Nucleon Emission Research Articles

Low-Energy Ternary Fission of Actinide Nuclei with the Emission of Nucleons and Light Nuclei

Low-Energy Ternary Fission of Actinide Nuclei with the Emission of Nucleons and Light Nuclei

Exploring Semi-Inclusive Two-Nucleon Emission in Neutrino Scattering: A Factorized Approximation Approach

The semi-inclusive cross-section of two-nucleon emission induced by neutrinos and antineutrinos is computed by employing the relativistic mean field model of nuclear matter and the dynamics of meson-exchange currents. Within this model, we explore a factorization approximation based on the product of an integrated two-hole spectral function and a two-nucleon cross-section averaged over hole pairs. We demonstrate that the integrated spectral function of the uncorrelated Fermi gas can be analytically computed, and we derive a simple, fully relativistic formula for this function, showcasing its dependency solely on both missing momentum and missing energy. A prescription for the average momenta of the two holes in the factorized two-nucleon cross-section is provided, assuming that these momenta are perpendicular to the missing momentum in the center-of-mass system. The validity of the factorized approach is assessed by comparing it with the unfactorized calculation. Our investigation includes the study of the semi-inclusive cross-section integrated over the energy of one of the emitted nucleons and the cross-section integrated over the emission angles of the two nucleons and the outgoing muon kinematics. A comparison is made with the pure phase-space model and other models from the literature. The results of this analysis offer valuable insights into the influence of the semi-inclusive hadronic tensor on the cross-section, providing a deeper understanding of the underlying nuclear processes.

Strangelets at finite temperature: Nucleon emission rates, interface, and shell effects

We investigate the properties of strangelets at finite temperature T, where an equivparticle model is adopted with both the linear confinement and leading-order perturbative interactions accounted for using density-dependent quark masses. The shell effects are examined by solving the Dirac equations for quarks in the mean-field approximation, which diminish with temperature as the occupation probability of each single-particle levels fixed by the Fermi-Dirac statistics, i.e., shell dampening. Consequently, instead of decreasing with temperature, the surface tension extracted from a liquid-drop formula increases with T until reaching its peak at T≈20–40 MeV with vanishing shell corrections, where the formula roughly reproduces the free energy per baryon of all strangelets. The curvature term, nevertheless, decreases with T despite the presence of shell effects. The neutron and proton emission rates are fixed microscopically according to the external nucleon gas densities that are in equilibrium with strangelets, which generally increase with T (≲50 MeV) for stable strangelets but decrease for those that are unstable against nucleon emission at T=0. The energy, free energy, entropy, charge-to-mass ratio, strangeness per baryon, and root-mean-square radius of β-stable strangelets obtained with various parameter sets are presented as well. The results indicated in this work are useful for understanding the products of binary compact star mergers and heavy-ion collisions. Published by the American Physical Society 2024

Neutron and gamma-ray signatures for the control of alpha-emitting materials in uranium production: A Nedis2m-MCNP6 simulation

The radiations emitted via 19F(α,nγ), 19F(α,pγ), and 19F(α,α′γ) reactions in compounds containing alpha-emitting elements and fluorine are used for non-destructive neutron and gamma-ray analyses in monitoring the contents of nuclear fissile materials during the manufacture, processing, and storage of nuclear fuels. As far as the literature survey confirms, the data available on the yield of (α,n) reaction on fluorine, which depends on alpha particle energy, have significant dispersion. For the accompanying gamma-rays, on the other hand, no reliable absolute data exist for the production yield of gamma-rays per number of interacting alpha particles or emitted nucleons as a function of alpha particle energy. Therefore, to improve the monitoring and control of nuclear materials and to ensure the safety of production processes, the present study was undertaken, where the neutron yields and energy spectra were calculated using the Nedis-2m program taking into account the updated values of the total cross-section for (α,n) reactions on 19F. The resulting Nedis-2m input datasets were used in the MCNP6 code to calculate the leakage multiplication factor and neutron energy spectrum.

In-medium nucleon-nucleon cross section in nuclear matter

In-medium nucleon-nucleon ($NN$) cross sections at various densities and isospin asymmetries are investigated systematically in the framework of the Brueckner-Hartree-Fock approach. The calculations are base on the exact treatment of the center-of-mass momentum instead of the total momentum approximation employed in previous works. The neutron-proton in-medium cross section is shown to exhibit nearly isospin asymmetry independence. For convenience in application, analytical formulas embodying the medium correction to free $NN$ cross section with parameters calibrated to the calculated results have been provided. Using these formulas, the transverse and elliptic flows of emitted nucleons in heavy-ion collisions are studied within the isospin-dependent Boltzmann-Uehling-Uhlenbeck transport model. We find the medium effect of the cross section from the $G$ matrix contributes to the transverse and elliptic flows oppositely to and obviously smaller than that from the effective mass. In addition, the present microscopic in-medium cross sections can significantly revise the predictions of the transverse and elliptic flows compared to the in-medium cross sections previously adopted in transport models for heavy-ion collisions.

Coupled-channel description for mirror mass-11 nuclei compared to shell-model structures

The spectra of mass-11 nuclei are unusual, and so pose a challenge for nuclear-structure theory. Relating to nucleon emission, the set of isobars range from being well-bound (^{11}B,^{11}C) through weakly bound (^{11}Li, ^{11}Be), to being proton unstable (^{11}N,^{11}O). To add complexity, the weakly bound ^{11}Li takes the form of a two-nucleon halo nucleus. A self-consistent approach to understand this set of nuclei is especially important as the mirror pair ^{11}Be-^{11}N exhibit a parity-inverted ground state compared to their neighboring nuclei. Herein, the Multi-Channel Algebraic Scattering method (MCAS) has been used to describe the low excitation spectra of those isobars in terms of nucleon-nucleus clusters. A collective model description of the low-excitation states of the mass-10 mass-10 core nuclei has been used to form the coupled-channel interactions required in the method. For comparison, and to understand the underlying configurations, a shell model approach has been used to obtain those spectra with no-core (0+2+4)hbar omega and (0+2)hbar omega shell-model spaces for the mass 10 and mass 11 nuclei respectively. The results of the calculations suggest the need of a strong coupling in the collective coupled-channel vibrational model. In particular, the strong coupling of the collective 2^+_1 state of ^{10}Be to the valence neutron plays a decisive role in forming the positive parity ground state in ^{11}Be; an effect confirmed by the shell-model results.

Nucleon–nucleon correlations in the extreme oxygen isotopes

There has been an upsurge of interest in two-nucleon decays thanks to the studies of nucleon–nucleon correlations. In our previous work (2021 Phys. Rev. Lett. 126 142501), based on a novel time-dependent three-body approach, we demonstrated that the energy and angular correlations of the emitted nucleons can shed light on the structure of nucleonic pairs formed inside the nucleus. In this work, we apply the new framework to study the decay dynamics and properties of some extreme proton-rich and neutron-rich oxygen isotopes, including two-proton (2p) decays of 11,12O and two-neutron (2n) decay of 26O. Here we show that the low-ℓ components of 11,12O wave functions, which are affected by continuum and configuration-interaction effects, strongly impact decay dynamics and asymptotic correlations. In the calculated wave functions of 11,12O, diproton and cigarlike structures merge together during the tunneling process and the resulting energy and angular correlations are very consistent with the experimental data. The asymptotic correlations of the 2n decay of 26O dramatically change as the 2n decay energy approaches the zero-energy threshold. The small reported value of Q 2n suggests that the 2n decay of this nucleus can be understood in terms of the universal phase-space limit.

Thermodynamic approach to proton number fluctuations in baryon-rich heavy-ion matter created at moderate collision energies

We develop a framework to relate proton number cumulants measured in heavy-ion collisions within a momentum space acceptance to the susceptibilities of baryon number, assuming that particles are emitted from a fireball with uniform distribution of temperature and baryochemical potential, superimposed on a hydrodynamic flow velocity profile. The rapidity acceptance dependence of proton cumulants measured by the HADES Collaboration in sNN=2.4 GeV Au-Au appears to be consistent with thermal emission of nucleons from a grand-canonical heat bath, with the extracted baryon number susceptibilities exhibiting an hierarchy χ4B≫−χ3B≫χ2B≫χ1B. Naively, this could indicate large non-Gaussian fluctuations that might point to the presence of the QCD critical point close to the chemical freeze-out at T∼70 MeV and μB∼850−900 MeV. However, the description of the experimental data at large rapidity acceptances becomes challenging once the effect of exact baryon number conservation is incorporated, suggesting that more theoretical and experimental studies are needed to reach a firm conclusion.

Secondary emission in pre-equilibrium reaction effects on differential cross-section

The differential cross-sections of the pre-equilibrium reactions were calculated within the exciton model (EM) and Feshbach–Kerman–Konin (FKK) theory for some selected reactions targets ([Formula: see text] and [Formula: see text] with [Formula: see text] and [Formula: see text] reactions at low and high energies 10, 14, 16, 25 and 57 MeV using PRECO-2006 to investigate the importance of secondary emission in the pre-equilibrium reactions. The results showed that the emission of secondary nucleons showed a behavior in the resulting spectra, the most prominent of which is its significant contribution to adding to the spectra obtained through practical experiments. As multistep direct (MSD) and multistep compound (MSC) are the main two components in the pre-equilibrium reaction, it was shown that the MSD dominated above 5[Formula: see text]MeV emission energy rather than MSC reaction.

Examining measuring the degree of linear photon-beam polarization in the energy range up to 100 MeV with the use of He4(γ⃗,p)H3 and He4(γ⃗,n)He3 reactions

The paper gives physical justification of a new method for measuring the degree of linear photon-beam polarization, which relies on the experimental data for the total cross section of spin $S$ = 1 transitions in the $^{4}\mathrm{He}(\ensuremath{\gamma},p)^{3}\mathrm{H}$ and $^{4}\mathrm{He}(\ensuremath{\gamma},n)^{3}\mathrm{He}$ reactions. The experimental information on the cross section was obtained from both the mentioned reactions and the reactions of radiative capture of protons by tritium nuclei. The total cross section for transitions with spin $S$ = 1 is $\ensuremath{\approx}1%$ of the total cross section for the reaction. The ratio of the differential cross section in the collinear geometry, specified by the $S$ = 1 transitions, to the differential reaction cross section at the nucleon emission angle ${\ensuremath{\theta}}_{N}={90}^{\ensuremath{\circ}}$ in the energy range 20 $\ensuremath{\le}{E}_{\ensuremath{\gamma}}\ensuremath{\le}$ 100 MeV is independent of the photon energy within the experimental error. The total cross section for the $S$ = 1 transitions can be also determined from the experimental data on the cross-section asymmetry of the reaction with linearly polarized photons $\mathrm{\ensuremath{\Sigma}}({\ensuremath{\theta}}_{N})$. However, the total cross section for transitions with spin $S$ = 1, calculated from the asymmetry of the cross section $\mathrm{\ensuremath{\Sigma}}({\ensuremath{\theta}}_{N})$, is several times greater than that calculated from the differential cross section. This inconsistency of the experimental data may be due, in particular, to the overestimate of the degree of linear photon-beam polarization. The above-mentioned reactions seem to be more convenient for measuring the degree of linear photon-beam polarization than the deuteron photodisintegration reactions.

Initial-State Temperature of Light Meson Emission Source From Squared Momentum Transfer Spectra in High-Energy Collisions

The squared momentum transfer spectra of light mesons, π0, π+, η, and ρ0, produced in high-energy virtual photon-proton (γ*p) → meson + nucleon process in electron-proton (ep) collisions measured by the CLAS Collaboration are analyzed by the Monte Carlo calculations, where the transfer undergoes from the incident γ* to emitted meson or equivalently from the target proton to emitted nucleon. In the calculations, the Erlang distribution from a multi-source thermal model is used to describe the transverse momentum spectra of emitted particles. Our results show that the average transverse momentum (⟨pT⟩) and the initial-state temperature (Ti) increase from lower squared photon virtuality (Q2) and Bjorken variable (xB) to higher one. This renders that the excitation degree of emission source, which is described by ⟨pT⟩ and Ti, increases with increasing of Q2 and xB.

Multiplicity scaling of light nuclei production in relativistic heavy-ion collisions

Using the nucleon coalescence model based on kinetic freeze-out nucleons from the 3D MUSIC+UrQMD and the 2D VISHNU hybrid model with a crossover equation of state, we study the multiplicity dependence of deuteron (d) and triton (t) production from central to peripheral Au+Au collisions at sNN= 7.7, 14.5, 19.6, 27, 39, 62.4 and 200 GeV and Pb+Pb at sNN=2.76 TeV, respectively. It is found that the ratio NtNp/Nd2 of the proton yield Np, deuteron yield Nd and triton yield Nt exhibits a scaling behavior in its multiplicity dependence, i.e., decreasing monotonically with increasing charged-particle multiplicity. A similar multiplicity scaling of this ratio is also found in the nucleon coalescence calculation based on kinetic freeze-out nucleons from a multiphase transport (AMPT) model. The scaling behavior of NtNp/Nd2 can be naturally explained by the interplay between the sizes of light nuclei and the nucleon emission source. We further argue that the multiplicity scaling of NtNp/Nd2 can be used to validate the production mechanism of light nuclei, and the collision energy dependence of this yield ratio can further serve as a baseline in the search for the QCD critical point in relativistic heavy-ion collisions.

Influence of the treatment of initialization and mean-field potential on the neutron to proton yield ratios

In this work, we first investigate how to reproduce and how well one can reproduce the Woods-Saxon density distribution of initial nuclei in the framework of the improved quantum molecular dynamics model. Then, we propose a new treatment for the initialization of nuclei which is correlated with the nucleonic mean-field potential by using the same potential energy density functional. In the mean field potential, the three-body force term is accurately calculated. Based on the new version of the model, the influences of precise calculations of the three-body force term, the slope of symmetry energy, the neutron-proton effective mass splitting, and the width of the wave packet on heavy ion collision observables, such as the neutron to proton yield ratios for emitted free nucleons $[R(n/p)]$ and for coalescence invariant nucleons $[{R}_{ci}(n/p)]$ for $^{124}\mathrm{Sn}+^{112}\mathrm{Sn}$ at the beam energy of 200 MeV per nucleon, are discussed. Our calculations show that the spectra of neutron to proton yield ratios $[R(n/p)]$ can be used to probe the slope of symmetry energy ($L$) and the neutron-proton effective mass splitting. In detail, the $R(n/p)$ in the low kinetic energy region can be used to probe the slope of symmetry energy ($L$). With a given $L$, the inclination of $R(n/p)$ to kinetic energy (${E}_{k}$) can be used to probe the effective mass splitting. In the case where the neutron-proton effective mass splitting is fixed, $R(n/p)$ at high kinetic energy can also be used to learn the symmetry energy at suprasaturation density.

Fermion Pair Dynamics in Open Quantum Systems.

Three-body decay is a rare decay mode observed in a handful of unbound rare isotopes. The angular and energy correlations between emitted nucleons are of particular interest, as they provide invaluable information on the interplay between structure and reaction aspects of the nuclear open quantum system. To study the mechanism of two-nucleon emission, we developed a time-dependent approach that allows us to probe emitted nucleons at long times and large distances. We successfully benchmarked the new method against the Green's function approach and applied it to low-energy two-proton and two-neutron decays. In particular, we studied the interplay between initial-state nucleon-nucleon correlations and final-state interaction. We demonstrated that the time evolution of the two-nucleon wave function is strongly impacted by the diproton/dineutron dynamics and that the correlations between emitted nucleons provide invaluable information on the dinucleon structure in the initial state.

Final state interaction in the pn and nn decay channels of $$^4_\varLambda $$He

We study the effects of final state interactions in the non-mesonic weak decay $\Lambda N \rightarrow nN$ (n is a neutron and N is either a neutron or a proton) of the hypernucleus $_\Lambda^4$He. Using a three-body model the effects of distortion of the interaction of the emitted nucleon pair with the residual nucleus is considered. We also study the influence of the final state interaction between the emitted nucleons using the Migdal-Watson model. The effect of spin symmetries in the final state of the pair is also considered. Based on our calculations, we conclude that final state interactions play a minor role in the kinetic energy spectrum of the emitted nucleon pair.

Two-stage description of charge distributions in 56Fe+p spallation reactions at 0.3-1.5 GeV/A

Spallation reactions are studied in the framework of a two-stage model. The intra-nuclear cascade (INC) stage of the reaction is simulated with the code ISABEL and a Constrained Molecular Dynamics (CoMD) code. The de-excitation of the highly excited pre-fragments is described with the multi-sequential binary decay code MECO, based on a generalized Weisskopf-Ewing formalism. Emission of nucleons, gamma rays and IMFs in their ground, excited bound and unbound states is considered. Calculated cross sections are compared with the experimental charge distributions of 56Fe + p spallation reaction products studied at GSI with the fragment separator FRS in the energy-range 0.3 − 1.5 GeV/A. At all bombarding energies, a good description of the experimental data is obtained with ISABEL-MECO using a global set of parameters. A preliminary calculation with CoMD-MECO at 1GeV/A is discussed.

Sequential binary decay of light nuclear systems: An assessment of statistical model codes

The sequential decay of excited nuclei is described as a succession of binary processes involving fragments in their ground, excited-bound and unbound states. Primary together with secondary decays lead to the final mass and charge distributions. Asymmetric mass splittings involving nucleon emission up to symmetric binary ones are treated within the Weisskopf evaporation formalism, in a unified manner. This procedure was imple- mented in the Monte-Carlo multi-step statistical model code MECO (Multisequential Evaporation COde). We study the evolution of the calculated final mass and charge distributions from 40Ar* as a function of the excitation energy, up to complete dissociation. Our results are compared with the predictions of statistical evaporation codes based on different assumptions for the compound nucleus decay.

Sequential binary decay of highly excited 40Ar*

The sequential statistical binary decay of the highly excited compound nucleus 40Ar* is described with an extended evaporation formalism implemented in a Monte-Carlo multi-step statistical model code. Asymmetric mass splittings involving nucleon emission up to symmetric binary ones are treated within the evaporation formalism, in a unified manner. Emission of heavy fragments in their ground and excited (particle-bound or unbound) states is considered. The evolution of the final mass distributions from 40Ar* is studied as a function of the initial excitation energy, in the range from 45 up to 405 MeV. The population of final states originating from the decay of intermediate mass fragments in particle-bound and particle-unbound states (side-feeding) is discussed. Results are compared with an alternative description in which the time-dependent decay process is described by rate equations for the generation of different fragment species.

Wounded nucleon, quark, and quark-diquark emission functions versus experimental results from the BNL Relativistic Heavy Ion Collider at sNN=200 GeV

Using the wounded nucleon, quark, and quark-diquark models, we extract the wounded source emission functions from the PHOBOS data for d+Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV. We apply these models to compute charged particle multiplicity distributions as functions of pseudorapidity for p+p, p+Al, p+Au, d+Au, $^3$He+Au, Cu+Cu, Cu+Au, Au+Au, and U+U collisions at the same energy and compare them with experimental data from the PHOBOS and PHENIX collaborations. In symmetric collisions of heavy nuclei, the obtained distributions differ among the tested models. On the other hand, in asymmetric collisions, all three models give essentially the same distributions. The wounded quark-diquark and quark models are in reasonable agreement with data for all the investigated systems.

High pt squeezed-out n/p ratio as a probe of Ksym of the symmetry energy

By involving the constraints of the slope of nuclear symmetry energy $L$ into the question of determination of the high-density symmetry energy, one needs to probe the curvature of nuclear symmetry energy $K_{\rm{sym}}$. Based on the Isospin-dependent Boltzmann-Uehling-Uhlenbeck (IBUU) transport model, effects of the curvature of nuclear symmetry energy on the squeezed-out nucleons are demonstrated in the semi-central Au+Au reaction at 400 and 600 MeV/nucleon. It is shown that the squeezed-out isospin-dependent nucleon emissions at high transverse momenta are sensitive to the curvature of nuclear symmetry energy. The curvature of nuclear symmetry energy at saturation density thus can be determined by the high momentum squeezed-out isospin-dependent nucleon emissions experiments from the semi-central Au+Au reaction at 400 or 600 MeV/nucleon.