Valence Neutrons Research Articles

16O + 16O + valence neutrons in molecular orbitals structures of positive- and negative-parity superdeformed bands in 34S

The structures of superdeformed (SD) states in 34S are investigated using the antisymmetrized molecular dynamics and generator coordinate method (GCM). The GCM basis wave functions are calculated via energy variation with a constraint on the quadrupole deformation parameter β. By applying the GCM after parity and angular momentum projections, the coexistence of two positive- and one negative-parity SD bands are predicted, and low-lying states and other deformed bands are obtained. The SD bands have structures of 16O + 16O + two valence neutrons in molecular orbitals around the two 16O cores in a cluster picture. The configurations of the two valence neutrons are δ2 and π2 for the positive-parity SD bands and π1δ1 for the negative-parity SD band.

Study of cluster structure in 13C with AMD+HON-constraint method

The 3α + n cluster states of 13C are discussed on the basis of antisymmetrized molecular dynamics with the constraint on the harmonic oscillator quanta. We predict two different kinds of the cluster states, the hoyle analogue state and the linear-chain state. The former is understood as the 0+2 state (Hoyle state) of 12C accompanied by a valence neutron occupying the s-wave. The latter constitute the parity doublet bands of Kπ = 1/2± owing to its parity asymmetric intrinsic structure.

Coupled-channel picture of the core nucleus for the expansion of the nuclear size in the drip-line region

We study the energy and radius of neutron rich nuclei toward to the drip-line region. We employ the cluster-orbital shell model approach to calculate the radius of the oxygen isotopes. The result shows that the calculated radius underestimates the observed one at 23O, though the binding energy and the drip-line of the oxygen isotopes are reproduced. Therefore, we propose a coupled-channel approach to explain the sudden increase of the radius of the oxygen isotopes at 23O. As a result, the sudden change of the radius can be descried in terms of the competition between the available valence neutron orbits and occupied orbits in the core nucleus. For the 23O case, we show that the excitation to the 1s1/2-orbit in the core nucleus is a key configuration to reproduce the radius.

Level densities and thermodynamical properties of Pt and Au isotopes

The nuclear level densities of $^{194-196}$Pt and $^{197,198}$Au below the neutron separation energy have been measured using transfer and scattering reactions. All the level density distributions follow the constant-temperature description. Each group of isotopes is characterized by the same temperature above the energy threshold corresponding to the breaking of the first Cooper pair. A constant entropy excess $\Delta S=1.9$ and $1.1$ $k_B$ is observed in $^{195}$Pt and $^{198}$Au with respect to $^{196}$Pt and $^{197}$Au, respectively, giving information on the available single-particle level space for the last unpaired valence neutron. The breaking of nucleon Cooper pairs is revealed by sequential peaks in the microcanonical caloric curve.

O16+O16molecular structures of positive- and negative-parity superdeformed bands inS34

The structures of excited states in $^{34}\mathrm{S}$ are investigated using the antisymmetrized molecular dynamics and generator coordinate method (GCM). The GCM basis wave functions are calculated via energy variation with a constraint on the quadrupole deformation parameter $\ensuremath{\beta}$. By applying the GCM after parity and angular momentum projections, the coexistence of two positive- and one negative-parity superdeformed (SD) bands are predicted, and low-lying states and other deformed bands are obtained. The SD bands have structures of $^{16}\mathrm{O}+^{16}\mathrm{O}$ + two valence neutrons in molecular orbitals around the two $^{16}\mathrm{O}$ cores in a cluster picture. The configurations of the two valence neutrons are ${\ensuremath{\delta}}^{2}$ and ${\ensuremath{\pi}}^{2}$ for the positive-parity SD bands and ${\ensuremath{\pi}}^{1}{\ensuremath{\delta}}^{1}$ for the negative-parity SD band. The structural changes of the yrast states are also discussed.

He6nucleus in halo effective field theory

Background: In recent years properties of light rare isotopes have been measured with high accuracy. At the same time, the theoretical description of light nuclei has made enormous progress, and properties of, e.g., the Helium isotopes can now be calculated {\it ab initio}. These advances make those rare isotopes an ideal testing ground for effective field theories (EFTs) built upon cluster degrees of freedom. Purpose: Systems with widely separated intrinsic scales are well suited to an EFT treatment. The Borromean halo nucleus $^6$He exhibits such a separation of scales. In this work an EFT in which the degrees of freedom are the valence neutrons ($n$) and an inert $^4$He-core ($\alpha$) is employed. The properties of ${}^6$He can then be calculated using the momentum-space Faddeev equations for the $\alpha nn$ bound state to obtain information on ${}^6$He at leading order (LO) within the EFT. Results: The $nn$ virtual state and the $^2$P$_{3/2}$ resonance in $^5$He give the two-body amplitudes which are input to our LO three-body Halo EFT calculation. We find that without a genuine three-body interaction the two-neutron separation energy $S_{2n}$ of ${}^6$He is strongly cutoff dependent. We introduce a $nn \alpha$ "three-body" operator which renormalizes the system, adjusting its coefficient to reproduce the $S_{2n}$ of $^6$He. The Faddeev components are then cutoff independent for cutoffs of the order of, and above, the breakdown scale of the Halo EFT. Conclusions: As in the case of a three-body system where only resonant s-wave interactions are present, one three-body input is required for the renormalization of the EFT equations that describe $^6$He at LO. However, in contrast to the s-wave-only case, the running of the LO $nn\alpha$ counterterm does not exhibit discrete scale invariance, due to the presence of the p-wave $n\alpha$ interaction.

Quadrupole transition strength in the (74)Ni nucleus and core polarization effects in the neutron-rich Ni isotopes.

The reduced transition probability B(E2;0(+)→2(+)) has been measured for the neutron-rich nucleus (74)Ni in an intermediate energy Coulomb excitation experiment performed at the National Superconducting Cyclotron Laboratory at Michigan State University. The obtained B(E2;0(+)→2(+))=642(-226)(+216) e(2) fm(4) value defines a trend which is unexpectedly small if referred to (70)Ni and to a previous indirect determination of the transition strength in (74)Ni. This indicates a reduced polarization of the Z=28 core by the valence neutrons. Calculations in the pfgd model space reproduce well the experimental result indicating that the B(E2) strength predominantly corresponds to neutron excitations. The ratio of the neutron and proton multipole matrix elements supports such an interpretation.

Ab InitioCoupled-Cluster Effective Interactions for the Shell Model: Application to Neutron-Rich Oxygen and Carbon Isotopes

We derive and compute effective valence-space shell-model interactions from ab initio coupled-cluster theory and apply them to open-shell and neutron-rich oxygen and carbon isotopes. Our shell-model interactions are based on nucleon-nucleon and three-nucleon forces from chiral effective-field theory. We compute the energies of ground and low-lying states, and find good agreement with experiment. In particular, our computed 2(+) states are consistent with N = 14,16 shell closures in (22,24)O, and a weaker N=14 shell closure in (20)C. We find good agreement between our coupled-cluster effective-interaction results with those obtained from standard single-reference coupled-cluster calculations for up to eight valence neutrons.

Erratum: Investigation of the3/22−state ofLi9with microscopic structure and reaction models [Phys. Rev. C87, 064320 (2013)

The low-lying states of the $^{9}$Li nucleus are investigated with a unified framework of microscopic structure and reaction models. In the structure model, the wave function is fully antisymmetrized and the $^{9}$Li nucleus is described as an $\alpha$ + $t$ + $n$ + $n$ four-body system, and low-lying 1/2$^{-}$, 3/2$^{-}$, 5/2$^{-}$, and 7/2$^{-}$ states are obtained by the stochastic multi-configuration mixing method. Using these wave functions, the quasi-elastic cross section at $E/A$ = 60 MeV and the elastic and inelastic cross sections at $E/A$ = 50 MeV on the $^{12}$C target are calculated in the framework of the microscopic coupled channel (MCC) method. The characteristic inelastic angular distribution is seen in the 3/2$_{2}^{-}$ state, whose $\alpha+t$ cluster structure and valence neutron configurations are discussed in detail. We find the possibility of triaxial deformation and mixing of di-neutron components in the $^{9}$Li nucleus.

Three-body model calculation of the2+state inO26

We discuss the energy of the excited $2{}^{+}$ state in the unbound nucleus $^{26}\mathrm{O}$ using the three-body model of ${}^{24}\mathrm{O}+n+n$. This model fully takes into account the continuum effects, as well as the dineutron correlation of the valence neutrons. The present calculation yields the energy of 1.354 MeV, which is slightly smaller than the unperturbed energy, 1.54 MeV. The energy shifts for the ground and the $2{}^{+}$ states with respect to the unperturbed energy suggest that $0{}^{+}$ and the $2{}^{+}$ states in $^{26}\mathrm{O}$ show a typical spectrum well described by a short-range pairing interaction acting on two valence nucleons in the same orbit.

Nuclear Halos and Efimov Effect: A Three-Body Approach

We present a brief review of our search for Efimov states and their evolutions in two-neutron halo nuclei 32Ne and 38Mg. We have tried to generalise the results by extending the calculations to a hypothetical case of a very heavy nucleus of mass 102 with a core of mass 100 and two valence neutrons.

Correlation between ground state lifetime and valence nucleons for isotopic chains

The variation of β-decay lifetime of the ground state of various isotope chains display a systematic behavior with respect to the ratio of the number of valence neutron particles or holes and the maximum possible number of neutron particles or holes in that shell. Lifetimes, depending on the particle or hole nature of the protons and neutrons, as well as whether they belong to the same shell or not, are found to exhibit similar increasing or decreasing trends and display similar behavior with respect to variation of the above-mentioned ratio.

Influence of the single-particle structure on the nuclear surface and the neutron skin

We analyze the influence of the single-particle structure on the neutron density distribution and the neutron skin in Ca, Ni, Zr, Sn, and Pb isotopes. The nucleon density distributions are calculated in the Hartree-Fock+BCS approach with the SLy4 Skyrme force. A close correlation is found between the quantum numbers of the valence neutrons and the changes in the position and the diffuseness of the nuclear surface, which in turn affect the neutron skin thickness. Neutrons in the valence orbitals with low principal quantum number and high angular momentum mainly displace the position of the neutron surface outwards, while neutrons with high principal quantum number and low angular momentum basically increase the diffuseness of the neutron surface. The impact of the valence shell neutrons on the tail of the neutron density distribution is discussed.

Precise Lifetime Measurements in Light Nuclei for Benchmarking Modern Ab-initio Nuclear Structure Models

A new generation of ab-initio calculations, based on realistic two- and three-body forces, is having a profound impact on our view of how nuclei work. To improve the numerical methods, and the parameterization of 3-body forces, new precise data are needed. Electromagnetic transitions are very sensitive to the dynamics which drive mixing between configurations. We have made a series of precise (< 3%) measurements of electromagnetic transitions in the A=10 nuclei 10C and 10Be by using the Doppler Shift Attenuation method carefully. Many interesting features can be reproduced including the strong α clustering. New measurements on 8Be and 12Be highlight the interplay between the alpha clusters and their valence neutrons.

Beyond the Neutron Drip-Line

One of the major challenge in experimental and theoretical nuclear physics is the understanding of the properties of nuclei with extreme neutron-to-proton ratios. The most extreme asymmetric nuclei are located at and beyond the neutron drip-line. The first question arising here is the location of the drip-line itself, i.e., where are the limits of nuclear stability. Experimentally, this question is answered today only for the lightest elements up to oxygen [1] with 24O (Z = 8) being the heaviest bound isotope. For fluorine (Z = 9), the drip-line lies considerably further out, with the heaviest bound isotope observed experimentally being 31F [2]. Figure 1 gives an overview on the nuclear chart for the light nuclides for elements up to the fluorine isotopes. Besides the stable (black) and bound (red and blue) isotopes, unbound nuclei that have been observed experimentally as resonance states are indicated as well by the green fields. The apparent anomalous behavior of the location of the drip-line for the oxygen isotopes has been recently explained by the impact of three-nucleon (3N) forces, which contribute repulsively to the interaction among the valence neutrons [3, 4].

Test of the Grodzins product rule inN= 88 isotones and the role of theZ= 64 subshell

Background: The increase of collectivity in the nuclear spectra, with increasing numbers of valence proton and neutron pairs, is a well-known phenomenon, yielding a decreasing $E({2}_{1}{}^{+})$ and increasing $B$($E$2)\ensuremath{\uparrow}, which is the basis of Grodzins $E({2}_{1}{}^{+})\ifmmode\times\else\texttimes\fi{}B(E2)\ensuremath{\uparrow}$ product constancy rule.Purpose: In the $N$ = 88 isotones, this product varies sharply with $Z$. This breakdown of the product rule in the Ba to Dy region is illustrated and its origin is analyzed.Method: Empirical data on energy level structure in various forms along with the $E$2 transition rates, vis-\`a-vis the level structure and the nature of the $Z$ = 64 subshell effect, are presented.Results: The complex nuclear structure of $N$ = 88 isotones is highlighted and the genesis of the underlying physics is made more transparent.Conclusion: Besides the static shape of the nucleus, it involves the dynamics of the nucleus, as reproduced in the dynamic pairing plus quadrupole model.

New Limit on Lorentz-Invariance- andCPT-Violating Neutron Spin Interactions Using a Free-Spin-PrecessionHe3-Xe129Comagnetometer

We report on the search for a CPT and Lorentz invariance violating coupling of the 3He and 129Xe nuclear spins (each largely determined by a valence neutron) to background tensor fields which permeate the universe. Our experimental approach is to measure the free precession of nuclear spin polarized 3He and 129Xe atoms in a homogeneous magnetic guiding field of about 400 nT using LTC SQUIDs as low-noise magnetic flux detectors. As the laboratory reference frame rotates with respect to distant stars, we look for a sidereal modulation of the Larmor frequencies of the co-located spin samples. As a result we obtain an upper limit on the equatorial component of the background field interacting with the spin of the bound neutron bn< 6.7*10^-34 GeV (68% C.L.). Our result improves our previous limit (data measured in 2009) by a factor of 30 and the world's best limit by a factor of 5.

$${^6_{\Lambda} \rm{H}}$$ Λ 6 H Modeled as $${^4_{\Lambda} \rm{H}}$$ Λ 4 H + n + n

A three-body calculation for the $${^4_{\Lambda} \rm{He}}$$ and $${^6_{\Lambda}{\rm H}}$$ hypernuclei has been undertaken. The respective cores are $${^4_{\Lambda}{\rm H}}$$ . The interactions in the $${^6_{\Lambda}{\rm He}}$$ system, modeled as $${^4_{\Lambda} {\rm H+p+n}}$$ , are reasonably well known. For example, the p n interaction is well determined by the p n scattering data, the $${^4_{\Lambda}{\rm H}}$$ –p interaction can be fitted to the $${^5_{\Lambda}{\rm He}}$$ binding energy. The $${^4_{\Lambda}{\rm He}}$$ –n interaction can be fitted to α–n scattering data. For the 4He–n system the s-wave can be modeled alternatively as a repulsive potential or as an attractive potential with a forbidden bound state. We explore these alternatives in 6He, because the interaction comes into play in modeling $${^6_{\Lambda}{\rm He}}$$ as well as in our $${^4_{\Lambda}{\rm H}}$$ + n + n model of $${^6_{\Lambda}{\rm H}}$$ , where the valence neutrons are Pauli blocked from the s-shell of the core nucleus.

Correlated two-neutron emission in the decay of the unbound nucleus &lt;span class="aps-inline-formula"&gt;&lt;math&gt;&lt;msup&gt;&lt;mrow&gt;&lt;/mrow&gt;&lt;mn&gt;26&lt;/mn&gt;&lt;/msup&gt;&lt;mi mathvariant="normal"&gt;O&lt;/mi&gt;&lt;/math&gt;&lt;/span&gt;

We study the two-neutron decay of the unbound ${}^{26}\mathrm{O}$ nucleus with a three-body model assuming an inert ${}^{24}\mathrm{O}$ core and two valence neutrons. We first discuss the importance of the neutron-neutron final state interaction in the observed decay energy spectrum. We then show that the energy and and angular distributions for the two emitted neutrons manifest a clear evidence for the strong neutron-neutron correlation in the three-body resonance state. In particular, we find an enhancement of two-neutron emission in back-to-back directions. This is interpreted as a consequence of dineutron correlation, with which the two neutrons are spatially localized before the emission.

Microscopic self-consistent study of neon halos with resonant contributions

Recent reaction measurements have been interpreted as evidence of a halo structure in the exotic neutron-rich isotopes 29,31Ne. While theoretical studies of 31Ne generally agree on its halo nature, they differ significantly in their predictions of its properties and underlying cause (e.g., that 31Ne has an inverted ordering of p–f orbitals). We have made a systematic theoretical analysis of possible Neon halo signatures – the first using a fully microscopic, relativistic mean field approach that properly treats positive energy orbitals (such as the valence neutron in 31Ne) self-consistently with bound levels, as well as the pairing effect that keeps the nucleus loosely bound with negative Fermi energy. Our model is the analytical continuation of the coupling constant (ACCC) method based on a relativistic mean field (RMF) theory with Bardeen–Cooper–Schrieffer (BCS) pairing approximation. We calculate neutron- and matter-radii, one-neutron separation energies, p- and f-orbital energies and occupation probabilities, and neutron densities for single-particle resonant orbitals in 27–31Ne. We analyze these results for evidence of neutron halo formation in 29,31Ne. Our model predicts a p-orbit 1n halo structure for 31Ne, based on a radius increase from 30Ne that is 7–8 times larger than the increase from 29Ne to 30Ne, as well as a decrease in the neutron separation energy by a factor of ∼10 compared to that of 27–30Ne. In contrast to some other studies, our inclusion of resonances yields an inverted ordering of p and f orbitals for spherical and slightly deformed nuclei. Furthermore, we find no evidence of an s-orbit 1n halo in 29Ne as recently claimed in the literature.