Date of Completion

10-21-2013

Embargo Period

4-2-2014

Major Advisor

Richard T. Jones

Associate Advisor

Henry R. Weller

Associate Advisor

Jeffrey S. Schweitzer

Field of Study

Physics

Degree

Doctor of Philosophy

Open Access

Open Access

Abstract

A low-lying Jπ = 2+ state in 12C was predicted over fifty years ago to exist as an excitation of the Hoyle state. The exact energy, width, and electromagnetic transition probability of this state is related to the structure of the Hoyle state, and measuring these parameters will help to constrain the various models which attempt to describe the clustering phenomenon in light nuclei. The second 2+ state in was directly observed in the 12C(γ,α)8Be reaction using the intense, nearly mono-energetic γ-ray beams available at the High Intensity γ-ray Source (HIγS) facility. The α particles produced by the photodisintegration of 12C were detected using an optical time projection chamber (OTPC), which allowed for the complete angular distributions necessary to definitively confirm the 2+ nature of the state. This unique combination of a Compton-backscattered γ-ray beam and and an active-target system made possible the first unambiguous identification of this 2+ state. The second 2+ state in 12C was found at an excitation energy of Ex = 10.13(6) MeV, with a width of Γ = 2080(300) keV, and with a γ-decay width to the ground state of Γγ = 135(14) meV. The previously-measured first 1− state in 12C was found at an excitation energy of Ex = 10.913(19) MeV, with a width of Γ = 305(38) keV, and an isospin-forbidden γ-decay width to the ground state of Γγ = 32.4(21) meV. These results are compared with predictions made by the various models that describe the structure of the excited states in 12C. This 2+ state increases astrophysical helium-burning reaction rates at the high temperatures which are thought to occur during core-collapse supernovae. New triple-α reaction rates have been calculated based on these results, and their possible impact on explosive nucleosynthesis is discussed.

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