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Problems with uranium series dating sim


Uranium-series dating techniques require the isolation "Problems with uranium series dating sim" radionuclides in high yields and in fractions free of impurities. Within this context, we describe a novel-rapid method for the separation and purification of U, Th, and Pa.

The method takes advantage of differences in the chemistry of U, Th, and Pa, utilizing a commercially-available extraction chromatographic resin TEVA and standard reagents.

The elution behavior of U, Th, and Pa were optimized using liquid scintillation counting techniques and fractional purity was evaluated by alpha-spectrometry. The overall method was further assessed by isotope dilution alpha-spectrometry for the preliminary age determination of an ancient carbonate sample obtained from the Lake Bonneville site in western Utah United States. Preliminary evaluations of the method produced elemental purity of greater than Radiometric age-dating techniques are powerful tools that are used often to understand geological events; describe geochemical processes; and more recently, to develop understanding of materials for nuclear forensic analysis.

For environmental science applications, this information can be used to estimate geomorphic growth rates Sims et al.

For nuclear forensic applications, this information represents a powerful tool that can be combined with other forensic evidence to develop a more detailed understanding of the process, time, and location from which the material may have originated Morgenstern et al. A mainstay for the last four decades in age-dating materials for various geochronology applications involves the disequilibria and daughter ingrowth of radionuclides in the natural U and Th decay series U, U, Th Bourdon et al.

Several excellent reviews provide detailed descriptions of the underlying assumptions that form the foundation for various time-dependent phenomena that can be described, and the time frame within which specific parent—daughter relationships can be most effectively employed.

Because Th and Pa ages can be confirmatory, radioanalytical methods developed for U-series radiochronometry applications have sought to combine the analysis of Th, Pa, and U in a single analytical run. Methods of analytical quantification for these analyses include isotope dilution alpha-spectrometry and more recently mass spectrometry Mola et al. Numerous approaches to radiochemical separations have been established for U-series radiochronometry Morgenstern et al. Early methods using standard ion-exchange technologies and liquid—liquid extraction Problems with uranium series dating sim effective, but often Problems with uranium series dating sim from relatively low radiochemical yields in the analysis of more complex matrices.

Problems with uranium series dating sim more recent methods have improved radiochemical yields, in general, two or more columns are usually employed, which complicates the process and can result in relatively large volumes of chemical waste Koornneef et al.

Within this context, we explored the potential to develop a new method, which might combine improved elemental purity of U, Th, and Pa fractions, with fewer steps, and less waste. In this paper, we describe this new method and present the application of the approach for the isotope dilution alpha-spectrometry analysis and preliminary age determination of an ancient carbonate sample obtained from the Lake Bonneville site in western Utah Makarova et al.

The method is relatively rapid; produces only small amounts of chemical waste; utilizes a commercially-available extraction chromatographic resin TEVA; Eichrom Technologies, Inc.

Radiochemical yields for Pa analysis were achieved by isotope dilution techniques using Pa tracer, prepared by solvent extraction from Np CRM Acids and salts used for radiochemical separations included: Chemicals used for electrodeposition included: Solutions containing HF and HClO 4 are potentially dangerous and Problems with uranium series dating sim personal protective equipment should be used when using these acids. Similarly, use of radioactive materials is potentially hazardous and appropriate ALARA principals should be considered prior to conducting experiments using radioactive materials.

Radioactive standard solutions were prepared by serial dilutions, which were performed volumetrically with gravimetric and radiometric confirmationswith dilutions of 5- and fold performed in 1. Radiotracers and control spikes were added using calibrated volumetric pipets according to our routine procedures.

Briefly, the glass ampoule containing the Np solution in 0. At the time of preparation, this solution was transferred and taken to dryness in a Teflon beaker and redissolved in a minimum volume of 6 M HCl Ultra-pure, Fluka. This process was repeated three times to ensure that the solution was fully converted to the chloride form and the Np was redissolved in 50 mL 6 M HCl Ultrapure, Fluka and transferred to a mL glass-separatory funnel.

1. Introduction

Extraction of Pa was achieved by the addition of 50 mL octanol pre-equilibrated in Xylenes. The aqueous layer, containing Np, was collected and returned to the storage bottle for future Pa tracer preparations.

Washings were repeated a total of three times to remove any residual Np. Each time, the aqueous-acid layer was discarded. Back-extraction of purified Pa into an aqueous solution that would be suitable for tracer additions, was accomplished by adding 50 mL of water containing 3 g Na 2 SO 4 in 5 mL concentrated H 2 SO 4 to the separatory funnel.

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The aqueous layer, containing Pa, was transferred to a mL Erlenmeyer flask and heated on a hot plate at medium heat. Schematic illustrating the liquid—liquid extraction of Pa from Np liquid-standard CRM for use as a radiometric tracer. A Np standard in 50 mL of 6 M HCl was transferred to a mL separation funnel containing 50 mL of octanol preequilibrated in xylenes. Once separated the aqueous phase was collected for Np, and an additional 50 mL of 6 M HCl was added to the separation funnel.

The aqueous layer was discarded and two more washes of 6 M HCl were added. Contents were taken dry on medium heat with 1—2 drops of a solution containing 1: HNO 3 to remove any potential organic.

To confirm the purity of the Pa fraction i. These sources were also used as standards for yield monitoring by beta counting of the Pa radiochemical yields for method evaluation as described previously "Problems with uranium series dating sim," ; Burnett and Yeh, To confirm quantitative electrodeposition of Pa to the stainless steel planchettes for use in yield determinationsthe supernatant of the electrodeposited sources were also analyzed for residual Pa by High Purity Germanium HPGe gamma-spectrometry.

No residual Pa could be detected in the supernatant solutions, based on examination of the count rate of Pa gamma emissions peaks Currie, Further, no Np could be detected by alpha-spectrometry absence of 4.

Development of the analytical method presented here was carried out using an extraction chromatographic resin, TEVA, which has been described in detail previously Horwitz et al.

Columns were prepared by slurrying the TEVA resin in water to a concentration of 0. Pre-manufactured frits provided with the empty columns were inserted on top and beneath the resin and a 25 mL reservoir AC, Eichrom was attached and the column Fig.

The column is preconditioned 4 M HCl. In step 3, the remaining Th is collected 10 mL 4 M HCl and combined with the previous fraction to account for Th activity in the sample. To prevent any potential Th cross contamination in the Pa or U fraction, an additional rinse 25 mL 4 M HCl is added to the column and is discarded. In step 5, Pa is stripped from the column and collected 4 M HCl The final rinse 5 mL 0. To arrive at the optimum separation strategy, the elution profiles for Th, Pa, and U were determined by step-wise elution of 1-mL aliquots of eluent solutions directly to mL standard plastic or glass LS vials for direct counting by LSC.

These experiments were undertaken to determine the elution peak maximum at test acid concentrations and no attempt was made in these experiments to assess or correct for scintillant quench or background contribution. Nonetheless, because the signal is proportional to the activity of the samples, the LSC experiments provide the necessary information to determine the elution peak maximum values of Pa. Once each 1-mL aliquot was collected Fig. Elution curves that describe the final separation procedure as a function of elution Problems with uranium series dating sim. The procedure for the separation of U, Th, and Pa was performed in 1 mL aliquots.

The final 15 mL were needed to remove residual Th in the Pa fraction observed in early experiments. The next 15 mL 4 M HCl A final 5 mL 0. The sample was loaded in 5 mL of 4 M HCl to the column via transfer pipette directly onto the frit at the top of the column. The eluent of the load solution was collected for Th and the remaining Th was collected in the same beaker with an additional 10 mL rinse with 4 M HCl Fig. Following the removal of residual Th, Pa was selectively stripped from the column i.

Finally, U was eluted from the column with 5 mL of 0. Problems with uranium series dating sim fractions were collected, they were set aside for source preparation and analysis. Instrumental methods employed for the studies presented here were carried out by LSC, alpha-spectrometry, and beta counting.

Problems with uranium series dating sim analyzed by alpha-spectrometry and beta counting were prepared via electrodeposition according to an approach developed previously Kressin,using a model EP-4 electrodeposition module Phoenix Scientific Sales, Roswell, GA USA.

Briefly, following separation and purification the Th, Pa, U, each analyte fraction was slowly taken to dryness. The beaker was then covered with a watch glass and allowed to reflux for 20 min.

Our experience indicated that neglecting the oxidation of organic matter step resulted in a visible cake on the Th planchettes that decreased yield and degraded spectral resolution. Once the module was assembled, a platinum electrode was inserted and the current was adjusted to 0.

The planchets were then removed from the cell and rinsed with minimal volumes of NH 4 OH, ethanol, and acetone to clean and dry the counting source. Once dry, the sources could be analyzed by alpha-spectrometry and beta counting.

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Thus, radiochemical yield determinations were obtained by standard calculations using efficiencies determined using standards prepared from NIST SRM A Th in an identical geometry.

Prevention of alpha-daughter recoil contamination of alpha-detectors was accomplished by use of thin films, prepared as described previously using a mixture of iso-amyl acetate and collodion. These thin films have been demonstrated to have no effect on the alpha emission detection efficiency or spectra resolution. Alpha-counting sources were Problems with uranium series dating sim for approximately h, with a matched Problems with uranium series dating sim background subtraction of each region of interest ROI applied to obtain background corrected integrated count rates.

Standard isotope dilution techniques were used to calculate the apparent activity of added controls Th and U-NAT radionuclides based on the ratio of control to added U and Th integrated counts Makarova et al. The count rate was compared to sources of Pa with known activity to determine the radio-chemical yield.

Preliminary evaluation was carried by validation Problems with uranium series dating sim in which control standards U-NAT and Th were used to simulate the analysis of natural samples. A nominal 5 mL volume of 4 M HCl was added and the process was repeated three times to convert the matrix to the chloride form for separations.

After conversion of the carrier to the chloride form, the tracer and analyte radionuclides were redissolved in the load solution 5—15 mL 4 M HCl for TEVA separation and purification Fig.

A complete description of the material and its preparation will be described elsewhere. For these further evaluation runs, 0. To this slurry, the radiometric tracers were added 0.

The slurry was diluted to approximately 10 mL of concentrated HCl, covered with a Teflon watch glass, and refluxed under low heat over night. The pH was kept low to minimize adsorption of Pa and Th to the beaker or particulate matter. Following centrifugation, samples were transferred back to Teflon beakers and taken dry.