Projectile-like Fragments Research Articles

Yield ratios and directed flows of light fragments from reactions induced by neutron-rich nuclei at intermediate energy**Supported by National Natural Science Foundation of China (11405025)

The yield ratios of neutron/proton and 3H/3He and the directed flow per nucleon for these projectile-like fragments at large impact parameters are studied for 50Ca + 40Ca and 50Cr + 40Ca for comparison at 50 MeV/u using the isospin-dependent quantum molecular dynamics (IQMD) model. It is found that the yield ratios and the directed flows per nucleon are different for reactions induced by the neutron-rich nucleus 50Ca and the stable isobaric nucleus 50Cr, and depend on the hardness of the EOS. The ratios of neutron/proton and 3H/3He and the difference of directed flow per nucleon of neutron-proton are suggested to be possible observables to investigate the isospin effects.

Elastic, inelastic, and 1-nucleon transfer channels in theLi7+Sn120system

Background: Simultaneous description of major outgoing channels for a nuclear reaction by coupled-channels calculations using the same set of potential and coupling parameters is one of the difficult tasks to accomplish in nuclear reaction studies.Purpose: To measure the elastic, inelastic, and transfer cross sections for as many channels as possible in $^{7}\mathrm{Li}+^{120}\mathrm{Sn}$ system at different beam energies and simultaneously describe them by a single set of model calculations using fresco.Methods: Projectile-like fragments were detected using six sets of Si-detector telescopes to measure the cross sections for elastic, inelastic, and 1-nucleon transfer channels at two beam energies of 28 and 30 MeV. Optical model analysis of elastic data and coupled-reaction-channels (CRC) calculations that include around 30 reaction channels coupled directly to the entrance channel, with respective structural parameters, were performed to understand the measured cross sections.Results: Structure information available in the literature for some of the identified states did not reproduce the present data. Cross sections obtained from CRC calculations using a modified but single set of potential and coupling parameters were able to describe simultaneously the measured data for all the channels at both the measured energies as well as the existing data for elastic and inelastic cross sections at 44 MeV.Conclusions: Non-reproduction of some of the cross sections using the structure information available in the literature which are extracted from reactions involving different projectiles indicates that such measurements are probe dependent. New structural parameters were assigned for such states as well as for several new transfer states whose spectroscopic factors were not known.

Observation of different isoscaling behavior between emitted fragments and residues

The traditional isoscaling technique has been used to analyze all isotopically identified particles from 70Zn+70Zn and 64Zn+64Zn collisions at 35 MeV/u. Two additional techniques, using an energy-averaged ratio and using the data present in the tail of energy spectra, are compared to the traditional method and show similar results. Isoscaling fit parameters α and β are found both globally and for each individual series of constant Z and N. The data are then split up between emitted fragments and projectile-like fragments. Isoscaling values for the two different types of fragments are shown to be different, emphasizing the importance of experimentally distinguishing between projectile-like fragments and emitted fragments in order to achieve accurate isoscaling parameters.

Production of neutron-rich exotic nuclei in projectile fragmentation at Fermi energies

Isotopic fragment yields of projectile fragmentation in peripheral collisions of \(^{86}\hbox {Kr}\) on \(^{124,112}\hbox {Sn}\) and the mean neutron-to-proton ratios of the fragments are calculated, theoretically, within the ensemble approach of statistical multifragmentation model. Obtained data are compared to the experimental cross-section measurements for the projectile-like fragmentation in the reaction of 25 MeV/nucleon \(^{86}\hbox {Kr}\)+\(^{124,112}\hbox {Sn}\) at Texas A&M University. We show the enhancement in the production of neutron-rich isotopes close to the projectile as observed in the experiments. We also demonstrate the universality of the limitation of the excitation energy induced in the projectile residues.

The study of neutron-rich nuclei production in the region of the closed shell N=126 in the multi-nucleon transfer reaction 136Xe+208Pb

The unexplored area of heavy neutron rich nuclei is extremely important for nuclear astrophysics investigations and, in particular, for the understanding of the r-process of astrophysical nucleogenesis. For the production of heavy neutron rich nuclei located along the neutron closed shell N=126 (probably the last "waiting point" in the r-process of nucleosynthesis) the low-energy multi-nucleon transfer reaction 136Xe+208Pb at Elab=870MeV was explored. Due to the stabilizing effect of the closed neutron shells in both nuclei, N=82 and N=126, and the rather favorable proton transfer from lead to xenon, the light fragments formed in this process are well bound and the Q-value of the reaction is nearly zero.Measurements were performed with the PRISMA spectrometer in coincidence with an additional time-of-flight (ToF) arm on the +20 beam line of the PIAVE-ALPI accelerator in Legnaro, Italy. The PRISMA spectrometer allows identification of the A, Z and velocity of the projectile-like fragments (PLF), while the second arm gives access to the target-like fragments (TLF). Details on the experimental setup and preliminary results are reported.

Dynamical fission following peripheral heavy-ion collisions

A closed-form theoretical approach describing in a single picture both the evaporation component and the fast nonequilibrium component of the sequential fission of projectile-like fragments in a peripheral heavy-ion collision is derived and then applied to the dynamical fission observed in the [Formula: see text]+[Formula: see text] semiperipheral collision at 35A[Formula: see text]MeV. Information on the reaction mechanism is obtained such as the opposite polarization effects and the estimate of the “formation-to-fast fission lifetimes” of the fissioning fragment.

Semiclassical approach to sequential fission in peripheral heavy-ion collisions

A closed-form theoretical approach describing in a single picture both the evaporation component and the fast nonequilibrium component of the sequential fission of projectilelike fragments in a semiperipheral heavy-ion collision is derived and then applied to the dynamical fission observed in the 124 Sn + 64 Ni semiperipheral collision at 35A MeV. Information on opposite polarization effects of the fissioning projectilelike fragments and on their “formation-to-fast fission lifetimes” are obtained.

Sensitivity of N/Z ratio in projectile break-up of isobaric systems

The binary break-up of projectile-like fragments in non central heavy-ion collisions follows different decay patterns, from equilibrated emission towards dynamical (prompt) fission. Recently, comparing two systems with different N/Z in the entrance channel, it has been shown that the dynamical emission cross-section is enhanced for the most neutron rich system while the statistical emission cross-section is independent from the isotopic composition. In order to understand this dependence and disentangle it from the initial size of the nuclei, we have studied the two isobaric systems 124 Xe +64 Zn and 124 Xe +64 Ni at 35 A MeV (InKiIsSy experiment), in comparison with the previous studied reactions (124 Sn +64 Ni and 112 Sn +58 Ni ) at the same bombarding energy. We present the first results evidencing a striking similar effect in the dynamical decay as a function of the N/Z of the target for equal size systems.

Determination of Impact Parameters in Aligned Breakup of Projectile-like Fragments in $^{197}$Au + $^{197}$Au Collisions at 23$A$\,MeV

Author(s): Cap, T; Siwek-Wilczynska, K; Wilczynski, J; Amorini, F; Auditore, L; Cardella, G; De Filippo, E; Geraci, E; Grassi, L; Grzeszczuk, A; La Guidara, E; Han, J; Kozik, T; Lanzalone, G; Lombardo, I; Najman, R; Nicolis, NG; Pagano, A; Papa, M; Piasecki, E; Pirrone, S; Planeta, R; Politi, G; Rizzo, F; Russotto, P; Skwira-Chalot, I; Trifiro, A; Trimarchi, M; Verde, G; Zipper, W

Velocity-dependent transverse momentum distribution of fragments produced fromAr40+Be9at 95 MeV/nucleon

Transverse momentum $({P}_{\mathrm{T}})$ distributions of projectilelike fragments produced at intermediate energy are investigated. ${P}_{\mathrm{T}}$ distributions of fragments with mass ${A}_{\mathrm{F}}\phantom{\rule{4pt}{0ex}}=10--37$, produced from an $^{40}\mathrm{Ar}$ beam with $E=95\phantom{\rule{0.16em}{0ex}}\mathrm{MeV}/\mathrm{nucleon}$ and a Be target, are observed as a function of fragment velocity by using the Rikagaku Kenkyusho (RIKEN) Projectile Fragment Separator at RIKEN. The width of ${P}_{\mathrm{T}}$ distribution $({\ensuremath{\sigma}}_{\mathrm{T}})$ decreases monotonically with longitudinal momentum $({P}_{\mathrm{L}})$. The decreasing trend shows a clear dependence on fragment mass. The observed ${\ensuremath{\sigma}}_{\mathrm{T}}$ is analyzed with a linear function and successfully reproduced by a parameter to characterize the decreasing trend. ${\ensuremath{\sigma}}_{\mathrm{T}}$ at the primary beam velocity shows good agreement with the Goldhaber formula, which is obtained on the basis of the contribution of the Fermi momentum. In contrast, the ${\ensuremath{\sigma}}_{\mathrm{T}}$ at the center of the ${P}_{\mathrm{L}}$ distribution as determined by using the fits and parametrized energy loss in the fragmentation process is consistent with the width, which includes an additional contribution from the orbital deflection of the projectile. The antisymmetrized molecular-dynamics calculation indicates that the contribution of impact parameters is an important factor in understanding the behavior of the ${P}_{\mathrm{T}}$ distribution. In addition, the practical formulation for ${\ensuremath{\sigma}}_{\mathrm{T}}$ obtained in the present study enables a reliable characterization of the fragments, which would be applicable to various research fields involving, for example, radioactive nuclear beams.

Theoretical studies on shaking processes in nuclear transfer reactions

The probabilities of shaking processes during nuclear transfer reactions have been studied using the Mukoyama formalism after the re-examination of formalism for β-decay processes. Electron shakeoff probabilities have been calculated for the α-transfer reaction in the range of Z=10–50. The Z-dependence on the shakeoff probabilities so obtained has been represented by an analytical equation with two parameters. The formalism has been applied on a typical nuclear transfer reaction Fe2656+C612→Ni2860+Be48 and it is found that electron shakeup, shakedown and shakeoff probabilities dominate for low l quantum number of the respective shells of the projectile-like fragment ion. However, for a particular value of l these processes show high probabilities for low values of n quantum number.

Xe136+Pb208reaction: A test of models of multinucleon transfer reactions

The yields of over 200 projectile-like fragments (PLFs) and target-like fragments (TLFs) from the interaction of (E$_{c.m.}$=450 MeV) $^{136}$Xe with a thick target of $^{208}$Pb were measured using Gammasphere and off-line $\gamma$-ray spectroscopy, giving a comprehensive picture of the production cross sections in this reaction.The measured yields were compared to predictions of the GRAZING model and the predictions of Zagrebaev and Greiner using a quantitative metric, the theory evaluation factor, {\bf tef}. The GRAZING model predictions are adequate for describing the yields of nuclei near the target or projectile but grossly underestimate the yields of all other products. The predictions of Zagrebaev and Greiner correctly describe the magnitude and maxima of the observed TLF transfer cross sections for a wide range of transfers ($\Delta$Z = -8 to $\Delta$Z = +2). However for $\Delta$Z =+4, the observed position of the maximum in the distribution is four neutrons richer than the predicted maximum. The predicted yields of the neutron-rich N=126 nuclei exceed the measured values by two orders of magnitude. Correlations between TLF and PLF yields are discussed.

“Phoswich Wall”: A charged-particle detector array for inverse-kinematic reactions with the Gretina/GRETA γ-ray arrays

A high-efficiency, forward-hemisphere detector system for light charged particles and low-Z heavy ions, as obtained in an accelerator experiment, is described. It consists of four 8×8 pixel multianode photomultiplier tubes with 2.2-mm thick CsI(Tl) and 12-μm thick fast-plastic scintillation detectors. Its phoswich structure allows individual Z resolution for 1H, 4He, 7Li, 4He+4He, 9Be, 11B, 12C, and 14N ions, which are target-like fragments detected in strongly inverse kinematics. The device design has been optimized for use with a 4π γ-ray array, and the main applications are transfer reactions and Coulomb excitation. A high-angular resolution for the detection of the target-like fragments is achieved which permits angular distributions to be measured in the rest frame of the projectile-like fragment with a resolution of ~2°.

Effect of neutron skin thickness on projectile fragmentation

The fragment production in collisions of $^{48,50}$Ca+$^{12}$C at 50 MeV/nucleon are simulated via the Isospin-Dependent Quantum Molecular Dynamics (IQMD) model followed by the {GEMINI code}. {By changing the diffuseness parameter of neutron density distribution to obtain different neutron skin size, the effects of neutron skin thickness (${\delta}_{np}$) on projectile-like fragments (PLF) are investigated. The sensitivity of isoscaling behavior to neutron skin size is studied, from which it is found that the isoscaling parameter $\alpha$ has a linear dependence on ${\delta}_{np}$. A linear dependence between ${\delta}_{np}$ and the mean $N/Z$ [N(Z) is neutron(proton) number] of PLF is obtained as well.} The results show that thicker neutron skin will lead to smaller {isoscaling parameter} $\alpha$ and N/Z. Therefore, it may be probable to extract information of neutron skin thickness from {isoscaling parameter} $\alpha$ and N/Z.

Production cross sections for intermediate mass fragments from dynamical and statistical decay of projectile-like fragments inSn124+Ni64andSn112+Ni58collisions at35AMeV

Background: In heavy-ion collisions at intermediate energies the production of light charged particles and intermediate mass fragments (IMFs) is due to different reaction mechanisms and different time scales, ranging from fast dynamical processes to statistical emission from the equilibrated system.Purpose: We compare the IMF statistical and dynamical emission probabilities in collisions of a neutron-rich $^{124}\mathrm{Sn}+^{64}\mathrm{Ni}$ system and a neutron-poor $^{112}\mathrm{Sn}+^{58}\mathrm{Ni}$ system at the laboratory energy of 35 $A$ MeV.Method: The IMFs production mechanism in semiperipheral reactions has been investigated in our previous works. In this paper, the analysis is expanded for the same set of data and production cross sections have been evaluated for dynamical and statistical emission in a coherent way for light and heavy fragments.Results: The data analysis has evidenced a strong competition between dynamical and statistical emission mechanisms. Probability of the dynamical emission of IMFs is strongly influenced by the ($N/Z$) ratio of the colliding system.Conclusions: It is demonstrated that the statistical emission is equally probable for the two systems, while the dynamical emission is enhanced for the neutron-rich system, especially for heavy fragments ($Z\ensuremath{\ge}6$). The observed effect points at a high sensitivity of the prompt-dynamical emission to the ($N/Z$) ratio of a given system.

Investigating energy dissipation through nucleon transfer reactions

Nucleon and cluster transfer probabilities have been measured in the collisions of 16,18 O, 19 F with 204,208 Pb, 209 Bi for charge stripping channels down to ΔZ = - 3. Strong evidence of correlated nucleon transfer has been observed in particular channels, and neighbouring systems are seen to differ significantly in their behaviour. New measurements were made using an improved ΔE-E telescope. The back-scattered projectilelike fragments were measured in the telescope at θ lab = 160.6°, and in combination with monitor detectors at forward angles allowed determination of absolute transfer probabilities. The improved design allows isotopic yields to be measured with greater precision.

Inverse-kinematics study of78Kr +40Ca at 10 AMeV

The CHIMERA multi-detector array at LNS Catania has been used to study the inverse-kinematics reaction of 78 Kr + 40 Ca at a bombarding energy of 10 AMeV. Analysis of the experimental data focused on a class of selected events consistent with the complete fusion and subsequent binary split of the of the reacting system. This class of events features a broad A, Z distribution of fission fragments centered about symmetric fission while exhibiting relative velocities significantly higher than given by Viola systematics. The center-ofmass angular distribution (dσ/dΘ) of the fission fragments exhibit an unexpected anisotropy inconsistent with a compound-nucleus reaction and indicates a dynamic fusion-fission like process. The observed angular distribution features an asymmetric forward-backward peaking most prevalent for mass-asymmetric events. Furthermore, the more massive fragment of mass-asymmetric events appears to emerge preferentially in the forward direction, along the beam axis, in analogy to dynamic fragmentation of projectile-like fragments. Analysis of the angular distribution of alpha particles emitted from these fission fragments suggests the events are associated mostly with central collisions. The study of low-energy heavy-ion reaction dynamics in the energy domain moderately in excess of the Coulomb barrier offers a unique insight into the large-scale collective motion of the nucleons in the nucleus. Fusion-fission constitutes an important reaction channel for many such nuclear reactions and is an important probe into the nuclear structure of matter. As the bombarding energy increases, the role of dynamically-induced decay modes

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.

Systematic study of neutron-rich rare isotope production in peripheral heavy-ion collisions below the Fermi energy

Detailed calculations of the yields of projectilelike fragments (with focus on the neutron-rich isotopes) are presented for the interaction of $^{86}\mathrm{Kr}$ (15 MeV/nucleon) with $^{64}\mathrm{Ni}$, $^{58}\mathrm{Ni}$, and $^{124}\mathrm{Sn}$, $^{112}\mathrm{Sn}$, as well as $^{86}\mathrm{Kr}$ (25 MeV/nucleon) with $^{124}\mathrm{Sn}$ and compared with our recently published experimental data for these reactions. The calculations are based on a two-step approach: the dynamical stage of the collision was described with the microscopic constrained molecular dynamics (CoMD) model, as well as the phenomenological deep-inelastic transfer (DIT) model and its modified (DITm) version. The deexcitation of the hot projectile fragments was performed with the statistical multifragmentation model (SMM) and the binary-decay model gemini, which provided nearly similar results for the neutron-rich products from the reactions studied. An overall good agreement of the calculations with the experimental results, especially for near-projectile isotopes was observed using both the CoMD model and the DITm model for the dynamical stage. The successful description of the production of neutron-rich isotopes with the CoMD model is of particular importance, due to the predictive power of the microscopic approach that essentially does not depend on the reaction dynamics. Our studies to date suggest that peripheral heavy-ion collisions at this energy range (i.e., well above the Coulomb barrier, but below the Fermi energy), if induced by neutron-rich rare-isotope beams of adequate intensity may offer a unique route to access extremely neutron-rich rare isotopes toward the astrophysical $r$-process path and the presently uncharted neutron drip line.

Semiclassical approach to sequential fission in peripheral collisions

A closed-form semiclassical approach describing in a single picture both the evaporation component and the fast nonequilibrium component of the sequential fission of projectile-like (PL) fragments in a peripheral heavy-ion collision is derived and then applied to the dynamical fission observed in the 124 Sn +64 Ni peripheral collision at 35A MeV. Information on opposite polarization effects of the fissioning PL fragments and on "dynamical fission lifetimes" are obtained.