A study of 8Be, 12C and 27Al with state-of-the-art detector arrays

This thesis reports on the results of four different experiments carried out
at either Argonne National Laboratory or Aarhus University.
The first experiment was a cross section measurement of the 23 Na(α, p)
reaction at astrophysical energies. This reaction acts as a proton source for
the 25 Mg(p, γ) 26 Al reaction in asymptotic giant branch stars and 26 Al is
an direct observable of ongoing nucleosynthesis. Prior to this experiment a
different group published the suprising result, that the cross section should
be scaled up by a factor of 40. However, the present experiment was
unable to reproduce this, and was instead a confirmation of the established
statistical model.
The second experiment was a measurement of the β feeding of the
Hoyle state from the 12 B ground state. This experiment was carried out at
Gammasphere in order to provide independent confirmation of a previous
result obtained using charged particle spectroscopy, which disagreed from
measurement performed in the seventies by roughly a factor of 2. The value
of β feeding is an important constraint in the β decay studies of the 2 +
excitation of the Hoyle state. The result of the present experiment was a
confirmation of the previous result.
In the third experiment the 8 Be excitation spectrum, following the γ
decay of the 17.6 MeV 1 + state, was measured using charged particles. This
technique yields unparalleled detection efficiency, resolution, and dynamic
range allowing the resulting spectrum to be analyzed using R-matrix theory.
The results of this analysis were improved γ decay widths and a tentative
broad 0 + resonance at 12 MeV.
The last experiment was a measurement of the 11 B(p, 3α) cross section
at an energy corresponding to the 16.1 MeV 2 + state in 12 C. For this state
there exist multiple inconsistent measurements of its decay properties, and
a majority of these inconsistencies can be traced to ambiguities in the nor-
malization. With this experiment the normalization issue was circumvented
by measuring the final state in full kinematics. The result was an improved
cross section for this resonance and a suggestion to rescale the recommended
cross section for 11 B(p, 3α) by a factor of 2/3 over the full energy range.
This impacts the recommended reaction rate by NACRE and NACRE II,
which should be scaled by roughly a factor of 2.
In addition to these experiments, this thesis reports on the technical
development carried out at Aarhus University. Specifically, an analysis
library for the analysis of nuclear physics experiment with silicon detectors
was developed. This library is built on the concept of an analysis pipeline. Secondly, the Aarhus data acquisition has been rebuilt from the ground
up. The different components and recent development allowing significantly
higher rates will be presented.