The exploration of the unique properties of stored and cooled beams of highly-charged ions as provided by heavy-ion storage rings has opened novel and fascinating research opportunities in the realm of atomic and nuclear physics research. Since the late 1980s, pioneering work has been performed at the CRYRING at Stockholm (Abrahamsson et al. 1993) and at the Test Storage Ring (TSR) at Heidelberg (Baumann et al. 1988). For the heaviest ions in the highest charge-states, a real quantum jump was achieved in the early 1990s by the commissioning of the Experimental Storage Ring (ESR) at GSI Helmholtzzentrum fur Schwerionenforschung (GSI) in Darmstadt (Franzke 1987) where challenging experiments on the electron dynamics in the strong field regime as well as nuclear physics studies on exotic nuclei and at the borderline to atomic physics were performed. Meanwhile also at Lanzhou a heavy-ion storage ring has been taken in operation, exploiting the unique research opportunities in particular for medium-heavy ions and exotic nuclei (Xia et al. 2002).

/pdf/physics-book-cryring-esr-51aj6htcu3.pdf

Physics book: CRYRING@ESR

The recent high-precision measurements of $\ensuremath{\alpha}$-particle-induced-reaction data below the Coulomb barrier ($B$) make possible the understanding of limits and possible improvement of a previous optical model potential (OMP) for $\ensuremath{\alpha}$ particles on nuclei within the mass number range $45\ensuremath{\le}A\ensuremath{\le}209$ [M. Avrigeanu et al., Phys. Rev. C 82, 014606 (2010)]. An updated version of this potential is given in the present work concerning mainly an increased surface imaginary-potential depth well below $B$ for $Ag130$. Moreover, underestimation of reaction cross sections for well-deformed nuclei is removed by using $\ensuremath{\sim}7%$ larger radius for the surface imaginary part of this spherical OMP. Improved input parameters based on recent independent data, particularly $\ensuremath{\gamma}$-ray strength functions, but no empirical rescaling factor of the $\ensuremath{\gamma}$ and/or neutron widths have been involved within statistical model calculation of the corresponding $(\ensuremath{\alpha},x)$ reaction cross sections.

Further explorations of the α -particle optical model potential at low energies for the mass range A ≈45 -209

Beyond iron, a small fraction of the total abundances in the Solar System is made of proton-rich isotopes, the p nuclei. The clear understanding of their production is a fundamental challenge for nuclear astrophysics. The p nuclei constrain the nucleosynthesis in core-collapse and thermonuclear supernovae. The $\gamma$ process is the most established scenario for the production of the p nuclei, which are produced via different photodisintegration paths starting on heavier nuclei. A large effort from nuclear physics is needed to access the relevant nuclear reaction rates far from the valley of stability. This review describes the production of the heavy proton-rich isotopes by the $\gamma$ process in stars, and explores the state of the art of experimental nuclear physics to provide nuclear data for stellar nucleosynthesis.

The production of proton-rich isotopes beyond iron: The $\gamma$ process in stars

He-accreting white dwarfs with sub-Chandrasekhar mass are revisited. The impact of the use of an extended reaction network on the predicted energy production and characteristics of the detonating layers is studied. It is shown that the considered scenario can be the site of an $\alpha$p-process combined with a p-process and with a variant of the rp-process we refer to as the pn-process. We define the conditions under which the derived distribution of the abundances of the p-nuclides in the ejecta, including the puzzling light Mo and Ru isotopes, mimics the solar-system one.

/pdf/he-detonation-in-sub-chandrasekhar-co-white-dwarfs-a-new-2wuoijg07v.pdf

He-detonation in sub-Chandrasekhar CO white dwarfs: a new insight into energetics and p-process nucleosynthesis

The γ-process in core-collapse and/or type Ia supernova explosions is thought
to explain the origin of the majority of the so-called p nuclei (the 35 protonrich
isotopes between Se and Hg). Reaction rates for γ-process reaction network
studies have to be predicted using Hauser–Feshbach statistical model
calculations. Recent investigations have shown problems in the prediction of
α-widths at astrophysical energies which are an essential input for the statistical
model. It has an impact on the reliability of abundance predictions in the
upper mass range of the p nuclei. With the measurement of the 164,166Er(α,n)
167,169Yb reaction cross sections at energies close to the astrophysically relevant
energy range we tested the recently suggested low energy modification of
the α+nucleus optical potential in a mass region where γ-process calculations
exhibit an underproduction of the p nuclei. Using the same optical potential
for the α-width which was derived from combined 162Er(α,n) and 162Er(α,γ)
measurement makes it plausible that a low-energy modification of the optical
α+nucleus potential is needed.

Measurement of (α,n) reaction crosssections of erbium isotopes for testingastrophysical rate predictions

Model calculations within the framework of the so-called γ process show an underproduction of the p nucleus with the highest isotopic abundace 92 Mo. This discrepancy can be narrowed by taking into account the alternative production site of a type Ia supernova explosion. Here, the nucleus 92 Mo can be produced by a sequence of proton-capture reactions. The amount of 92 Mo nuclei produced via this reaction chain is most sensitive to the reactions 90 Zr(p, γ ) and 91 Nb(p, γ ). Both rates have to be investigated experimentally to study the impact of this nucleosynthesis aspect on the long-standing 92 Mo-problem. We have already measured the proton-capture reaction on 90 Zr using high-resolution in-beam γ -ray spectroscopy. In this contribution, we will present our preliminary results of the total cross sections as well as the partial cross sections. Furthermore, we plan to measure the 91 Nb(p, γ ) reaction soon. Due to the radioactive target material, the 91 Nb nuclei have to be produced prior to the experiment. The current status of this production will be presented in this contribution.

/pdf/p-process-nucleosynthesis-via-proton-capture-reactions-in-gy7hpygsqr.pdf

p-process nucleosynthesis via proton-capture reactions in thermonuclear supernovae explosions

The FRANZ facility is currently under construction at Goethe Universit?t Frankfurt a.M., Germany. It is designed to produce the world's highest neutron intensities in the astrophysically relevant energy range between 10 keV and 1 MeV and consists of a high-intensity proton linac providing energies close to the threshold of the 7Li(p,n) reaction at Ep = 1880 keV. The high intensities of both the proton and the neutron beam allow the investigation of reactions of unstable target isotopes since the needed amount of target material is significantly reduced. We will present two examplary reactions relevant for the s process and the nucleosynthesis of p nuclei, respectively.

Experiments with radioactive target samples at FRANZ

Background: Optical potentials are crucial ingredients for the prediction of nuclear reaction rates needed in simulations of the astrophysical $\ensuremath{\gamma}$ process. Associated uncertainties are particularly large for reactions involving $\ensuremath{\alpha}$ particles. This includes $(\ensuremath{\gamma},\ensuremath{\alpha})$ reactions which are of special importance in the $\ensuremath{\gamma}$ process.Purpose: The measurement of $(\ensuremath{\alpha},n)$ reactions allows for an optimization of currently used $\ensuremath{\alpha}$-nucleus potentials. The reactions $^{165}\mathrm{Ho}$$(\ensuremath{\alpha},n)$ and $^{166}\mathrm{Er}$$(\ensuremath{\alpha},n)$ probe the optical model in a mass region where $\ensuremath{\gamma}$ process calculations exhibit an underproduction of $p$ nuclei which is not yet understood.Method: To investigate the energy-dependent cross sections of the reactions $^{165}\mathrm{Ho}$$(\ensuremath{\alpha},n)$ and $^{166}\mathrm{Er}$$(\ensuremath{\alpha},n)$ close to the reaction threshold, self-supporting metallic foils were irradiated with $\ensuremath{\alpha}$ particles using the FN tandem Van de Graaff accelerator at the University of Notre Dame. The induced activity was determined afterwards by monitoring the specific $\ensuremath{\beta}$-decay channels.Results: Hauser-Feshbach predictions with a widely used global $\ensuremath{\alpha}$ potential describe the data well at energies where the cross sections are almost exclusively sensitive to the $\ensuremath{\alpha}$ widths. Increasing discrepancies appear towards the reaction threshold at lower energy.Conclusions: The tested global $\ensuremath{\alpha}$ potential is suitable at energies above 14 MeV, while a modification seems necessary close to the reaction threshold. Since the $\ensuremath{\gamma}$ and neutron widths show non-negligible impact on the predictions, complementary data are required to judge whether or not the discrepancies found can be solely assigned to the $\ensuremath{\alpha}$ width.

/pdf/experimental-cross-sections-of-ho-165-a-n-tm-168-and-er-166-32fq7ocb30.pdf

Experimental cross sections of Ho 165 ( α , n ) Tm 168 and Er 166 ( α , n ) Yb 169 for optical potential studies relevant for the astrophysical γ process

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p-process nucleosynthesis via proton-capture reactions in thermonuclear supernovae explosions

Experiments with radioactive target samples at FRANZ

Experimental cross sections of Ho 165 ( α , n ) Tm 168 and Er 166 ( α , n ) Yb 169 for optical potential studies relevant for the astrophysical γ process