Date of Award

Spring 5-15-2017

Author's School

Graduate School of Arts and Sciences

Author's Department

Physics

Degree Name

Doctor of Philosophy (PhD)

Degree Type

Dissertation

Abstract

In 1987 presolar grains were first isolated from meteorites, opening up a new line of data about the stars that produced them. Based on anomalies in isotopic ratios, identification and classification of presolar grains has borne great fruit in understanding nucleosynthesis, stellar evolution, and mass loss from the stellar objects in which these grains originated: primarily, but not exclusively, supernovae and asymptotic giant branch stars.Meteoritic nanodiamonds were the first type of presolar grain identified, but more than three decades later, their origins remain unclear. Anomalies in the ratios of Xe isotopes carried by the nanodiamonds suggest the nanodiamonds formed from supernova material, but, measured in bulk, the ratios of 12C/13C and 14N/15N are consistent with formation in the solar system. Nanodiamonds are ~3 nm in diameter and contain only a few thousand atoms each, such that it is impossible to measure the isotopic ratios of single grains with traditional techniques.A multi-part experimental approach has allowed me to investigate the origins of meteoritic nanodiamonds. I use statistical studies with nanoscale secondary ion mass spectrometry of thousands of small aggregates of nanodiamonds to put upper limits on the fraction of them that can have non-solar ratios of the stable isotopes 12C and 13C and to detect isotopically anomalous statistical outliers. I also continue a collaborative work to measure the ratio of 12C/13C in individual nanodiamonds. This work adapts the experimental technique of atom-probe tomography from materials science to presolar grain research, and to that end my collaborators and I have worked extensively to mature the experimental procedures. I use focused ion beam sample preparation and correlated secondary and transmission electron microscopy to characterize samples before and after atom-probe isotopic analysis.These studies characterize the likelihood of various origins for individual and small clusters of nanodiamonds and accompanying disordered C, based on ratios of 12C/13C isotopes. The results are consistent with solar system formation for most nanodiamonds, although they do not necessarily rule out a large fraction of supernova grains with isotopic anomalies averaging close to the solar system value. The data suggest that a small subset of nanodiamonds have large isotopic enrichments in 13C relative to 12C. Supernovae are favored due to their production of the Xe isotopes, although J-star or novae could also produce this isotopic anomaly.

Language

English (en)

Chair and Committee

Ryan Ogliore & Christine Floss

Committee Members

Erik Henriksen, Dieter Isheim, Martin Israel, Rita Parai,

Comments

Permanent URL: https://doi.org/10.7936/K7BG2MF6

Download

Share

COinS