Isospin Transport Research Articles

Experimental study of isospin transport with Ca40,48+Ca40,48 reactions at 35 MeV/nucleon

We investigate the isospin transport with 40,48 Ca+ 40,48 Ca reactions at 35 MeV/nucleon, measured with the coupling of the VAMOS high acceptance spectrometer and the INDRA charged particle multidetector. Using the quasi-projectile remnant measured with VAMOS and carefully selected light-charged-particles measured in coincidence with INDRA, a reconstruction method is applied to estimate the excited quasi-projectile (QP) on an event-by-event basis. The isospin diffusion is investigated using the isospin transport ratio with the asymmetry $\delta$ = (N -- Z)/A of the projectile as an isospin-sensitive observable and the total transverse energy of Z $\le$ 2 nuclei for experimental centrality sorting. The experimental isospin transport ratios present, for both the reconstructed QP and its remnant, a clear evolution towards isospin equilibration with increasing dissipation of the collision while the full equilibration is not reached. A smoother evolution with less discrepancies between the two mixed systems in the case of the reconstructed QP is also observed. The isospin migration is investigated using the neutron-to-proton ratio of the light-charged-clusters and the velocity of the QP remnant as a sorting parameter. More particularly, we focused on an angular region centered around the mid-rapidity of the reaction so as to characterize the low-density neck emissions. A systematic neutron-enrichment is observed and interpreted as a consequence of isospin migration, more particularly for the symmetric systems which present no isospin gradient between the projectile and the target. We also noticed that the ^{2}H and ^{4}He particles exhibit very close multiplicities independently of the sorting variable for the symmetric systems.

First results from the INDRA-FAZIA apparatus on isospin diffusion in Ni58,64+Ni58,64 systems at Fermi energies

An investigation of the isospin equilibration process in the reactions Ni58,64+Ni58,64 at two bombarding energies in the Fermi regime (32 and 52 MeV/nucleon) is presented. Data have been acquired during the first experimental campaign of the coupled INDRA-FAZIA apparatus in GANIL. Selecting from peripheral to semicentral collisions, both the neutron content of the quasiprojectile residue and that of the light ejectiles coming from the quasiprojectile evaporation have been used as probes of the dynamical process of isospin diffusion between projectile and target for the asymmetric systems. The isospin transport ratio technique has been employed. The relaxation of the initial isospin imbalance with increasing centrality has been clearly evidenced. The isospin equilibration appears stronger for the reactions at 32 MeV/nucleon, as expected due to the longer projectile-target interaction time than at 52 MeV/nucleon. Coherent indications of isospin equilibration come from the quasiprojectile residue characteristics and from particles ascribed to the quasiprojectile decay.6 MoreReceived 23 May 2022Accepted 26 July 2022DOI:https://doi.org/10.1103/PhysRevC.106.024603©2022 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasEquations of state of nuclear matterLow & intermediate energy heavy-ion reactionsNuclear fragmentationNuclear reactionsTechniquesSpectrometers & spectroscopic techniquesNuclear Physics

Constraining the density dependence of the symmetry energy: the isospin transport ratio revisited

The isospin diffusion of the quasi-projectile formed in the on reactions in the Fermi energy domain is investigated in the framework of the Boltzmann–Uehling–Uhlenbeck transport model. The well known isospin transport ratio observable is revisited, with the aim of insuring an optimal comparison between experimental data and theoretical calculations and reducing the present uncertainties in the extraction of empirical equation of state parameters. We show that isospin transport ratios are sensitive to all the low order isovector parameters (E sym, L sym and K sym, but the quantitative results depend on the choice of the isospin sensitive observable. We demonstrate that realistic models of the equation of state, covering the uncertainty that presently affects the theoretical description of neutron stars static observables, can be effectively discriminated by isospin diffusion experiments, provided the neutron to proton ratio of the projectile remnant is precisely measured as a function of centrality.

Isospin diffusion measurement from the direct detection of a quasiprojectile remnant

The neutron-proton equilibration process in 48 Ca+ 40 Ca at 35 MeV/nucleon bombarding energy has been experimentally estimated by means of the isospin transport ratio. Experimental data have been collected with a subset of the FAZIA telescope array, which permitted to determine Z and N of detected fragments. For the first time, the QP evaporative channel has been compared with the QP break-up one in a homogeneous and consistent way, pointing out to a comparable n-p equilibration which suggests close interaction time between projectile and target independently of the exit channel. Moreover, in the QP evaporative channel n-p equilibration has been compared with the prediction of the Antisymmetrized Molecular Dynamics (AMD) model coupled to the GEMINI statistical model as an afterburner, showing a larger probability of proton and neutron transfers in the simulation with respect to the experimental data.

Isospin transport phenomena for the systems Kr80+Ca40,48 at 35 MeV/nucleon

Experimental data concerning isospin transport phenomena for the systems Kr80+Ca40,48 at 35 MeV/nucleon are presented. Data have been collected with four FAZIA blocks; this data set is the same analyzed in S. Piantelli , Phys. Rev. C 101, 034613 (2020)2469-998510.1103/PhysRevC.101.034613. The isotopic composition of the QuasiProjectile residue (up to Z around 25) and of its decay products in the two reactions are compared, finding a neutron enrichment when the target is Ca48. The isotopic composition of the emitted light charged particles (LCP) and intermediate mass fragments (IMF) was also investigated in different windows of velocity. The obtained results have been compared with the prediction of the transport model AMD followed by the statistical code GEMINI (used as an afterburner), with different recipes for the density dependence of the symmetry energy term in the nuclear equation of state. A weak indication in favor of a stiff symmetry energy emerges when the fragments emitted at midvelocity are examined.

Influence of fast emissions and statistical de-excitation on the isospin transport ratio

Isospin transport ratio is a powerful method to estimate the neutron-proton (n-p) equilibration in heavy-ion collisions, and extensively used to obtain information on the asy-stiffness of the nuclear Equation of State. In fact such a ratio is expected to bypass any perturbations introducing a linear transformation of the chosen observable. In particular, it is supposed to overcome contributions due to emission, either of dynamical or statistical nature, from the primary fragments formed during the collisions. In this paper we explore the validity of this assumption, looking at the quasi-projectile n-p ratio ($N/Z$) in peripheral and semi-peripheral events for Ca+Ca reactions at 35\amev{}, simulated via the Antisymmetrized Molecular Dynamics transport model, coupled to different statistical decay codes. The statistical de-excitation of the primary fragments introduces a linear transformation at relatively high excitation energies (above 2\amev{}) when the residue approaches the Evaporation Attractor Line, while some effect is produced at lower excitation energies due to the occurrence of some non-linearities. As for fast emissions after the end of the projectile-target interaction it is shown that they introduce a non-linear transformation too.

Constraints on symmetry energy and n/p effective mass splitting with improved quantum molecular dynamics model

A new version of improved quantum molecular dynamics model that includes standard Skyrme interactions has been developed. Based on the new code,four commonly used parameter sets,SLy4,SkI2,SkM*and Gs are adopted in the improved quantum molecular dynamics model and the isospin sensitive observables,namely isospin transport ratios,single and double ratios of the yields of neutrons and protons are investigated. The isospin transport ratios are strongly sensitive to the slope of symmetry energy,and are not very sensitive to the nucleon effective mass splitting. On the other hand,the high energy neutrons and protons yields ratios from reactions at different incident energies provide a good observable to the momentum dependence of nucleon effective mass splitting. By comparing our calculations with the data,we find that the constrained L value(the slope of density dependence of symmetry energy) is about ~46 MeV when the Skyrme type interaction is considered in transport models,and the isospin diffusion data prefer to mn*mp*,but it is not a strong constraint with deep χ2minimum.

Reexamining the isospin-relaxation time in intermediate-energy heavy-ion collisions

Isospin relaxation times characterizing isospin transport processes between the projectile and the target with different $N/Z$ ratios and that between the neck and the spectator with different isospin asymmetries and densities in intermediate-energy heavy-ion collisions are studied within an isospin-dependent Boltzmann-Uehling-Uhlenbeck transport model using the lattice Hamiltonian approach. The respective roles and time scales of the isospin diffusion and drift as the major mechanisms of isospin transport in intermediate-energy heavy-ion collisions are discussed. Effects of nuclear symmetry energy and neutron-proton effective mass splitting on the isospin relaxation times are examined.

OSCAR: A new modular device for the identification and correlation of low energy particles

A new modular and high versatility hodoscope, OSCAR, has been developed and characterized. The aim of this hodoscope is to work as an ancillary detector of present large acceptance heavy ion detectors in specific angular regions where low thresholds and high granularities are needed. We discuss the capabilities of OSCAR in the ΔE–E identification of very low energy light particles, providing a precise map of the thickness uniformity of the ΔE (SSSSD, 20μm) stage and showing how the thickness gradient affects the identification of particles. Energy spectra of light identified particles produced in Ca+Ca collisions at 35AMeV are used to investigate isospin transport phenomena involving the emission of low energy particles from the quasi-target (QT) source in semi-peripheral nuclear collisions. The possibility to explore particle–particle correlations are also discussed.

Long-time drift of the isospin degree of freedom in heavy ion collisions

The drift of the isospin degree of freedom (IDOF) is experimentally visualized through the wide-range angular distribution and the beam energy dependence of the isospin composition of light charged particles (LCP) produced in heavy ion collisions. It is evidenced that the net isospin transport is presented in the whole process from early dynamic emission to the statistical evaporation lasting to very late stage. Due to the long-time accumulation of the isospin effect, the angular distribution of the relative neutron excess summing over the LCPs is utilized as a sensitive probe to the symmetry energy. With S fixed at 28.3 MeV, a soft density dependence with L=33–61 MeV (0.95 confidential level) at ρ0 is circumstantially deduced using an improved quantum molecular dynamics transport model attached with a statistical emission afterburner.

FAZIA applications

The present status and the perspectives of the FAZIA project are presented. The main achievements in terms of identification thresholds and isotopic resolution are discussed, together with the adopted technical solutions. The detector is particularly well suited for the investigation of isospin transport phenomena at intermediate beam energies; perspectives to reduce the identification thresholds to cope with lower energy ISOL beams are briefly introduced. Some experimental results concerning isospin transport effects obtained with a test telescope are presented. The study of isospin transport phenomena can give information on the symmetry energy term of the nuclear equation of state by comparing the experimental results on isospin related observables with the predictions of transport codes.

The InKiIsSY experiment at LNS: A study of size vs. isospin effects with124Xe+64Zn and124,112Sn+64,58Ni reactions

The systems 124Sn+64Ni and 112Sn+58Ni at 35 AMeV were analyzed in previous experiments, by using the 4π multi-detector CHIMERA in order to investigate the time-scale and mechanisms of fragments formation. By comparing the two systems, effects that have been correlated to Isospin transport in reaction dynamics and symmetry energy term of nuclear equation of state at sub-saturation density were found. Recently, in order to disentangle the Isospin effects from size effects the 124Xe+64Ni reaction was investigated at the same beam energy of 35 AMeV.

Isospin transport phenomena and odd-even staggering in84Kr+112,124Sn collisions at 35 AMeV

Experimental results concerning isospin transport phenomena and odd-even staggering in Z and N distributions are presented. Data refer to 84Kr+112,124Sn collisions at 35AMeV and they were collected with a high resolution FAZIA telescope, able to isotopically resolve ions up to Z ∼ 20. Evidences of isospin diffusion and drift obtained from the /Z behaviour of the detected fragments are discussed. The odd-even staggering both in Z and N is compared with experimental data available in literature, finding that it shows a common trend in different reaction types.

Symmetry energy dependence of long-timescale isospin transport

Isospin transport occurring within dinuclear projectile-like fragments (PLFs) produced in heavy- ion collisions is explored as a probe of the nuclear symmetry energy. Within the framework of the Constrained Molecular Dynamics model (CoMD), the existence of the long-lived dinuclear PLFs, for up to 800 fm/c, is observed. It is demonstrated that changes in the <N/Z> of the two fragments resulting from the breakup of the dinuclear PLF is due to isospin transport. The rate of the transport between the two fragments is shown to be dependent on the slope of the symmetry energy at saturation density. Comparison of the CoMD calculations with experimental data establish that the evolution of <N/Z> could be used to constrain the density dependence of the symmetry energy.

Learning about the nuclear symmetry energy through the lens of isospin transport

Learning about the nuclear symmetry energy through the lens of isospin transport

Timescale for isospin equilibration in projectile breakup

Isospin transport within a nuclear system is influenced by the density dependence of the symmetry energy. Studying isospin transport over relatively long times can therefore be a powerful tool to learn about the symmetry energy. The transiently deformed PLF* formed in an intermediate energy heavy-ion collision presents an opportunity to study isospin transport for a variety of initial configurations. Isospin transport is found to persist over several hundred fm/c , establishing the timescale for isospin equilibration.

The FAZIA project in Europe: R&amp;D phase

The goal of the FAZIA Collaboration is the design of a new-generation 4π detector array for heavy-ion collisions with radioactive beams. This article summarizes the main results of the R&D phase, devoted to the search for significant improvements of the techniques for charge and mass identification of reaction products. This was obtained by means of a systematic study of the basic detection module, consisting of two transmission-mounted silicon detectors followed by a CsI(Tl) scintillator. Significant improvements in ΔE-E and pulse-shape techniques were obtained by controlling the doping homogeneity and the cutting angles of silicon and by putting severe constraints on thickness uniformity. Purposely designed digital electronics contributed to identification quality. The issue of possible degradation related to radiation damage of silicon was also addressed. The experimental activity was accompanied by studies on the physics governing signal evolution in silicon. The good identification quality obtained with the prototypes during the R&D phase, allowed us to investigate also some aspects of isospin physics, namely isospin transport and odd-even staggering. Now, after the conclusion of the R&D period, the FAZIA Collaboration has entered the demonstrator phase, with the aim of verifying the applicability of the devised solutions for the realization of a larger-scale experimental set-up.

Isospin transport in84Kr+112,124Sn reactions at Fermi energies

Isospin transport phenomena in dissipative heavy ion collisions have been investigated at Fermi energies with a beam of 84Kr at 35AMeV. A comparison of the <N>/Z of light and medium products forward-emitted in the centre of mass frame when the beam impinges on a n-poor 112Sn and a n-rich 124Sn targets is presented. Data were collected by means of a three-layer telescope with very good performances in terms of mass identification (full isotopic resolution up to Z about 20 for ions punching through the first detector layer) built by the FAZIA Collaboration and located just beyond the grazing angle for both reactions. The <N>/Z of the decay products emitted when the n-rich target is used is always higher than that associated to the n-poor one. Since the detector was able to measure only fragments coming from the QuasiProjectile decay and/or neck emission, the observed behaviour can be ascribed to the isospin diffusion, driven by the isospin gradient between QuasiProjectile and QuasiTarget. Moreover, for light fragments the <N>/Z as a function of the lab velocity of the fragment increases when we move from the QuasiProjectile velocity to the centre of mass (neck zone). This effect can be interpreted as an evidence of isospin drift driven by the density gradient between the QuasiProjectile zone (at normal density) and the more diluted neck zone.

Equation of State Effects on Nucleon Transport

Previous studies have examined the nucleon transport in charge symmetric beam-on-target reaction systems as a probe for the asymmetry energy of nuclear matter. Using the iBUU transport model, isospin transport in charge symmetric as well as mass symmetric systems of Zn and Ni isotopes at 35 MeV/nucleon will be explored. Comparisons of the behavior of charge symmetric (70,64Zn + 70,64Zn) to mass symmetric (64Zn,64Ni + 64Zn,64Ni) reaction systems will be studied. The effect of different nucleon interactions will also be studied. This set of reaction systems will be used to determine what effect the difference in charge between projectile and target may have on the proton transport as compared to the neutron transport. These studies will be carried out with a view for use in the analysis of experimental data acquired using the NIMROD-ISiS array and TAMU Neutron Ball at the Cyclotron Institute at Texas A&M.

Probing the symmetry energy with isospin ratio from nucleons to fragments

Within the framework of ImQMD05, we study several isospin sensitive observables, such as DR(n/p) ratios, isospin transport ratio (isospin diffusion), yield ratios for LCPs between the projectile region and mid-rapidity region for the reaction systems Ni+Ni, Zn+Zn, Sn+Sn at low-intermediate energies. Our results show that those observables are sensitive to the density dependence of symmetry energy, and also depend on the cluster formation mechanism. By comparing these calculations to the data, the information of the symmetry energy and reaction mechanism is obtained.