Doctoral Dissertations

Date of Award

12-2019

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Physics

Major Professor

Alfredo Galindo-Uribarri Dr.

Committee Members

Adriana Moreo Dr., Kenneth F. Read Jr Dr., Lawrence W. Townsend Dr., Soren P. Sorensen Dr.

Abstract

In this thesis, two methods for ultra-trace analysis were investigated: the negative-ion based Accelerator Mass Spectrometry (AMS) and positive-ion based Resonant Ionization Mass Spectrometry (RIMS). The detection, monitoring and quantification of ultra-low levels of actinides is of considerable importance and represents a challenge in many fields of modern science: neutrino physics, dark matter searches, nuclear astrophysics, environmental science, etc. These fields demand more sensitive and more efficient techniques and are pushing the limits of conventional techniques for ultra-trace analysis. Employing the 14 MV tandem accelerator of the Australian National University, we demonstrated an AMS method capable of detecting ultra-low traces of Th and U from ultrapure Cu made underground for the Majorana Demonstrator. We made significant advancements on the actinide detection efficiency and sensitivity with the laser-based technique RIMS. We demonstrated that RIMS is a powerful analytical technique for actinides since it does not require large amount of material, has high selectivity, high efficiency and high sensitivity. Several three-step ionization schemes were studied and numerous high-lying and autoionizing levels were observed for Pu, Th, and U. We obtained efficiencies up to 51% for Pu, 32% for Th and 9% for U. These efficiencies represent the largest efficiencies reported for actinides and surpass significantly the efficiencies obtained in AMS for actinides which are between 0.1% to 1%. We performed sensitivity studies with a setup consisting of a Resonant Ionization Laser Ion Source (RILIS), a mass spectrometer based on the Holifield Radioactive Ion Beam Facility radioactive injector IRIS2 and a single ion counter. We obtained a detection limit of 103 atoms for 240Pu which is 1-2 orders of magnitude below previously reported. Atomic spectroscopic studies for Pu were done revealing four Rydberg series in the energetically high-lying region in the atomic spectrum. The first ionization potential (IP) of Pu was precisely determined as 48605.748 (95)stat(49)sys cm−1 using two methods. This value is a factor of three more precise than the one reported in the NIST database and constitutes a benchmark for modern relativistic atomic theories.

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