Nucleon Emission Research Articles

Isospin effect in the nuclear reaction induced by neutron-halo nuclei

We study the average property of the isospin effect of reaction induced by halo-neutron nuclei 8He and 10He in the intermediate energy heavy ion collisions using the isospin-dependent quantum molecular dynamics model (IQMD). This study is based on the extended neutron density distribution for the halo-neutron nuclei, which includes the average property of the isospin effect of reaction mechanism and loose inner structure. The extended neutron density distribution brings an important isospin effect into the average property of reaction mechanism because the interaction potential and nucleon-nucleon(N-N)cross section in IQMD model depend sensitively on the density distribution of colliding system. In order to see clearly the average properties of reaction mechanism induced by halo-neutron nuclei we also compare the results for the neutron-halo colliding systems with those for the corresponding stable colliding systems under the same incident channel condition. We found that the extended density distribution for the neutron-halo projectile brings an important isospin effect to the reaction mechanism, which leads to the decrease of nuclear stopping R, yet induces obvious increase of the neutron-proton ratio of nucleon emissions and isospin fractionation ratio for all beam energies studied in this work, compared to the corresponding stable colliding system. In this case, nuclear stopping, the neutron-proton ratio of nucleon emissions and isospin fractionation ratio induced by halo-neutron nuclei can be used as possible probes for studying the average property of the isospin effect of reaction mechanism and extracting the information of symmetry potential and in-medium N-N cross section by the neutron-halo nuclei in heavy ion collisions.

Evidence for Strong Dominance of Proton-Neutron Correlations in Nuclei

We analyze recent data from high-momentum-transfer (p, pp) and (p, ppn) reactions on carbon. For this analysis, the two-nucleon short-range correlation (NN-SRC) model for backward nucleon emission is extended to include the motion of the NN pair in the mean field. The model is found to describe major characteristics of the data. Our analysis demonstrates that the removal of a proton from the nucleus with initial momentum 275-550 MeV/c is 92(+8/-18) % of the time accompanied by the emission of a correlated neutron that carries momentum roughly equal and opposite to the initial proton momentum. This indicates that the probabilities of pp or nn SRCs in the nucleus are at least a factor of 6 smaller than that of pn SRCs. Our result is the first estimate of the isospin structure of NN-SRCs in nuclei, and may have important implication for modeling the equation of state of asymmetric nuclear matter.

Average Property of Isospin Effects Induced by Neutron-HaloNuclei

We study the average property of the isospin effects of reactionmechanism induced by neutron-halo nuclei within theisospin-dependent quantum molecular dynamics model. Wefind that the extended neutron density distribution for theneutron-halo projectile brings an important isospin effect intothe reaction mechanism, which induces the decrease of nuclearstopping R; however, it induces the obvious increases of theneutron-proton ratio of nucleon emissions (n/p)nucl for allof the beam energies in this work, compared to the same mass stablecolliding system.

Coincidence measurement of the nonmesonic weak decay of 12ΛC

We have measured the angular correlation of the pair nucleons np and nn emitted from the nonmesonic weak decay (NMWD) of ^{12}_{Lambda}C produced via the (pi^+,K^+) reaction in coincidence measurement. The Lambda p -> np and Lambda n -> nn modes were clearly identified by measuring the back-to-back correlation of the emitted nucleon pairs which is the characteristic of two-body kinematics. From the measured nucleon pair numbers N_{nn} and N_{np}, the ratio Gamma_n/Gamma_p of the partial decay widths Gamma_n(Lambda n -> nn) and Gamma_p(Lambda p -> np) of ^{12}_{Lambda}C was extracted to be 0.51+-0.13(stat) +- 0.05(syst); this result is almost free from the ambiguity due to the nuclear final state interaction and 3-body decay process, which were inherent in the previous results. The obtained Gamma_n/Gamma_p ratio of ^{12}_{Lambda}C (p-shell) is close to that of ^5_{Lambda}He (s-shell). The results are consistent with those of recent theoretical calculations.

Neutron and Proton Transverse Emission Ratio Measurements and the Density Dependence of the Asymmetry Term of the Nuclear Equation of State

Recent measurements of preequilibrium neutron and proton transverse emission from (112,124)Sn+(112,124)Sn reactions at 50 MeV/A have been completed at the National Superconducting Cyclotron Laboratory. Free nucleon transverse emission ratios are compared to those of A=3 mirror nuclei. Comparisons are made to Boltzmann-Uehling-Uhlenbeck (BUU) transport calculations and conclusions concerning the density dependence of the asymmetry term of the nuclear equation of state at subnuclear densities are made. Comparison to BUU model predictions indicate a density dependence of the asymmetry energy that is closer to a form in which the asymmetry energy increases as the square root of the density for the density region studied. A coalescent-invariant analysis is introduced as a means of reducing suggested difficulties with cluster emission in total nucleon emission.

Probing the symmetry energy and the degree of isospin equilibrium

The rapidity dependence of the single and double neutron-to-proton ratios of nucleon emission from isospin-asymmetric but mass-symmetric reactions Zr+Ru and Ru+Zr at the energy range $100A\ensuremath{\sim}800A$ MeV and impact parameter range $0\ensuremath{\sim}8$ fm is investigated. The reaction system with isospin asymmetry and mass symmetry has the advantage of simultaneously showing the dependence on symmetry energy and the degree of the isospin equilibrium. We find that the beam-energy and impact-parameter dependences of the slope parameter of the double neutron-to-proton ratio (${F}_{D}$) as a function of rapidity are quite sensitive to the density dependence of symmetry energy, especially at energies ${E}_{b}\ensuremath{\sim}400A$ MeV and reduced impact parameters around 0.5. Here, the symmetry energy effect on ${F}_{D}$ is enhanced, as compared to the single neutron-to-proton ratio. The degree of equilibrium with respect to isospin (isospin mixing) in terms of ${F}_{D}$ is addressed, and its dependence on symmetry energy is also discussed.

Inclusive nucleon emission induced by quasi-elastic neutrino-nucleus interactions

A previous model on inclusive charged-current quasi-elastic nuclear reactions is extended to include neutral- and charged-current nucleon emission reactions. The problem of outgoing nucleon propagation is treated by means of a Monte Carlo simulation.

Inclusive nucleon emission induced by quasielastic neutrino-nucleus interactions

A previous model on inclusive charged-current quasi-elastic nuclear reactions [J. Nieves, J.E. Amaro and M. Valverde, Phys. Rev. C 70 (2004) 055503] is extended to include neutral- and charged-current nucleon emission reactions. The problem of outgoing nucleon propagation is treated by means of a Monte Carlo simulation.

Double neutron/proton ratio of nucleon emissions in isotopic reaction systems as a robust probe of nuclear symmetry energy

The double neutron/proton ratio of nucleon emissions taken from two reaction systems using four isotopes of the same element, namely, the neutron/proton ratio in the neutron-rich system over that in the more symmetric system, has the advantage of reducing systematically the influence of the Coulomb force and the normally poor efficiencies of detecting low energy neutrons. The double ratio thus suffers less systematic errors. Within the IBUU04 transport model the double neutron/proton ratio is shown to have about the same sensitivity to the density dependence of nuclear symmetry energy as the single neutron/proton ratio in the neutron-rich system involved. The double neutron/proton ratio is therefore more useful for further constraining the symmetry energy of neutron-rich matter.

Fast nucleon emission as a probe of the momentum dependence of the symmetry potential

In this article we investigate the structure of the non-local part of the symmetry term, that leads to a splitting of the effective masses of protons and neutrons in asymmetric matter. Based on microscopic transport simulations we suggest some rather sensitive observables in collisions of neutron-rich (unstable) ions at intermediate ($RIA$) energies. In particular we focus the attention on pre-equilibrium nucleon emissions. We discuss interesting correlations between the N/Z content of the fast emitted particles and their rapidity or transverse momentum, that show a nice dependence on the prescription used for the effective mass splitting.

Influence of reaction channel on the isomeric cross-section ratio

Summary The influence of reaction channel on the isomeric cross-section ratio was investigated by analysing the experimental data on the reactions 52Cr(p, n)52m,gMn, 52Cr(3He, t)52m,gMn, 54Fe(d, α)52m,gMn, 54Fe(n, t)52m,gMn and 54Fe(3He, α p)52m,gMn over the incident particle energy range extending up to 35 MeV. The influence is most pronounced when the channels differ widely, for example (p, n) and (3He, t) processes, i.e. when the reaction mechanisms are different. The nuclear model calculational code EMPIRE-II described the isomeric cross-section ratio rather well in the case of a simple nucleon emission reaction, but not when complex reaction channels were involved.

Investigation of pn correlations in 4Hep interactions at a momentum of 5 GeV/c

Proton-neutron correlations in 4Hep interactions are studied in an exclusive experiment by using a 2-m bubble chamber exposed to a 5-GeV/c beam of α particles (the kinetic energy of the protons in the nucleus rest frame is Tp = 620 MeV). Data on the production of pn pairs in 4π geometry for three channels, where it is possible to reconstruct the neutron momentum unambiguously, are used to determine the pn correlation function in 4Hep interactions. The experimental results are compared with the predictions of a modified Lednicky-Lyuboshitz model. The value obtained for the root-mean-square radius of the pn-emission region is Rpn = 2.1 ± 0.3 fm. The dependence of the correlation function on the modulus of the total momentum of the emitted nucleon pair and on the direction of the momentum transfer is studied. An indication that the emission of a pn pair proceeds predominantly through the production of a virtual deuteron is obtained.

Several observables sensitive to the symmetry energy in heavy-ion collisions induced by high energy radioactive beams

Using an isospin- and momentum-dependent hadronic transport model, effects of the symmetry energy on several observables in heavy-ion collisions induced by radioactive beams at intermediate energies are studied. It is found that rapidity distribution of the isospin asymmetry of nucleon emissions, transverse momentum distribution of the ratio of free neutrons to protons at mid-rapidity, kinetic energy distribution of the ratio of π−/π+ as well as evolution of the isospin fractionation are all sensitive to the symmetry energy.

EXCITATION FUNCTIONS OF ALPHA INDUCED REACTIONS ON NATURAL NICKEL UP TO 50 MeV

Excitation functions have been measured for alpha-particle induced reactions on natural nickel leading to the production of 62 Zn , 61 Cu , 56,57 Ni and 56,58 Co radioisotopes up to 50 MeV by the activation method and gamma-ray spectrometry. Most of these radioisotopes are produced by one or two alpha-particle emissions and a few nucleon emissions. The excitation functions were compared with semi-classical models codes, like ALICE/90, which includes a shell corrected level density formulation and the COMPLET, which takes into account angular momentum removal effects. In the case of multi-nucleon emission channels, ALICE/90 gives fairly good agreement. The COMPLET code on the other hand, explains the α-particle emission channels in a more convincing manner.

Isospin dependence of nucleon emission and radial flow in heavy-ion collisions induced by high energy radioactive beams

Using an isospin- and momentum-dependent transport model we study the emission of free nucleons and nuclear radial flow in central heavy-ion collisions induced by high-energy radioactive beams. The midrapidity neutron/proton ratio and its transverse momentum dependence are found to be very sensitive to the high-density behavior of nuclear symmetry energy. The nuclear radial flow, however, depends only weakly on the symmetry energy.

Spin-dependent neutrino-induced nucleon knockout

We study neutrino-induced nucleon knockout off atomic nuclei and examine the polarization properties of the ejectile. A detailed study of the spin dependence of the outgoing nucleon is presented. The numerical results are derived within a nonrelativistic plane-wave impulse-approximation approach. Our calculations reveal large polarization asymmetries and clear dissimilarities between neutrino- and antineutrino-induced reactions. They reflect the fact that neutrino-induced nucleon knockout is dominated by the transverse axial current and gains its major contributions from forward nucleon emission and backward lepton scattering.

Double-Differential Cross Section of 5He Emission

The probability of 5He particle emission has been affirmedtheoretically [J.S. Zhang, Science in China G47 (2004) 137].In order to describe the 5He emission, thetheoretical formula of the double-differential cross sectionof emitted 5He is to be established. Based on the pick-up mechanism,used for calculating the formula of d, t, 3He, αemissions, the theoretical formula of double-differential crosssection of 5He is obtained, which is expressed in the form ofLegendre coefficients. In the case of low incident energies,the configuration [J.S. Zhang, Science in China G47 (2004) 137;J.S. Zhang, Commun. Theor. Phys. (Beijing, China)39 (2003) 83] is the dominant part in the reactionprocesses. The calculated result indicates that the forward peakedangular distribution of the composite particle emission is weakerthan that of the emitted single nucleon due to pick-up nucleonfrom the Fermi sea. As an example, the reactions of n+14N havebeen calculated, and the Legendre coefficients of d, t, 3He,α, 5He emissions are obtained respectively. The resultsshow that the forward tendency is decided by the average momentum pernucleon in the emitted composite particles. The larger the averagemomentum is, the stronger the forward tendency is.

Probing the density dependence of the symmetry potential with peripheral heavy-ion collisions

The peripheral heavy-ion collisions of $^{112, 124}Sn+ ^{86}Kr$ at $E_{b}= 25AMeV$ are studied by means of the Improved Quantum Molecular Dynamics Model(ImQMD). It is shown that the slope of the average N/Z ratio of emitted nucleons vs impact parameters for these reactions is very sensitive to the density dependence of the symmetry energy. Our study also shows that the yields of $^{3}H$ and $^{3}He$ decrease with impact parameters and slope of the yield of $^{3}H$ vs impact parameters as well as the ratio of Y($^{3}H$)/Y($^{3}He$) depend on the symmetry potential strongly for peripheral heavy-ion collisions.

Weak response of nuclei

We discuss some differences and similarities between electron and neutrino scattering off atomic nuclei. We find that, in the giant resonance region, the two processes excite different nuclear modes, therefore the weak and the electromagnetic nuclear responses are rather different. In any case, the scattering of electrons and photons is the best guide we have to test the validity of our nuclear models and their prediction power. The experience in describing electromagnetic excitations of the nucleus, suggests that, when the nucleus is excited in the continuum, the re-interaction between the emitted nucleon and the remaining nucleus should not be neglected. A simple model taking into account this final state interaction is proposed, and applied to the neutrino scattering off 16O nucleus.

Effects of rescattering in(e,e′p)reactions within a semiclassical model

The contribution of rescattering to final state interactions in (e,e'p) cross sections is studied for medium and high missing energies using a semiclassical model. This approach considers two-step processes that lead to the emission of both nucleons. The effects of nuclear transparency are accounted for in a Glauber inspired approach and the dispersion effects of the medium at low energies are included. It is found that rescattering is strongly reduced in parallel kinematics. At high missing energy and momenta, the distortion of the short-range correlated tail of the spectral function is dominated by a rearrangement of that strength itself. In perpendicular kinematics, a further enhancement of the experimental yield is due to strength that is originally in the mean field region. This contribution becomes negligible at large missing momenta.