Award Date

May 2018

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry and Biochemistry

First Committee Member

Ralf Sudowe

Second Committee Member

Spencer Steinberg

Third Committee Member

Daniel Koury

Fourth Committee Member

William Culbreth

Number of Pages

242

Abstract

The determination of the low-energy neutron capture cross-section of 171Tm with greater accuracy is important for both astrophysics and Stockpile Stewardship. Thulium-171 is a branching point in the s-process of stellar nucleosynthesis. Branching point nuclei, with half-lives in the order of 1 – 100 years are places where neutron capture and beta decay competes. This isotope is also the second order (n, γ) reaction product of 169Tm, which was used as a radiochemical detector in nuclear weapon experiments. It was placed at strategic locations in nuclear devices. Following the test, samples were analyzed in the collected samples for products of nuclear reactions on 169Tm. Measurement of the neutron capture cross-section of 171Tm will provide better insight into physical processes in stars as well as nuclear devices. Thulium-171 is produced through a neutron capture reaction on isotopically enriched stable 170Er followed by beta decay. After irradiation, milligram amounts of thulium will need to be separated from the remaining target material of about half a gram. A highly efficient and very selective separation based on ion exchange chromatography has been developed to separate these neighboring lanthanides in preparative quantities.

Lanthanides primarily exist in +3 oxidation state in solutions. Since their ionic radii are very similar and gradually decreasing due to the lanthanide contraction throughout the group, separation of neighboring members are especially challenging. Separation of them on the analytical scale has been widely studied since the 1950s but very few publications deal with the separation on the preparative (several hundred milligrams to gram) scale. Even fewer attempts have been made to separate one of them from its neighbor in great excess. The separation of the Er/Tm pair has been studied and optimized in the past on the analytical scale. This work focuses on studying the effect of temperature, eluent flow rate and eluent composition on the retention time and separation resolution when separating samples consisting of Tm/Er mixture.

After selecting a cation exchanger, BioRad AG 50W-X8, with enough capacity to handle the mass of the 170Er sample, separation was achieved using small amounts. The method was stepwise scaled up to separate the 0.5-gram sample in a single pass through with 99+% purity and 95+% recovery using the results of the analytical scale experiments. Verification of the separation and purity of the thulium fraction was determined using ICP-AES.

The obtained thulium fraction also contained substantial amounts of the chelating agent, which can interfere with the deposition of the thulium during the subsequent preparation of the cross section measurement target. Thus four extraction chromatographic resins (TRU, RE2, n-DGA and b-DGA) were studied using 171Tm tracer and gamma spectrometric detection in batch experiments to determine the effect of varying concentrations of the chelating agent on the retention of thulium in different concentrations of nitric acid. The results were used to develop a separation procedure to remove the chelating agent from the thulium fraction after it was separated from erbium. The procedure was tested in column setting and achieved a thulium recovery of 97.2(34)%.

The developed preparative separation method exceeded the goals of this project. This method could also be applied with adjustment of the concentration of the chelating reagent to other pairs of neighboring heavy lanthanides such as the Ho/Dy pair.

Keywords

Erbium; Extraction chromatography; Ion exchange chromatography; Preparative HPLC; Thulium; Tm-171

Disciplines

Analytical Chemistry | Chemistry | Radiochemistry

Language

English

Available for download on Saturday, May 15, 2021


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