous traces).
Analytical methods such as Laser Ablation ICP MS or Electron Microprobe working on thin-sections usually require that the locations of the mineral grains to be analysed are known. In the case of accessory phases such as e.g. zircon, autoradiographic imaging allows simultaneous screening and visualization of up to 90 thin-sections for locations of such radionuclide-rich mineral grains. This helps tremendously when pinpointing the grains under the microscope later on, particularly in samples containing only a few such grains.
In a project investigating hominid sites in South Africa, U-Pb dating was applied to finely layered speleothems in an attempt to obtain a numerical age stratigraphy for the fossil finds (Pickering & Kramers, 2010; Pickering et al., 2010). With concentrations of these target elements in the speleothems mostly at the lower end or below the analytical capability, a means was required to identify U-rich layers in order to expedite analytical work. Autoradiographic imaging of speleothem slabs allowed to localization of such layers to within less than 1 mm, thereby avoiding the need to sample and analyse hundreds of layers in vain.
Autoradiograpy of a stalagmite showing uranium-rich layers by high signal intensity, while the pure calcite shows no signal above the background intensity.
In luminescence age dating the investigated signal in a sample is built up over time by the radioactive dose from its surrounding material. Recent developments of analytical capabilities now permit measurements of smaller sample amounts – down to single mineral grains – and of spatially-resolved luminescence signals. As such it has become increasingly important to not only accurately quantify the signal producing dose rates, but to also assess whether this irradiation is homogeneous or not, in order to distinguish whether different signal intensities in single mineral grains stem from heterogeneous irradiation or are the result of another process.
With small, strong radiation emitters, such as radionuclide-rich accessory minerals like zircon, are the primary source of spatial dose rate heterogeneity. Autoradiography can be used to at least qualitatively assess whether this phenomenon must be taken into consideration when evaluating luminescence data (Rufer & Preusser, 2009).