Speaker
Jeff C Blackmon
(for the RIBENS Collaboration, Oak Ridge National Laboratory)
Description
Radioactive nuclei play an important role in many astrophysical phenomena,
particularly in stellar explosions where the rates of nuclear reactions can be much
faster than the lifetimes of most radioactive isotopes. Accelerated beams of
radioactive ions are being used to address uncertainties in some key reaction rates.
We will review recent progress in the field, focusing on results from the Holifield
Radioactive Ion Beam Facility (HRIBF) at Oak Ridge National Laboratory.
The production of gamma-rays from electron-positron annihilation in novae is
particularly sensitive to the rate of the 18F(p,alpha)15O reaction. Uncertainties
have remained in the 18F(p,alpha)15O reaction rate due to the uncertain contributions
of low-energy resonances and the nature of interferences between resonances. We
will report on recent measurements of the 18F(p,alpha)15O cross section at the HRIBF
using an experimental approach similar to previous measurements. [1] The new data
provide some of the first constraints on the nature of interferences between
resonances in the 18F(p,alpha)15O reaction.
We will present a novel experimental technique that we have developed to allow
improved sensitivity for studies of low energy (p,alpha) resonances. In this new
approach a heavy ion beam bombards a large, windowless chamber filled with hydrogen
gas. Alpha particles and recoiling heavy ions are detected in coincidence in arrays
of silicon strip detectors operating inside the hydrogen gas. While each element of
the detector array simultaneously views reaction products over a wide range of
angles, the relative kinematics of the two reaction products allows the vertex of the
reaction to the accurately determined on an event-by-event basis. We will present
results from a measurement applying this technique to the 183-keV resonance in the
17O(p,alpha)14N reaction that was first reported using a more conventional approach
last year. [2] The strength of this resonance is also crucial for understanding the
production of 18F in novae as well as the Galactic origins of the rare 17O isotope.
This approach will next be applied to low energy resonances in 18F(p,alpha)15O.
Results from recent measurements using 7Be beams at the HRIBF will also be presented.
Accurate measurements of the neutrino flux originating from the decay of 8B in the
solar core provide a powerful probe of the properties of the solar interior and
neutrinos themselves. [3] While recent measurements have substantially improved our
understanding of the 7Be(p,gamma)8B reaction rate that impacts the interpretation of
solar neutrino observations [4], the experimental situation is less than completely
resolved. We are performing direct measurements of the 7Be(p,gamma)8B cross section
in inverse kinematics, using a radioactive 7Be beam on a windowless hydrogen gas
target. This alternative approach at direct measurement of the 7Be(p,gamma)8B cross
section is interesting since it provides an independent check on potential systematic
uncertainties. We will present results from the first measurements at the HRIBF that
have demonstrated the advantages and limitations of the approach, though not yet
competitive statistically with 7Be target experiments. We will also present
measurements of 7Be+p elastic and inelastic scattering cross sections at
center-of-mass energies ranging between 0.5 to 3.4 MeV that provide new information
on the properties of excited states in 8B and 7Be+p s-wave phase shifts, which can
influence extrapolations of the 7Be(p,gamma)8B cross section to solar energies in
some models. [5]
*for the RIBENS Collaboration. Oak Ridge National Laboratory is managed by
UT-Battelle, LLC, for the U.S. Department of Energy under Contract No. DE-AC05-00OR22725.
[1] D.W. Bardayan et al., Phys. Rev. Lett. 89, 262501 (2002).
[2] A. Chafa et al., Phys. Rev. Lett. 95, 031101 (2005).
[3] S.N. Ahmed et al., Phys. Rev. Lett. 92, 181301 (2004).
[4] A.R. Junghans et al., Phys. Rev. C 65, 065803 (2003), and references therein.
[5] P. Descouvemont, Phys. Rev. C 70, 065802 (2004).
Author
Jeff C Blackmon
(for the RIBENS Collaboration, Oak Ridge National Laboratory)
