A precision measurement of the 7Li(d, p)8Li reaction cross section

Date of Completion

January 2001

Keywords

Physics, Astronomy and Astrophysics|Physics, Nuclear

Degree

Ph.D.

Abstract

We have measured the 7Li(d, p)8Li reaction cross section using a 7Li beam and a deuterium target. Our methodolgy has two unique strengths. First, we measure the ratio of the cross section of the 7Li(d, p)8Li reaction to the well-known elastic scattering cross section of deuterium off 7Li, which eliminates many sources of systematic uncertainty associated with the beam-target luminosity. This type of systematic uncertainty dominated in previous measurements. Second, we require α-α coincidence in a closed geometry yielding clean spectra through the exclusion of background counts and resulting in small systematic uncertainty due to detector efficiency. We have measured the value of the 7Li(d, p)8Li reaction cross section at the center of mass energies 667, 778, 889 and 1000 keV to be 130, 103, 127 and 71 mb, respectively, with a systematic uncertainty of ±5%. The values obtained are consistent with previous measurements by other groups over the energy ranged covered. ^ The value of the 7Li(d, p)8Li reaction cross section has important application to the study of solar neutrinos, which provide a unique way to observe nuclear reactions in the solar core and understand solar evolution. There exists, however, a “solar neutrino problem:” a deficit of observed solar neutrinos unexplained by the current standard solar model and the standard model of particle physics. The least certain input to the standard solar model is S17(0), the astrophysical cross section factor associated with the 7Be(p, γ) 8B reaction cross section; existing measurements of S 17(0) vary by as much as a factor of two and have large systematic uncertainties. Most of the measurements of S17(0) rely on knowledge of the cross section of the 7Li(d, p)8Li reaction for absolute normalization, but this cross section in itself is not known with high precision and accuracy. Thus an accurate measurement of the 7Li(d, ph8Li reaction cross section is critical for normalizing the measurements of the 7Be(p, γ) 8B reaction cross section (and thus S17(0)) and for understanding the solar neutrino problem and therefore the nature of the neutrino. In our research program, the measurement also serves as a precursor experiment with stable 7Li beams before measuring S17(0)with radioactive 7Be beams. We plan to use the current setup with a 7Be radioactive beam and hydrogen target at the Université Catholique de Louvain in Louvain-La-Neuve, Belgium, to directly measure the 7Be(p, γ)8B reaction cross section. ^

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