Featured Researches

Nuclear Theory

229m Th isomer from a nuclear model perspective

The physical conditions for the emergence of the extremely low-lying nuclear isomer 229m Th at approximately 8 eV are investigated in the framework of our recently proposed nuclear structure model. Our theoretical approach explains the 229m Th-isomer phenomenon as the result of a very fine interplay between collective quadrupole-octupole and single-particle dynamics in the nucleus. We find that the isomeric state can only appear in a rather limited model space of quadrupole-octupole deformations in the single-particle potential, with the octupole deformation being of a crucial importance for its formation. Within this deformation space the model-described quantities exhibit a rather smooth behaviour close to the line of isomer-ground state quasi-degeneracy determined by the crossing of the corresponding single-particle orbitals. Our comprehensive analysis confirms the previous model predictions for reduced transition probabilities and the isomer magnetic moment, while showing a possibility for limited variation in the ground-state magnetic moment theoretical value. These findings prove the reliability of the model and suggest that the same dynamical mechanism could manifest in other actinide nuclei giving a general prescription for the search and exploration of similar isomer phenomena.

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Nuclear Theory

? -admixed neutron stars: spinodal instabilities and dUrca processes

Within the covariant density functional theory of nuclear matter we build equations of state of ? -admixed compact stars. Uncertainties in the interaction of ?(1232) resonance states with nuclear matter, due to lack of experimental data, are accounted for by varying the coupling constants to scalar and vector mesonic fields. We find that, over a wide range of the parameter space allowed by nuclear physics experiments and astrophysical observations, cold catalyzed star matter exhibits a first order phase transition which persists also at finite temperature and out of β -equilibrium in the neutrino-transparent matter. Compact stars featuring such a phase transition in the outer core have small radii and, implicitly, tidal deformabilities. The parameter space is identified where simultaneously ? -admixed compact stars obey the astrophysical constraint on maximum mass and allow for dUrca processes, which is otherwise forbidden.

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Nuclear Theory

? -nucleus potential for ? ??quasifree production in the 9 Be( K ??, K + ) reaction

We study phenomenologically a ? ??production spectrum of the 9 Be( K ??, K + ) reaction at 1.8 GeV/ c within the distorted-wave impulse approximation using the optimal Fermi-averaged K ??p??K + ? ??amplitude. We attempt to clarify properties of a ? -nucleus potential for ? ??- 8 Li, comparing the calculated spectrum with the data of the BNL-E906 experiment. The results show a weak attraction in the ? -nucleus potential for ? ??- 8 Li, which can sufficiently explain the data in the ? ??quasifree region. The strength of V ? 0 = ??7±6 MeV is favored within the Woods-Saxon potential, accompanied by the reasonable absorption of W ? 0 = ?? MeV for ? ??p??? 0 n , ?? transitions in nuclear medium. It is difficult to determine the value of W ? 0 from the data due to the insufficient resolution of 14.7 MeV FWHM. The energy dependence of the Fermi-averaged K ??p??K + ? ??amplitude is also confirmed by this analysis, and its importance in the nuclear ( K ??, K + ) reaction is emphasized.

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Nuclear Theory

A Bayesian Analysis of Nuclear Deformation Properties with Skyrme Energy Functionals

In spite of numerous scientific and practical applications, there is still no comprehensive theoretical description of the nuclear fission process based solely on protons, neutrons and their interactions. The most advanced simulations of fission are currently carried out within nuclear density functional theory (DFT). In spite of being fully quantum-mechanical and rooted in the theory of nuclear forces, DFT still depends on a dozen or so parameters characterizing the energy functional. Calibrating these parameters on experimental data results in uncertainties that must be quantified for applications. This task is very challenging because of the high computational cost of DFT calculations for fission. In this paper, we use Gaussian processes to build emulators of DFT models in order to quantify and propagate statistical uncertainties of theoretical predictions for a range of nuclear deformations relevant to describing the fission process.

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Nuclear Theory

A Bayesian-Neural-Network Prediction for Fragment Production in Proton Induced Spallation Reaction

Fragments productions in spallation reactions are key infrastructure data for various applications. Based on the empirical parameterizations {\sc spacs}, a Bayesian-neural-network (BNN) approach is established to predict the fragment cross sections in the proton induced spallation reactions. A systematic investigation have been performed for the measured proton induced spallation reactions of systems ranging from the intermediate to the heavy nuclei and the incident energy ranging from 168 MeV/u to 1500 MeV/u. By learning the residuals between the experimental measurements and the {\sc spacs} predictions, the BNN predicted results are in good agreement with the measured results. The established method is suggested to benefit the related researches in the nuclear astrophysics, nuclear radioactive beam source, accelerator driven systems, and proton therapy, etc.

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Nuclear Theory

A Guided Tour of Ab Initio Nuclear Many-Body Theory

Over the last decade, new developments in Similarity Renormalization Group techniques and nuclear many-body methods have dramatically increased the capabilities of ab initio nuclear structure and reaction theory. Ground and excited-state properties can be computed up to the tin region, and from the proton to the presumptive neutron drip lines, providing unprecedented opportunities to confront two- plus three-nucleon interactions from chiral Effective Field Theory with experimental data. In this contribution, I will give a broad survey of the current status of nuclear many-body approaches, and I will use selected results to discuss both achievements and open issues that need to be addressed in the coming decade.

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Nuclear Theory

A New Radioactive Decay Mode, True Ternary Fission, the Decay of Heavy Nuclei Into Three Comparable Fragments

The ternary cluster decay of heavy nuclei has been observed in several experiments with binary coincidences between two fragments using detector telescopes (the FOBOS-detectors, JINR, Dubna) placed on the opposite sides from the source of fissioning nuclei. The binary coincidences at a relative angle of 180 0 deg. correspond to binary fission or to the decay into three cluster fragments by registration of two nuclei with different masses (e.g. 132 Sn, 52−48 Ca, 68−72 Ni). This marks a new step in the physics of fission-phenomena of heavy nuclei. These experimental results for the collinear cluster tripartition (CCT), refer to the decay into three clusters of comparable masses. In the present work we discuss the various aspects of this ternary fission (FFF) mode. The question of collinearity is analysed on the basis of recent publications. Further insight into the possible decay modes is obtained by the discussion of the path towards larger deformation, towards hyper-deformation and by inspecting details of the potential energy surfaces (PES). In the path towards the extremely deformed states leading to ternary fission, the concept of deformed shells is most important. At the scission configuration the phase space determined by the PES's leads to the final mass distributions. The possibility of formation of fragments of almost equal size ( Z i = 32, 34, 32, for Z =98) and the observation of several other fission modes in the same system can be predicted by the PES. The PES's show pronounced minima and valleys, namely for several mass/charge combinations of ternary fragments, which correspond to a variety of collinear ternary fission (multi-modal) decays. The case of the decay of 252 Cf(sf,fff) turns out to be unique due to the presence of deformed shells in the total system and of closed shells in all three nuclei in the decay.

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Nuclear Theory

A Novel Regularization Scheme for Nucleon-Nucleon Lattice Simulations with Effective Field Theory

We propose a new regularization scheme to study the bound state of two-nucleon systems in Lattice Effective Field Theory. Inspired by continuum EFT calculation, we study an exponential regulator acting on the leading-order (LO) and next-to-leading order (NLO) interactions, consisting of local contact terms. By fitting the low-energy coefficients (LECs) to deuteron binding energy and the asymptotic normalization coefficient (ANC) on a lattice simulation, we extract the effective range expansion (ERE) parameters in the 3 S 1 channel to order p 2 . We explore the impact of different powers of the regulator on the extracted ERE parameters for the lattice spacing a=1.97 fm. Moreover, we investigate how the implementation of the regularization scheme improves the predicted ERE parameters on the lattice spacing in the range of 1.4≤a≤2.6 fm. Our numerical analysis indicates that for lattice spacing greater than 2 fm, the predicted observables are very close to the experimental data.

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Nuclear Theory

A Semimicroscopic Algebraic Cluster Model for Heavy Nuclei

An extension of the Semimicroscopic Algebraic Cluster Model (SACM) is proposed, based on the pseudo-SU(3) model. The Hamiltonian and the spectroscopic factor operator of the model are presented and a procedure of constructing the model space. Because a huge number of SU(3) irreducible representations (irrep) appear, one has to be careful in designing a practical, consistent path to reduce the Hilbert space. The concept of forbiddenness, taking into account excitations of the clusters, is introduced and applied. The applications are to two systems with a low forbiddenness, namely to 236U -> 210Pb + 26Ne and 224Ra -> 210Pb + 14C, and to 236U -> 146Xe + 90Sr, which appears in the fission of 236U, which requires a large forbiddenness. Energies, electromagnetic transitions and spectroscopic factors are calculated.

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Nuclear Theory

A Statistical Description of Nuclear Reaction Models for Medical Radionuclides: the Paradigmatic Case of 47 Sc Production with Thick Vanadium Targets

We have introduced a tool to describe in a simple and efficient way the outcomes of known nuclear reaction codes. It differs from the customary use where typically a specific single model is selected and the remaining disregarded. The use of simple statistical procedures allows to introduce a more general theoretical evaluation with quantitative uncertainty, constructed on the variability of the built-in theoretical models. We apply the technique to study the production of 47 Sc (a radio-nuclide with potential theranostic applications in nuclear medicine) with a proton beam impinging on a thick natural Vanadium target. We find an energy range with significant production of 47 Sc, and a minimum co-production of 46 Sc, the radioactive contaminant that has to be avoided as much as possible because of its much longer half life than 47 Sc (83.79 d vs 3.3492 d).

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