Light Mass Nuclei Research Articles

From the lightest nuclei to the equation of state of asymmetric nuclear matter with realistic nuclear interactions

We present microscopic calculations of light and medium mass nuclei and the equation of state of symmetric and asymmetric nuclear matter using different nucleon-nucleon forces, including a new Argonne version that has the same spin/isospin structure as local chiral forces at next-to-next-to-leading order (N2LO). The calculations are performed using Auxiliary Field Diffusion Monte Carlo (AFDMC) combined with an improved variational wave function. We show that the AFDMC method can now be used to successfully calculate the energies of very light to medium mass nuclei as well as the energy of isospin-asymmetric nuclear matter, demonstrating microscopically the quadratic dependence of the energy on the symmetry energy.

Study of Reaction Cross Section of Light Mass Nuclei Using Glauber Formalisms

We study the reaction dynamics of some light mass nuclei (Be-Ar isotopes). We use the well-known Glauber formalism with densities obtained from a recently developed simple effective interaction, in the frameword of the microscopic non-relativistic Hartree–Fock formalisms. For comparison, we used also densities from the relativistic mean field formalism. In general, the study of the reaction dynamics suggests that both formalisms reproduce well the experimental data, with a slight superiority of the relativistic mean field densities over those of non-relativistic Hartree–Fock. This is so for the majority of the chosen cases, except for a few nuclear halo systems.

Fragment mass identification and related aspects in the decay of 40Ca* and 39K* nuclei

The dynamics involved in the decay of light mass nuclei formed in asymmetric channels 12 C + 28 Si , 11 B + 28 Si and 12 C + 27 Al have been investigated using the dynamical cluster-decay model (DCM). In reference to the experimentally measured charge particle cross-sections, the fragment masses contributing towards the decay of 40 Ca * and 39 K * nuclei have been identified using spherical choice of fragmentation. Also, the role of entrance channel has been investigated by studying the decay of 39 K * nuclear system formed in two different reactions at same excitation energy. The behavior of fragmentation potential, preformation probability, penetrability and emission time, is analyzed to figure out the favorable mass fragments, their relative emergence and the entrance channel effects observed in the decay of light mass nuclei. In addition to this, the cross-sections for the light particles (LPs) and heavier charge fragments have been estimated for the compound nucleus (CN) decay. Besides this, one of the noncompound nucleus (nCN) process, deep inelastic collision (DIC) has been addressed in context of DCM approach for the first time. The cross-sections obtained in framework of DCM for both CN and nCN processes are found to have nice agreement with the available experimental data.

Comparison of multi-ħωshell-model results with MCAS

A multi-channel algebraic scattering (MCAS) method has been used to obtain spectra of a number of light-mass nuclei, which are treated as a two-cluster system, here specifically a nucleon plus nucleus. To date, collective models have been used to specify the interactions between the nucleon and low-lying states of the nucleus that form the compound. For the case of the carbon isotopes, these studies have been complemented by sufficiently complex and complete shell-model calculations. Comparisons with the multi-ħω shell-model results provide new insights into the validity of those from MCAS.

Nuclear weak interactions, supernova nucleosynthesis and neutrino oscillation

We study the nuclear weak response in light-to-heavy mass nuclei and calculate neutrino-nucleus cross sections. We apply these cross sections to the explosive nucleosynthesis in core-collapse supernovae and find that several isotopes of rare elements 7Li, 11B, 138La, 180Ta and several others are predominantly produced by the neutrino-process nucleosynthesis. We discuss how to determine the suitable neutrino spectra of three different flavors and their anti-particles in order to explain the observed solar system abundances of these isotopes, combined with Galactic chemical evolution of the light nuclei and the heavy r-process elements. Light-mass nuclei like 7Li and 11B, which are produced in outer He-layer, are strongly affected by the neutrino flavor oscillation due to the MSW (Mikheyev-Smirnov-Wolfenstein) effect, while heavy-mass nuclei like 138La, 180Ta and r-process elements, which are produced in the inner O-Ne-Mg layer or the atmosphere of proto-neutron star, are likely to be free from the MSW effect. Using such a different nature of the neutrino-process nucleosynthesis, we study the neutrino oscillation effects on their abundances, and propose a new novel method to determine the unknown neutrino oscillation parameters, θ13 and mass hierarchy, simultaneously. There is recent evidence that some SiC X grains from the Murchison meteorite may contain supernova-produced neutrino-process 11B and 7Li encapsulated in the grains. Combining the recent experimental constraints on θ13, we show that although the uncertainties are still large, our method hints at a marginal preference for an inverted neutrino mass hierarchy for the first time.

Symmetry-Adapted Ab Initio Shell Model for Nuclear Structure Calculations

An innovative concept, the symmetry-adapted ab initio shell model, that capitalizes on partial as well as exact symmetries that underpin the structure of nuclei, is discussed. This framework is expected to inform the leading features of nuclear structure and reaction data for light and medium mass nuclei, which are currently inaccessible by theory and experiment and for which predictions of modern phenomenological models often diverge. We use powerful computational and group-theoretical algorithms to perform ab initio CI (configuration-interaction) calculations in a model space spanned by SU(3) symmetry-adapted many-body configurations with the JISP16 nucleon-nucleon interaction. We demonstrate that the results for the ground states of light nuclei up through A = 16 exhibit a strong dominance of low-spin and high-deformation configurations together with an evident symplectic structure. This, in turn, points to the importance of using a symmetry-adapted framework, one based on an LS coupling scheme with the associated spatial configurations organized according to deformation.

Scattering and resonances on p-shell nuclei

We review recent results on low energy nucleon scattering from some light-mass nuclei and particularly consider resonances seen or expected in cross sections. We present results found using the multichannel algebraic scattering (MCAS) method which was developed to analyze such cross sections. The method gives both bound and resonant spectra of the compound nuclei. In 2006, using this method, we predicted nontrivial narrow resonances due to non-valence p-shell effects in the nucleus 15F, a nucleus that lies beyond the proton drip-line. Subsequently, in 2009, those predictions were verified experimentally. Research on other such mirror systems is underway. In particular, the structure of 17C has been used to define an appropriate MCAS nuclear interaction for the n+16C system. Adding a Coulomb field to that interaction, an MCAS evaluation has been made for the mirror system p+16Ne, to specify the low excitation spectrum of the yet unobserved, particle-unstable, 17Na.We also investigated how the scattering cross-section changes when the spectra of the colliding nuclei have low-excitation particle-emitting resonances. As a test case, we considered 8Be, being particle-unstable, and analyzed neutron scattering cross sections and the spectra of the compound 9Be system. If the 8Be excited states have fixed-energy decay widths, we find that the bound states of the compound system are affected in an unphysical way. The shapes of the target resonances must vary from the usual Lorentzian in an energy-dependent way. Resonance width functions must go smoothly to zero at the elastic threshold. Ways of achieving this condition are being explored.

Fermi breakup and the statistical multifragmentation model

Abstract We demonstrate the equivalence of a generalized Fermi breakup model, in which densities of excited states are taken into account, to the microcanonical statistical multifragmentation model used to describe the disintegration of highly excited fragments of nuclear reactions. We argue that such a model better fulfills the hypothesis of statistical equilibrium than the Fermi breakup model generally used to describe statistical disintegration of light mass nuclei.

Symmetry-adapted Ab Initio Theory for Many-body Correlations in Nuclei

We demonstrate that no-core shell-model results for low-lying states of light and medium mass nuclei, whether they are dilute or dense systems, reveal a strong dominance of low-spin and high-deformation configurations. This result is independent of whether the system Hamiltonian is phenomenological in nature or derived from a realistic interaction. It implies that only a small fraction of the complete model space is required for a description of such states, and this in turn points to the importance of using a symmetry-adapted, no-core shell-model framework for describing such nuclei, one based on an LS coupling scheme with the associated spatial configurations organized according to deformation.These results confirm that the pioneering work of early developers of the field, J. P. Elliott with his SU(3) model and M. Moshinsky with his U(3) many-body oscillator work, extends to more open, multi-shell environments. Specifically, algebraic methods are both relevant in such an environment and they can be used to quell the combinatorial growth in dimensionality that comes with the addition of oscillator shells to a model space. Indeed, our findings demonstrate the utility of a symmetry-adapted, no-core shell-model approach, one that takes advantage of group theoretical as well as advanced computational methods. And importantly, what at first glance appear to be a daunting task – casting complex algebraic expressions of a symmetry-adapted scheme into a user-friendly and efficient shell-mode code, turns out to be not only doable, but a logical framework that embraces constructs that can be made to execute efficiently on massively parallel, multi-processor (and core) systems. Early results for some light p-shell nuclei are presented. In addition, we will show that the method can be extended to heavier nuclei of the sd-shell and beyond, including some cases of special astrophysical interest in the upper fp- and lower gds-shells, like isotopes of Ge, Se, and even Kr.

Systematics for ( n, 2 n) excitation functions in the neutron energy between 13.4 and 14.9 MeV

A systematics of ( n, 2 n) partial excitation functions was studied on the basis of our 58 sets of experimental data measured using intense neutron source facilities of FNS at the Japan Atomic Energy Agency and of OKTAVIAN at Osaka University. The empirically predictive formulas based on the evaporation theory are applicable for the neutron energy between 13.4 and 14.9 MeV, and in the mass range between 14 and 238. About 83% of the cross sections and the slopes of excitation functions can be reproduced within an uncertainty of 15%. The formulas were expressed in terms of a mass number A, an asymmetry parameter S = ( N − Z)/ A, and threshold energies of the ( n, 2 n) and ( n, 3 n) reactions, where N and Z are the neutron and proton numbers for the target nuclei, respectively. The excitation functions were well reproduced for the nuclei in the region with S < 0.05 and 14 ⩽ A ⩽ 58 for the first time. The general characteristics of the excitation functions, consisting of steep rising in the light mass nuclei, gently-sloping or nearly flat in the middle and heavy mass nuclei, and slow falling in the heavy mass regions with possible ( n, 3 n) reactions, are well expressed.

Generalized mass formula for non-strange, strange and multiply-strange nuclear systems

A simultaneous description of non-strange nuclei, hypernuclei and multiply-strange nuclear systems is provided by a single mass formula which is shown to be useful for estimating binding energies of nuclear systems over a wide mass range, including the light mass nuclei. It not only provides a good fit to the existing experimental data on hyperon-separation energies but also reproduces results of the relativistic mean field (RMF) calculations. In addition, it can provide the lambda (Λ), cascade-0 (Ξ0) and cascade-minus (Ξ−) drip lines. The existence of a range of bound pure-hyperonic systems without any neutron and proton is suggested among which 6Λ, 9Ξ0Ξ0, 10Ξ−Ξ−, 1Λ7Ξ0, 1Λ8Ξ−, 1Ξ09Ξ−, 1Ξ−8Ξ0 and 2Λ + 3Ξ0 + 3Ξ− represent the lightest species. In agreement with the RMF predictions, this generalized mass formula also predicts the nucleus to be bound. An exotic nucleus is also found to be bound. This new mass formula can be used in astrophysics for strange stellar objects, as well as in high energy physics for estimating the strangeness production yield in nucleus–nucleus or nucleon–nucleon collisions.

Comparison of DD, DT and Cf-252 neutron excitation of light and medium mass nuclei for field PGNAA applications

Prompt Gamma Ray Neutron activation analysis can offer significant cost and safety advantages in the identification of explosives and toxic chemicals. As an example, the US military examined over a thousand suspect chemical munitions with Idaho National Laboratory’s PINS Chemical Assay System last year. PGNAA requires, of course, a neutron source to excite the atomic nuclei of the item under test via neutron capture and inelastic neutron scattering reactions and the choice of neutron source can drastically affect PGNAA system performance. We have carried out Monte Carlo and laboratory experiments comparing DD, DT and Cf-252 neutrons incident on light and medium mass chemical elements, toward optimizing the design of future neutron-generator-based PGNAA systems for field use. We report the excitation of (n,γ) and (n,n′) gamma rays from these elements by each type of neutron source.

The s process in massive stars

In the past decades considerable progress has been made in the understanding of the various processes responsible for the nucleosynthesis of the elements in stars. This holds especially for the main s process in low-mass AGB stars, which is largely responsible for the production of about half of the elemental abundances in the mass range 90 ≤ A ≤ 209 . The weak s process, which produces elements with A ≤ 90 , however, is much less understood. A better characterization of the weak s component would help to disentangle the various processes contributing to element production in this region. In this respect, more accurate neutron capture cross sections in the mass range 56 ≤ A ≤ 90 are indispensable. Also, neutron captures on abundant light elements with A < 56 play an important role, since they act as neutron poisons and affect the stellar neutron balance. In this context, the impact of new results for neutron capture cross sections on light and medium mass nuclei is discussed.

Generalized mass formula for non-strange and hypernuclei with SU(6) symmetry breaking

A simultaneous description of non-strange nuclei and hypernuclei is provided by a single mass formula inspired by the spin–flavour SU(6) symmetry breaking. This formula is used to estimate the hyperon binding energies of lambda, double lambda, sigma, cascade and theta hypernuclei. The results are found to be in good agreement with the available experimental data on ‘bound’ nuclei and relativistic as well as quark mean-field calculations. This mass formula is useful to estimate binding energies over a wide range of masses including the light mass nuclei. It is not applicable to a repulsive potential.

Does the break-up process influence the fusion cross section?

We give an overall picture of our present understanding of the effect of the break-up of stable weakly bound nuclei on their fusion cross section with light, medium and heavy mass nuclei, at energies above the Coulomb barrier. The discussion is based mostly on recent data obtained by our group in collaborative experiments with ANU, USP and Tandar Laboratories. We conclude that there is complete fusion suppression for heavy targets, due to the loss of flux, corresponding to the occurrence of incomplete fusion of one of the break-up fragments. For medium and light mass targets, the incomplete fusion is negligible and therefore is no complete fusion suppression.

Microscopic analysis of the breathing mode in 40Ca and 58Ni

Recent experimental studies of the giant electric resonance region in 58Ni and 40Ca with inelastically scattered α-particles of energy Eα= 240 MeV are analyzed within a microscopic nuclear structure model. The model includes the continuum RPA and more complex 1p1h⊗phonon configurations. By superimposing the contributions of different multipoles up to L = 4 we obtain good agreement with the newest (reanalyzed) data for the isoscalar monopole strength and for the total (α,α′) cross section in 58Ni. Agreement with experiment for the isoscalar monopole resonance in 40Ca is obtained too. We emphasize the necessity of using microscopic transition densities and discuss consequences for the analyses of such experiments in light and medium mass nuclei. It is shown that the gross structure of the isoscalar monopole resonance in 40Ca is caused by the 1p1h⊗phonon configurations.

Microscopic model analyses of the elastic scattering of 65 MeV protons from targets of diverse mass

Nonlocal coordinate space optical potentials for the scattering of 65 MeV protons from nuclei ranging in mass from ${}^{6}\mathrm{Li}$ to ${}^{238}\mathrm{U}$ have been defined by folding a complex, medium-dependent effective interaction with the density matrix elements of each target. The effective interaction is based upon solutions of the Lippmann-Schwinger and Brueckner-Bethe-Goldstone equations having the Paris potential as input. The nuclear structure information required in our folding model is the one-body density matrix elements for the target and the single- nucleon bound state wave functions that they weight. For light mass nuclei, very large basis shell model calculations have been made to obtain the one-body density matrix elements. For medium and heavy mass nuclei, a very simple shell model prescription has been used. The bound-state single-particle wave functions that complete the nuclear density matrices are either Woods-Saxon or harmonic oscillator functions. The former are employed in most cases when large basis structure is available. For light nuclei $(Al~16)$ Woods-Saxon potential parameters and harmonic oscillator lengths are determined from fits to electron scattering form factors. For all other nuclei, we use harmonic oscillator functions with the oscillator lengths set from an ${A}^{1/6}$ mass law. Using this microscopic model, optical potentials result from which differential cross sections, analyzing powers, and spin rotations are found. In general the calculated results compare very well with data when the effective interactions are determined from a mapping of nucleon-nucleon g matrices. This is not the case when effective interactions built from a mapping of (free) t matrices are used.

Triaxial deformation of unstable nuclei in the relativistic mean-field theory

We study the properties of unstable nuclei including the possibility of triaxial deformation in the relativistic mean-field theory. We calculate the energy surfaces of various light mass nuclei: 16O, 40Ca, 20Ne, 24Ne and 24Mg and a series of even mass sulfur isotopes, in the βγ plane. We find the well-known spherical solution for the 16O and 40Ca nuclei strong prolate solutions for 20Ne, 24Ne and 24Mg, which are in agreement with the experimental data and the non-relativistic density-dependent Hartree-Fock results. For the sulfur isotopes, we find that the energy minimum moves from a prolate shape below 46S through triaxial shapes for 48–52S to an oblate shape above 54S.

Shape changes and isospin purity in highly excited light mass nuclei

The statistical decay of the Giant Dipole Resonance built on highly excited states of light mass nuclei was studied in inclusive experiments. We report on results of the search for a shape change of hot, fast rotating 45Sc and the test of the isospin purity at high excitation in 28Si and 26Al.

Excitation function for α-particle-induced reactions in light-mass nuclei

The excitation functions of 51V [(α, xn), x = 1.3; (α, p3n); (α, α3n) (α, 2α)] and 59Co[(α, 2n); (α, αxn); x = 1, 2] reactions were measured up to 50 MeV by the stacked-foil activation technique and high-resolution Ce(Li)–HPGe spectroscopy method. Of these 51V(α, xnypzα) reactions were studied for the first time with better accuracy than earlier measurements, which were done with a poor resolution scintillation–Ge(Li) detectors some 20 years ago. In the case of 59Co, existing data show large discrepancies at low energy. Comparisons are presented with the prediction of hybrid and exciton models. While for the (α, xn) type of reactions, reasonably explainable pre-equilibrium contributions are found, there seem to be indications of direct inelastic scattering effects in the (α, αxn) type of reactions.