Nucleon Emission Research Articles

Cluster-Îş competition in the Iwamoto-Harada-Bisplinghoff model

The exciton model of pre-equilibrium decay was originally formulated for the emission of nucleons, and the other kinds of ejectiles have been introduced afterward. However, clusters and gammas have not been yet treated simultaneously in competition. In connection with series of experiments using the ʼʼOslo methodʼʼ, we feel that such extension of calculations may be useful and we try to fill in this gap. To this aim, we employ the updated version of the Iwamoto-Harada-Bisplinghoff model.

New BRC neutron evaluations of actinides with the TALYS code: Modelization and first validation tests

The reader may have a look on references [1–3,5] for more details.Over the last five years, new evaluations of plutonium and uranium have been performed at Bruyeeres-le-Châtel (BRC) from the resolved resonance region up to 30MeV. Only nuclear reactions models have been used to build these evaluations. Total, shape elastic and direct inelastic cross sections are obtained from a coupled channel model using a dispersive optical potential (BRC, [13]) devoted to actinides. All the other cross sections are calculated owing to the Hauser-Fesbach theory (TALYS code [4]).We take particular care over the fission channel. For uranium isotopes, a triple-humped barrier [3] is required in order to reproduce accurately the variations of the experimental fission cross sections. As not commonly expected, we show [5] that the effect of the class II or class III states located in the wells of the aforementioned fission barrier provide sometimes an anti-resonant transmission rather than a resonant. With increasing neutron incident energy, a lot of residual nuclei produced by nucleon emission lead to fission also. All available experimental data assigned to the various fission mechanisms of the same nucleus are used to define its fission barrier parameters. As a result of this approach, we are now able to provide consistent evaluations for a large series of isotopes. Of course, our new evaluations have been tested against integral data.

Neutron/proton ratio of nucleon emissions as a probe of neutron skin

The dependence between neutron-to-proton yield ratio (Rnp) and neutron skin thickness (δnp) in neutron-rich projectile induced reactions is investigated within the framework of the Isospin-Dependent Quantum Molecular Dynamics (IQMD) model. The density distribution of the Droplet model is embedded in the initialization of the neutron and proton densities in the present IQMD model. By adjusting the diffuseness parameter of neutron density in the Droplet model for the projectile, the relationship between the neutron skin thickness and the corresponding Rnp in the collisions is obtained. The results show strong linear correlation between Rnp and δnp for neutron-rich Ca and Ni isotopes. It is suggested that Rnp may be used as an experimental observable to extract δnp for neutron-rich nuclei, which is very significant to the study of the nuclear structure of exotic nuclei and the equation of state (EOS) of asymmetric nuclear matter.

Variation of nonequilibrium processes in thep+Nisystem with beam energy

The energy and angular dependence of double differential cross sections dsigma/dOmega dE were measured for p, d, t, 3,4He, 6,7Li, 7,9Be, and 10,11B produced in collisions of 0.175 GeV protons with Ni target. The analysis of measured dfferential cross sections allowed to extract total production cross sections for ejectiles listed above. The shape of the spectra and angular distributions indicate the presence of other nonequilibrium processes besides the emission of nucleons from the intranuclear cascade, and besides the evaporation of various particles from remnants of intranuclear cascade. These nonequilibrium processes consist of coalescence of nucleons into light charged particles during the intranuclear cascade, of the fireball emission which contributes to the cross sections of protons and deuterons, and of the break-up of the target nucleus which leads to the emission of intermediate mass fragments. All such processes were found earlier at beam energies 1.2, 1.9, and 2.5 GeV for Ni as well as for Au targets, however, significant differences in properties of these processes at high and low beam energy are observed in the present study.

Photoproduction of pion-nucleon pairs on nuclei in the Δ(1232)-resonance region

Data from three experiments performed at the linear accelerator in Saclay, the Tomsk synchrotron, and the MAMI accelerator in Mainz and devoted to studying single pion photoproduction on light nuclei that is accompanied by nucleon emission are analyzed. The behavior of respective experimental cross sections, which is inexplicable in the impulse approximation, is interpreted within a model where the formation of an intermediate quasibound isobar-nucleus state is followed by its decay to a final state involving a pion and a nucleon.

Instantaneous-shape sampling for calculation of the electromagnetic dipole strength in transitional nuclei

Electromagnetic dipole absorption cross sections of transitional nuclei with large-amplitude shape fluctuations are calculated in a microscopic way by introducing the concept of instantaneous-shape sampling, which is based on slow shape dynamics as compared with fast dipole vibrations. The dipole strength is calculated by means of the quasiparticle random-phase approximation (QRPA) for the instantaneous shapes, the probability of which is obtained by means of the interacting boson approximation. The calculations agree well with the experimental photoabsorption cross sections near the nucleon emission threshold, but they underestimate it at low energies. The cumulative cross sections for the region below the threshold are a factor of 2 too low.

The influence of Isospin Dependence of In-Medium NN Cross Sections on the Ratio of Emitted Neutrons to Protons in HICs

The influence of isospin dependence of in-medium nucleon-nucleon cross sections on the n/p ratios for emitted nucleons in reactions 96Zr+96Zr and 96Ru+96Ru at Eb = 400 AMeV is investigated by means of an improved quantum molecular dynamics model. Our results show that the high energy part of the spectra of the n/p ratios for emitted nucleons is sensitive to the isospin dependence of in-medium nucleon-nucleon cross sections for neutron-rich reaction systems. Therefore, we propose that the n/p ratio of emitted high energy nucleons in a very neutron-rich reaction system at several hundreds of AMeV can be taken as sensitive observables to constrain the isospindependence of in-medium nucleon-nucleon cross sections.

Momentum dependence of the nuclear mean field and multifragmentation in heavy-ion collisions

We report the consequences of implementing momentum dependent interactions (MDI) on multifragmentation in heavy-ion reactions over entire collision geometry. The evolution of a single cold nucleus using static soft equation of state and soft momentum dependent equation of state demonstrates that inclusion of momentum dependence increases the emission of free nucleons. However, no heavier fragments are emitted artificially. The calculations performed within the framework of quantum molecular dynamics approach suggest that MDI strongly influence the system size dependence of fragment production. A comparison with ALADiN experimental data justifies the use of momentum dependent interactions in heavy-ion collisions.

PHASE-OTI: A pre-equilibrium model code for nuclear reactions calculations

The present work focuses on a pre-equilibrium nuclear reaction code (based on the one, two and infinity hypothesis of pre-equilibrium nuclear reactions). In the PHASE-OTI code, pre-equilibrium decays are assumed to be single nucleon emissions, and the statistical probabilities come from the independence of nuclei decay. The code has proved to be a good tool to provide predictions of energy-differential cross sections. The probability of emission was calculated statistically using bases of hybrid model and exciton model. However, more precise depletion factors were used in the calculations. The present calculations were restricted to nucleon–nucleon interactions and one nucleon emission. Program summary Program title: PHASE-OTI Catalogue identifier: AEDN_v1_0 Program summary URL: http://cpc.cs.qub.ac.uk/summaries/AEDN_v1_0.html Program obtainable from: CPC Program Library, Queen's University, Belfast, N. Ireland Licensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.html No. of lines in distributed program, including test data, etc.: 5858 No. of bytes in distributed program, including test data, etc.: 149 405 Distribution format: tar.gz Programming language: Fortran 77 Computer: Pentium 4 and Centrino Duo Operating system: MS Windows RAM: 128 MB Classification: 17.12 Nature of problem: Calculation of the differential cross section for nucleon induced nuclear reaction in the framework of pre-equilibrium emission model. Solution method: Single neutron emission was treated by assuming occurrence of the reaction in successive steps. Each step is called phase because of the phase transition nature of the theory. The probability of emission was calculated statistically using bases of hybrid model [1] and exciton model [2]. However, more precise depletion factor was used in the calculations. Exciton configuration used in the code is that described in earlier work [3]. Restrictions: The program is restricted to single nucleon emission and nucleon–nucleon interactions. Running time: 5–30 minutes

Quark masses: An environmental impact statement

We investigate worlds that lie on a slice through the parameter space of the standard model over which quark masses vary. We allow as many as three quarks to participate in nuclei, while fixing the mass of the electron and the average mass of the lightest baryon flavor multiplet. We classify as congenial worlds that satisfy the environmental constraint that the quark masses allow for stable nuclei with charge one, six, and eight, making organic chemistry possible. Whether a congenial world actually produces observers capable of measuring those quark masses depends on a multitude of historical contingencies, beginning with primordial nucleosynthesis and including other astrophysical processes, which we do not explore. Such constraints may be independently superimposed on our results. Environmental constraints such as the ones we study may be combined with information about the a priori distribution of quark masses over the landscape of possible universes to determine whether the measured values of the quark masses are determined environmentally, but our analysis is independent of such an anthropic approach. We estimate baryon masses as functions of quark masses via first-order perturbation theory in flavor SU(3) breaking. We estimate nuclear masses as functions of the baryon masses using two separatemore » tools: for a nucleus made of two baryon species, when possible we consider its analog in our world, a nucleus with a similar binding energy, up to Coulomb contributions. For heavy nuclei or nuclei made of more than two baryons, we develop a generalized Weizsaecker semiempirical mass formula, in which strong kinematic flavor symmetry violation is modeled by a degenerate Fermi gas . We check for the stability of nuclei against fission, strong particle emission (analogous to {alpha} decay), and weak nucleon emission. For two light quarks with charges 2/3 and -1/3 , we find a band of congeniality roughly 29 MeV wide in their mass difference, with our own world lying comfortably away from the edges. We also find another, less robust region of congeniality with one light, charge -1/3 quark, and two heavier, approximately degenerate quarks with charges -1/3 and 2/3. No other assignment of light quark charges yields congenial worlds with two baryons participating in nuclei. We identify the region in quark mass space where nuclei would be made from three or more baryon species. We discuss issues relevant to the congeniality of such worlds, but a full characterization of them is left for future investigation.« less

Shell model formalism for all hypernuclei types: A guide to solving the nonmesonic weak decay puzzle

We extend to odd-odd core hypernuclei our independent particle shell model (IPSM) formalism developed previously for the evaluation of the ${\ensuremath{\Gamma}}_{\mathrm{NM}},{\ensuremath{\Gamma}}_{n/p}$, and ${a}_{\ensuremath{\Lambda}}$ hypernuclear weak decay observables. The present procedure reproduces the even-odd and even-even core results as particular cases. Adopting the standard strangeness-changing weak $\ensuremath{\Lambda}N\ensuremath{\rightarrow}\mathit{NN}$ transition potential with exchange of the complete pseudoscalar and vector meson octets ($\ensuremath{\pi},\ensuremath{\eta},K,\ensuremath{\rho},\ensuremath{\omega},{K}^{*}$) we get simple analytical expressions for all observables. Numerical values for ${}_{\ensuremath{\Lambda}}^{4}\mathrm{He},{}_{\ensuremath{\Lambda}}^{5}\mathrm{He},{}_{ \ensuremath{\Lambda}}^{11}\mathrm{B},{}_{ \ensuremath{\Lambda}}^{12}\mathrm{C},{}_{ \ensuremath{\Lambda}}^{16}\mathrm{O},{}_{ \ensuremath{\Lambda}}^{17}\mathrm{O}$, and ${}_{\ensuremath{\Lambda}}^{28}\mathrm{Si}$ hypernuclei are obtained and compared with available experimental data, putting special attention on the asymmetry parameter. We remark that, in the present form, the IPSM gives roughly the same value of ${a}_{\ensuremath{\Lambda}}$ for all hypernuclei in contradiction with experiments. We stress the necessity of introducing configuration mixing to go beyond the IPSM taking into account, in a more realistic way, nuclear structure effects. Moreover, one could to include more relevant degrees of freedom, even within the IPSM framework, like: (i) modifications of the exchange potential (two-pion, ${a}_{1}$ meson, $\ensuremath{\Delta}T=3/2$ terms of vector mesons, etc.), (ii) final state interactions accounting for the distortion of the plane waves of emitted nucleons, and (iii) two-nucleon induced decay, as possible ways to solve the puzzle.

SEQUENTIAL BINARY DECAY OF HIGHLY EXCITED NUCLEI

The decay of highly excited nuclei is described as a sequence of binary processes involving emission of fragments in their ground, excited-bound and unbound states. Primary together with secondary decay products lead to the final mass distributions. Asymmetric mass splittings involving nucleon emission up to symmetric binary ones are treated according to a generalized Weisskopf evaporation formalism. This procedure is implemented in the Monte-Carlo multi-step statistical model code MECO (Multisequential Evaporation COde). We examine the evolution of the calculated final mass distributions in the decay of a light compound nucleus, as the initial excitation energy increases towards the limits of complete dissociation. Comparisons are made with the predictions of the transition-stage theory, as well as a consistent Weisskopf treatment in which the decay process is described by rate equations for the generation of different fragment species.

On the mechanism of the photoproduction of pion-nucleon pairs on nuclei

The data from an experiment performed at the MAMI accelerator in Maintz in which the differential cross sections for the photoproduction of positive pions on the carbon nucleus accompanied by the emission of nucleons have been analyzed. The behavior of the experimental cross sections as functions of the pion energy and nucleon emission angle, which cannot be explained in the quasi-free approximation, is interpreted in a model where an intermediate quasi-bound isobar nuclear state is formed and decays with the emission of a pion and a nucleon.

Effect of entrance-channel asymmetry on the isospin dependence of nucleon emission in heavy ion collisions**Supported by National Natural Science Foundation of China (10575119, 10775061), Knowledge Innovation Project of Chinese Academy of Sciences (KJCX3-SYW-N2), Major State Basic Research Developing Program of China (2007CB815004), CAS/SAFEA International Partnership Program for Creative Research Teams (CXTD-J2005-1), and the Asia-Link Project of the European Commission (CN/ASIA-LINK/008(94791))

Using the isospin- and momentum-dependent hadronic transport model IBUU04, we have investigated the influence of the entrance-channel isospin asymmetry on the sensitivity of the pre-equilibrium neutron/proton ratio to symmetry energy in central heavy-ion collisions induced by high-energy radioactive beams. Our analysis and discussion are based on the dynamical simulations of the three isotopic reaction systems 132Sn+124Sn, 124Sn+112Sn and 112Sn+112Sn which are of the same total proton number but different isospin asymmetry. We find that the kinetic-energy distributions of the pre-equilibrium neutron/proton ratio are quite sensitive to the density-dependence of symmetry energy at incident beam energy E/A = 400 MeV, and the sensitivity increases as the isospin asymmetry of the reaction system increases.

Conservation properties in the time-dependent Hartree Fock theory

We discuss the conservation of angular momentum in nuclear time-dependent Hartree-Fock calculations for a numerical representation of wave functions and potentials on a three-dimensional Cartesian grid. Free rotation of a deformed nucleus performs extremely well even for relatively coarse spatial grids. Heavy ion collisions produce a highly excited compound system associated with substantial nucleon emission. These emitted nucleons reach the bounds of the numerical box which leads to a decrease of angular momentum. We discuss strategies to distinguish the physically justified loss from numerical artifacts.

Single- and double-pion production in nucleon collisions on the nucleon and on nuclei—the ABC effect and its origin in a dibaryonic resonance

The ABC effect–an intriguing low-mass enhancement in the π π invariant mass spectrum–is known from inclusive measurements of two-pion production in nuclear fusion reactions. First exclusive measurements carried out at CELSIUS-WASA for the fusion reactions leading to d or 3He reveal this effect to be a σ -channel phenomenon associated with the formation of a Δ Δ system in the intermediate state and combined with a resonance-like behavior in the total cross-section. Together with the observation that the differential distributions do not change in shape over the resonance region the features fulfill the criteria of an isoscalar s-channel resonance in p n and N N π π systems, if the two emitted nucleons are bound. It obviously is robust enough to survive in nuclei as a dibaryonic resonance configuration. In this context also the phenomenon of N Δ resonances is reexamined.

The quenching of nucleon yields in the nonmesonic weak decay of Λ -hypernuclei and the three-body weak decay process

Recent exclusive coincidence measurements of non-mesonic weak decays (NMWD) reported for the ratio of the partial decay width of neutron-induced-to-proton-induced NMWD, Γ n/Γ p , values of 0.45±0.11±0.03 and 0.51±0.13±0.04 for Λ 5 He and Λ 12 C, respectively. These observations agree well with the improved theoretical Γ n /Γ p ratios which are in the range of 0.3–0.7. It appears that the long-standing discrepancy between the experimental and theoretical values of Γ n /Γ p has finally been solved. However, when compared to the results of intra-nuclear cascade (INC) calculations, the observed numbers of both single nucleons and coincident nucleon pairs are strongly quenched. The quenching of the proton yield observed previously has been interpreted as an increase of the Γ n /Γ p ratio. On the other hand, significant contributions from the two-nucleon-induced three-body process ΛNN → nNN are predicted. Indeed, the angular correlation of the emitted nucleon pairs in the NMWD of Λ 12 C showed not only decay events in back-to-back kinematics, but also events with non-back-to-back kinematics. In this paper we show that the difficulties to extract the correct Γ n /Γ p ratio from the proton spectra is related to the three-body weak-interaction process which strongly quenches the nucleon yields.

Chiral invariant phase space event generator

The Geant4 CHIPS model simulates both decay and nuclear capture of negative muons. In hadron level models the muon is captured by a proton, p(μ, vμ)n, and the neutron transfers to the nucleus only 5 MeV, which is not enough to split a nucleon from the nucleus, while the measured spectra of emitted nucleons reach 80 MeV. In CHIPS, which considers asymptotically free quark-partons, the muon can be captured by a quark, u(μ, vμ)d, and transfers 52 MeV to the nucleus. This capture mechanism fits the main part of the nucleon spectra, while the high-energy part of the spectra is not fitted. A precise fit of the high-energy part of the nucleon spectra can be made if the muon decay μ → dŪν μ is taken into account.

Particle production in nucleon induced reactions above 14 MeV with an intranuclear cascade model

We present an intranuclear cascade (INC) model which includes the dynamics of the cascade particles and uses in-medium $\mathit{NN}$ elastic cross sections. Our model is still based on the impulse approximation and does not have the complexity of other sophisticated nuclear models. Nevertheless, its results of angle-energy distributions of emitted nucleons in continuum are in good agreement with experimental data for nucleon induced reactions from low intermediate energy to high energy, between 14 MeV and 1.6 GeV, and for light to heavy target nuclei. Comparisons with results of Bertini's INC model and of a previous version of our INC model show the clear contribution of the combined effects of better dynamics and the in-medium $\mathit{NN}$ cross sections. From our results, we conclude that our INC model effectively includes the preequilibrium emission.

Sensitive Probe for Symmetry Potential

Based on both very obvious isospin effect of the neutron–proton numberratio of nucleon emissions (n/p)nucl on symmetry potential and(n/p)nucl's sensitive dependence on symmetry potential in thenuclear reactions induced by halo-neutron projectiles, compared to thesame mass stable projectile, probing symmetry potential is investigatedwithin the isospin-dependent quantum molecular dynamics with isospin andmomentum-dependent interactions for different symmetry potentialsU1sym and U2sym. It is found that the neutron-haloprojectile induces very obvious increase of (n/p)nucl andstrengthens the dependence of (n/p)nucl on the symmetrypotential for all the beam energies and impact parameters, compared tothe same mass stable projectile under the same incident channelcondition. Therefore (n/p)nucl induced by the neutron-haloprojectile is a more favourable probe than the normal neutron-rich andneutron-poor projectiles for extracting the symmetry potential.