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Register a new userNeutron skins of atomic nuclei: per aspera ad astra
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Concettina Sfienti Prof. Dr.
Publication date
2019
fields
Physics
and research's language is
English
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The complex nature of the nuclear forces generates a broad range and diversity of observational phenomena. Heavy nuclei, though orders of magnitude less massive than neutron stars, are governed by the same underlying physics, which is enshrined in the nuclear equation of state. Heavy nuclei are expected to develop a neutron-rich skin where many neutrons collect near the surface. Such a skin thickness is strongly sensitive to the poorly-known density dependence of the symmetry energy near saturation density. An accurate and model-independent determination of the neutron-skin thickness of heavy nuclei would provide a significant first constraint on the density dependence of the nuclear symmetry energy. The determination of the neutron-skin thickness of heavy nuclei has far reaching consequences in many areas of physics as diverse as heavy-ion collisions, polarized electron and proton scattering off nuclei, precision tests of the standard model using atomic parity violation, and nuclear astrophysics. While a systematic and concerted experimental effort has been made to measure the neutron-skin thickness of heavy nuclei, a precise and model-independent determination remains elusive. How to move forward at a time when many new facilities are being commissioned and how to strengthen the synergy with other areas of physics are primary goals of this review.
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We report mass measurements of neutron-rich Ga isotopes $^{80-85}$Ga with TRIUMFs Ion Trap for Atomic and Nuclear science (TITAN). The measurements determine the masses of $^{80-83}$Ga in good agreement with previous measurements. The masses of $^{84}$Ga and $^{85}$Ga were measured for the first time. Uncertainties between $25-48$ keV were reached. The new mass values reduce the nuclear uncertainties associated with the production of A $approx$ 84 isotopes by the emph{r}-process for astrophysical conditions that might be consistent with a binary neutron star (BNS) merger producing a blue kilonova. Our nucleosynthesis simulations confirm that BNS merger may contribute to the first abundance peak under moderate neutron-rich conditions with electron fractions $Y_e=0.35-0.38$.
The neutron skin of nuclei is an important fundamental property, but its accurate measurement faces many challenges. Inspired by charge symmetry of nuclear forces, the neutron skin of a neutron-rich nucleus is related to the difference between the charge radii of the corresponding mirror nuclei. We investigate this relation within the framework of the Hartree-Fock-Bogoliubov method with Skyrme interactions. Predictions for proton skins are also made for several mirror pairs in the middle mass range. For the first time the correlation between the thickness of the neutron skin and the characteristics related with the density dependence of the nuclear symmetry energy is investigated simultaneously for nuclei and their corresponding mirror partners. As an example, the Ni isotopic chain with mass number $A=48-60$ is considered. These quantities are calculated within the coherent density fluctuation model using Brueckner and Skyrme energy-density functionals for isospin asymmetric nuclear matter with two Skyrme-type effective interactions, SkM* and SLy4. Results are also presented for the symmetry energy as a function of $A$ for a family of mirror pairs from selected chains of nuclei with $Z=20$, $N=14$, and $N=50$. The evolution curves show a similar behavior crossing at the $N=Z$ nucleus in each chain and a smooth growing deviation when $N eq Z$ starts. Comparison of our results for the radii and skins with those from the calculations based on high-precision chiral forces is made.
We present an inference of the nuclear symmetry energy magnitude $J$, the slope $L$ and the curvature $K_{rm sym}$ by combining neutron skin data on Ca, Pb and Sn isotopes and our best theoretical information about pure neutron matter (PNM). A Bayesian framework is used to consistently incorporate prior knowledge of the PNM equation of state from chiral effective field theory calculations. Neutron skins are modeled in a Hartree-Fock approach using an extended Skyrme energy-density functional which allows for independent variation of $J$, $L$ and $K_{rm sym}$ without affecting the symmetric nuclear matter equation of state. We discuss the choice of neutron skin data sets, and combining errors in quadrature we obtain 95% credible values of $J=31.3substack{+4.2 -5.9}$ MeV, $L=40substack{+34 -26}$ MeV and $K_{tau} = L - 6K_{rm sym}= -444substack{+100 -84}$ MeV using uninformative priors in $J$, $L$ and $K_{rm sym}$, and $J=31.9substack{+1.3 -1.3}$ MeV, $L=37substack{+9 -8}$ MeV and $K_{tau} = -480substack{+25 -26}$ MeV using PNM priors. The correlations between symmetry energy parameters induced by neutron skin data is discussed and compared with the droplet model. Neutron skin data alone is shown to place limits on the symmetry energy parameters as stringent as those obtained from chiral effective field theory alone, and when combined the 95% credible intervals are reduced by a factor of 4-5. Ahead of new measurements of lead and calcium neutron skins from parity-violating electron scattering experiments at Jefferson Lab and Mainz Superconducting Accelerator, we make predictions based on existing data on neutron skins of tin for the neutron skins of calcium and lead of 0.166$pm$0.008 fm and $0.169 pm 0.014$ fm respectively, using uninformative priors, and 0.167$pm$0.008 fm and $0.172 pm 0.015$ fm respectively, using PNM priors.
Both the incompressibility Ka of a finite nucleus of mass A and that ($K_{infty}$) of infinite nuclear matter are fundamentally important for many critical issues in nuclear physics and astrophysics. While some consensus has been reached about the $K_{infty}$, accurate theoretical predictions and experimental extractions of $K_{tau}$ characterizing the isospin dependence of Ka have been very difficult. We propose a differential approach to extract the Kt and Ki independently from the Ka data of any two nuclei in a given isotope chain. Applying this new method to the Ka data from isoscalar giant monopole resonances (ISGMR) in even-even Pb, Sn, Cd and Ca isotopes taken by U. Garg {it et al.} at the Research Center for Nuclear Physics (RCNP), Osaka University, Japan, we find that the $^{106}$Cd-$^{116}$Cd and $^{112}$Sn-$^{124}$Sn pairs having the largest differences in isospin asymmetries in their respective isotope chains measured so far provide consistently the most accurate up-to-date Kt value of $K_{tau}=-616pm 59$ MeV and $K_{tau}=-623pm 86$ MeV, respectively, largely independent of the remaining uncertainties of the surface and Coulomb terms in expanding the $K_{rm A}$, while the $K_{infty}$ values extracted from different isotopes chains are all well within the current uncertainty range of the community consensus for $K_{infty}$. Moreover, the size and origin of the Soft Sn Puzzle is studied with respect to the Stiff Pb Phenomenon. It is found that the latter is favored due to a much larger (by $sim 380$ MeV) Kt for Pb isotopes than for Sn isotopes, while the Ki from analyzing the Ka data of Sn isotopes is only about 5 MeV less than that from analyzing the Pb data.
Background: The high momentum distribution of atoms in two spin-state ultra-cold atomic gases with strong short-range interactions between atoms with different spins, which can be described using Tans contact, are dominated by short range pairs of different fermions and decreases as $k^{-4}$. In atomic nuclei the momentum distribution of nucleons above the Fermi momentum ($k>k_F approx 250$ Mev/c) is also dominated by short rangecorrelated different-fermion (neutron-proton) pairs. Purpose: Compare high-momentum unlike-fermion momentum distributions in atomic and nuclear systems. Methods: We show that, for $k>k_F$ MeV/c, nuclear momentum distributions are proportional to that of the deuteron. We then examine the deuteron momentum distributions derived from a wide variety of modern nucleon-nucleon potentials that are consistent with $NN$-scattering data. Results: The high momentum tail of the deuteron momentum distribution, and hence of the nuclear momentum distributions appears to decrease as $k^{-4}$. This behavior is shown to arise from the effects of the tensor part of the nucleon-nucleon potential. In addition, when the dimensionless interaction strength for the atomic system is chosen to be similar to that of atomic nuclei, the probability for finding a short range different-fermion pair in both systems is the same. Conclusions: Although nuclei do not satisfy all of the conditions for Tans contact, the observed similarity of the magnitude and $k^{-4}$ shape of nuclear and atomic momentum distributions is remarkable because these systems differ by about $20$ orders of magnitude in density. This similarity may lead to a greater understanding of nuclei and the density dependence of nuclear systems.
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