147 research outputs found
Nuclear structure beyond the neutron drip line: the lowest energy states in He via their T=5/2 isobaric analogs in Li
The level structure of the very neutron rich and unbound He nucleus has
been the subject of significant experimental and theoretical study. Many recent
works have claimed that the two lowest energy He states exist with spins
and and widths on the order of hundreds of keV.
These findings cannot be reconciled with our contemporary understanding of
nuclear structure. The present work is the first high-resolution study with low
statistical uncertainty of the relevant excitation energy range in the
He system, performed via a search for the T=5/2 isobaric analog states
in Li populated through He+p elastic scattering. The present data show
no indication of any narrow structures. Instead, we find evidence for a broad
state in He located approximately 3 MeV above the neutron
decay threshold
Study of the Mg waiting point relevant for x-ray burst nucleosynthesis via the Mg(,)Al reaction
The Mg(,)Al reaction rate has been identified as a
major source of uncertainty for understanding the nucleosynthesis flow in
Type-I x-ray bursts (XRBs). We report a direct measurement of the energy- and
angle-integrated cross sections of this reaction in a 3.3-6.9 MeV
center-of-mass energy range using the MUlti-Sampling Ionization Chamber
(MUSIC). The new Mg(,)Al reaction rate is a factor of
4 higher than the previous direct measurement of this reaction within
temperatures relevant for XRBs, resulting in the Mg waiting point of
x-ray burst nucleosynthesis flow to be significantly bypassed via the
() reactionComment: 6 pages, 3 figures, 1 tabl
Decay properties of resonances and their impact on -process nucleosynthesis
The astrophysical -process is one of the two main processes forming
elements heavier than iron. A key outstanding uncertainty surrounding
-process nucleosynthesis is the neutron flux generated by the
reaction during the He-core
and C-shell burning phases of massive stars. This reaction, as well as the
competing reaction, is
not well constrained in the important temperature regime from --~GK, owing to uncertainties in the nuclear properties of resonances
lying within the Gamow window. To address these uncertainties, we have
performed a new measurement of the reaction in inverse kinematics, detecting the outgoing
deuterons and recoils in coincidence. We have
established a new decay branching ratio of for the key
MeV resonance in , which results in a new
strength for this resonance of eV when combined with
the well-established strength of this resonance. We have
also determined new upper limits on the partial widths of
neutron-unbound resonances at , , and
MeV. Monte-Carlo calculations of the stellar and rates, which incorporate these results, indicate
that both rates are substantially lower than previously thought in the
temperature range from --~GK.Comment: 17 pages, 4 figures, accepted for publication in Phys. Lett.
Confirmation of a new resonance in 26Si and contribution of classical novae to the galactic abundance of 26Al
© 2023 The Author(s). Published by the American Physical Society. This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY), https://creativecommons.org/licenses/by/4.0/The 25Al(p ,γ ) reaction has long been highlighted as a possible means to bypass the production of 26Al cosmic γ rays in classical nova explosions. However, uncertainties in the properties of key resonant states in 26Si have hindered our ability to accurately model the influence of this reaction in such environments. We report on a detailed γ -ray spectroscopy study of 26Si and present evidence for the existence of a new, likely ℓ =1 , resonance in the 25Al + p system at Er=153.9 (15 ) keV. This state is now expected to provide the dominant contribution to the 25Al(p ,γ ) stellar reaction rate over the temperature range, T ≈0.1 −0.2 GK. Despite a significant increase in the rate at low temperatures, we find that the final ejected abundance of 26Al from classical novae remains largely unaffected even if the reaction rate is artificially increased by a factor of 10. Based on new, galactic chemical evolution calculations, we estimate that the maximum contribution of novae to the observed galactic abundance of 26Al is ≈0.2 M⊙ . Finally, we briefly highlight the important role that super-asymptotic giant branch stars may play in the production of 26Al.Peer reviewe
Optical potentials for the rare-isotope beam era
We review recent progress and motivate the need for further developments in
nuclear optical potentials that are widely used in the theoretical analysis of
nucleon elastic scattering and reaction cross sections. In regions of the
nuclear chart away from stability, which represent a frontier in nuclear
science over the coming decade and which will be probed at new rare-isotope
beam facilities worldwide, there is a targeted need to quantify and reduce
theoretical reaction model uncertainties, especially with respect to nuclear
optical potentials. We first describe the primary physics motivations for an
improved description of nuclear reactions involving short-lived isotopes,
focusing on its benefits for fundamental science discoveries and applications
to medicine, energy, and security. We then outline the various methods in use
today to build optical potentials starting from phenomenological, microscopic,
and ab initio methods, highlighting in particular the strengths and weaknesses
of each approach. We then discuss publicly-available tools and resources
facilitating the propagation of recent progresses in the field to
practitioners. Finally, we provide a set of open challenges and recommendations
for the field to advance the fundamental science goals of nuclear reaction
studies in the rare-isotope beam era.Comment: This paper is the outcome of the Facility for Rare Isotope Beams
Theory Alliance (FRIB - TA) topical program "Optical Potentials in Nuclear
Physics" held in March 2022 at FRIB. Its content is non-exhaustive, was
chosen by the participants and reflects their efforts related to optical
potential
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