Measurement of the

The cross section of the ^{13}C(α,n)^{16}O reaction is needed for nuclear astrophysics and applications to a precision of 10% or better, yet inconsistencies among 50 years of experimental studies currently lead to an uncertainty of ≈15%. Using a state-of-the-art neutron detection array, we have performed a high resolution differential cross section study covering a broad energy range. These measurements result in a dramatic improvement in the extrapolation of the cross section to stellar energies potentially reducing the uncertainty to ≈5% and resolving long standing discrepancies in higher energy data.

Direct Measurement of the Astrophysical

Fluorine is one of the most interesting elements in nuclear astrophysics, where the ^{19}F(p,α)^{16}O reaction is of crucial importance for Galactic ^{19}F abundances and CNO cycle loss in first generation Population III stars. As a day-one campaign at the Jinping Underground Nuclear Astrophysics experimental facility, we report direct measurements of the essential ^{19}F(p,αγ)^{16}O reaction channel. The γ-ray yields were measured over E_{c.m.}=72.4-344 keV, covering the Gamow window; our energy of 72.4 keV is unprecedentedly low, reported here for the first time. The experiment was performed under the extremely low cosmic-ray-induced background environment of the China JinPing Underground Laboratory, one of the deepest underground laboratories in the world. The present low-energy S factors deviate significantly from previous theoretical predictions, and the uncertainties are significantly reduced. The thermonuclear ^{19}F(p,αγ)^{16}O reaction rate has been determined directly at the relevant astrophysical energies.

New

Precise antineutrino measurements are very sensitive to proper background characterization. We present an improved measurement of the ^{13}C(α,n)^{16}O reaction cross section which constitutes significant background for large ν[over ¯] detectors. We greatly improve the precision and accuracy by utilizing a setup that is sensitive to the neutron energies while making measurements of the excited state transitions via secondary γ-ray detection. Our results shows a 54% reduction in the background contributions from the ^{16}O(3^{-},6.13 MeV) state used in the KamLAND analysis.

Constraining the External Capture to the

The ^{12}C(α,γ)^{16}O reaction is one of the most crucial reactions in nuclear astrophysics. The E2 external capture to the ^{16}O ground state (GS) has not been emphasized in previous analyses but may make a significant contribution to the ^{12}C(α,γ)^{16}O cross section depending on the value of the GS asymptotic normalization coefficient (ANC). In the present work, we determine this ANC to be 337±45 fm^{-1/2} through the ^{12}C(^{11}B,^{7}Li)^{16}O reaction using a high-precision magnetic spectrograph. This sheds light on the existing large discrepancy of more than 2 orders of magnitude between the previously reported ANC values. Based on the new ANC, we experimentally constrain the GS external capture and show that through interference with the high energy tail of the 2^{+} subthreshold state, a substantial enhancement in the GS S_{E2}(300) factor can be obtained (70±7 keV b) compared to that of a recent review (45 keV b), resulting in an increase of the total S factor from 140 to 162 keV b, which is now in good agreement with the value obtained by reproducing supernova nucleosynthesis calculations with the solar-system abundances. This work emphasizes that the external capture contribution for the ground state transition cannot be neglected in future analyses of the ^{12}C(α,γ)^{16}O reaction.

New Measurement of

Carbon and oxygen burning reactions, in particular, ^{12}C+^{12}C fusion, are important for the understanding and interpretation of the late phases of stellar evolution as well as the ignition and nucleosynthesis in cataclysmic binary systems such as type Ia supernovae and x-ray superbursts. A new measurement of this reaction has been performed at the University of Notre Dame using particle-γ coincidence techniques with SAND (a silicon detector array) at the high-intensity 5U Pelletron accelerator. New results for ^{12}C+^{12}C fusion at low energies relevant to nuclear astrophysics are reported. They show strong disagreement with a recent measurement using the indirect Trojan Horse method. The impact on the carbon burning process under astrophysical scenarios will be discussed.

First Direct Measurement of (12)C((12)C,n)(23)Mg at Stellar Energies.

Neutrons produced by the carbon fusion reaction (12)C((12)C,n)(23)Mg play an important role in stellar nucleosynthesis. However, past studies have shown large discrepancies between experimental data and theory, leading to an uncertain cross section extrapolation at astrophysical energies. We present the first direct measurement that extends deep into the astrophysical energy range along with a new and improved extrapolation technique based on experimental data from the mirror reaction (12)C((12)C,p)(23)Na. The new reaction rate has been determined with a well-defined uncertainty that exceeds the precision required by astrophysics models. Using our constrained rate, we find that (12)C((12)C,n)(23)Mg is crucial to the production of Na and Al in pop-III pair instability supernovae. It also plays a nonnegligible role in the production of weak s-process elements, as well as in the production of the important galactic γ-ray emitter (60)Fe.