Project Description: |
The presence of three relatively short-lived radionuclides (7Be, 137Cs, and 210Pb) in the environment has been used extensively to radiodate sedimentary records of lakes and streams. Each of these radioisotopes is distributed over the earth’s surface from the atmosphere, where they are each created by different processes. By careful measurements with sensitive detectors of the sediment or earth, an approximate age since last atmospheric exposure can be deduced for each isotope. Lead and cesium have half-lives between 22 and 30 years, which allows for radiodating of soils and sediments over a longer time constant than 7Be, which as a half-live of 52 days. The ratio between 7Be and 210Pb can therefore be used as an indicator of where within the soil horizon the sediment arises. Only the topmost layers of soil will have appreciable 7Be concentrations, while all layers within the first several inches will have appreciable 210Pb concentrations. Analysis of stream and lake sediment for these isotopes will allow a more precise determination of which sedimentation sources are contributing to the sediment load. In undisturbed soils, mostly surface soils should cause sediment loading. In tilled soils, there will be a much lower 7Be/210Pb ratio measured. This clever application of nuclear environmental measurements can be an important tool in the characterization of pollution sources (of sediment) in many local lakes.
For this research project we are looking to improve our measurement technique for all three radionuclides by pre-concentrating those elements in the samples to be measured. Currently these low-background radionuclide measurements require many hours to a day of acquisition time per sample, just to obtain sufficient statistics to determine an accurate age, or soil source. We will be working on analytical digestion of metals from soils using microwave-asssisted acid digestion techniques, and then pre-concentrating the metals eluted from each sample onto a commercial metal chelation disk. All metals, but specifically the radionuclides of interest will be extracted from larger soil samples and concentrated down to samples that can be radiodated much more quickly. The development of a robust pre-concentration method such as this, including the quantitative determination of extract efficiency for each radionuclide, will form the basis of a publication on the technique. Students involved in this project will be actively engaged in sample preparation and performing standard radiodating analyses of soil and sediment samples using High-purity Germanium detectors. In addition, analytical techniques of acid-assisted microwave digestion of replicate soil samples will be learned in order to extract the metals of interest in the soil/sediment. After preconcentration by filtration through commercial metal chelation disks, students will have the opportunity to measure absolute efficiencies for the technique and to prepare a publication on the method.
|
