Projectile-like Fragments Research Articles

Dynamical versus statistical production of Intermediate Mass Fragments at Fermi Energies

The emission probability of Intermediate Mass Fragments (IMFs) in non-central reactions has been investigated in collisions of heavy $$^{124}\hbox {Xe}$$ projectiles with the two different medium-mass targets of $$^{64}\hbox {Ni}$$ and $$^{64}\hbox {Zn}$$ at the laboratory energy of 35 A MeV. The two colliding systems differ only for the target atomic number Z and, consequently, for the isospin N/Z ratio. The probability of IMFs emission from the projectile-like fragment has been measured, showing an enhancement of the IMFs emission for the neutron rich $$^{64}\hbox {Ni}$$ target. Most of the observed projectile break-up yield is associated with the production of only one IMF, that is, a quasi-binary splitting of projectile in two fragments in a broad range of charge asymmetry. For the events with one IMF, the relative contributions of the dynamical and statistical emissions have been evaluated. We find an enhancement of dynamical break-up probability for the neutron rich target with respect to the neutron poor one. The analysis suggests influence of the target isospin in inducing the dynamical break-up of projectile-like fragments. The new data have been also compared with previous published results of $$^{112,124}\hbox {Sn}$$ + $$^{58,64}\hbox {Ni}$$ systems, in order to disentangle between isospin effects against system-size effects on the emission probability. The comparisons between previous and new data suggest that the dynamical break-up is determined by the N/Z content of both projectile and target; for the cases here investigated, the influence of the system size on the dynamical emission probability can be excluded.

Approaching neutron-rich nuclei towards the r-process path in 86Kr-induced collisions at 15 MeV/nucleon

The production cross sections of projectile-like fragments from collisions of 15 MeV/nucleon 86Kr with 64,58Ni and 124,112Sn have been measured using a magnetic separator with emphasis on the neutron-rich isotopes. Neutron pick-up isotopes (with up to 6-8 neutrons picked-up from the target) were observed with large cross sections. The present results were also compared with our previous data of the same reactions at 25 MeV/nucleon. The data at 15 MeV/nucleon show enhanced production of neutron-rich isotopes very close to the projectile, relative to the corresponding data at 25 MeV/nucleon. The large cross sections of such reactions involving peripheral nucleon exchange, indicate that these reactions offer a novel route to access extremely neutron-rich rare isotopes towards the the astrophysical r-process path and the neutron-drip line.

Determination of Mn53(n,xp) cross sections using the surrogate reaction ratio method

The $^{54}\mathrm{Mn}^{*}$ (surrogate of $n+^{53}\mathrm{Mn}$) and $^{61}\mathrm{Ni}^{*}$ (surrogate of $n+^{60}\mathrm{Ni}$) compound systems have been populated at overlapping excitation energies by transfer reactions $^{52}\mathrm{Cr}(^{6}\mathrm{Li},\ensuremath{\alpha})^{54}\mathrm{Mn}^{*}$ at ${E}_{\mathrm{lab}}=33.0\phantom{\rule{0.28em}{0ex}}\mathrm{MeV}$ and $^{59}\mathrm{Co}(^{6}\mathrm{Li},\ensuremath{\alpha})\phantom{\rule{0.16em}{0ex}}^{61}\mathrm{Ni}^{*}$ at ${E}_{\mathrm{lab}}=40.5\phantom{\rule{0.28em}{0ex}}\mathrm{MeV}$, respectively. The proton decay probabilities of the compound systems have been determined by measuring evaporated protons at backward angles in coincidence with projectilelike fragments detected around the grazing angle. The $^{53}\mathrm{Mn}(n,xp)$ cross sections in the equivalent neutron energy range of 8.2--16.4 MeV have been determined within the framework of the surrogate reaction ratio method using $^{60}\mathrm{Ni}(n,xp)$ cross-section values from the literature as a reference. The measured $^{53}\mathrm{Mn}(n,xp)$ cross-section values are found to be consistent with the predictions of the talys-1.8 statistical model code using microscopic level densities and results of various evaluated nuclear data libraries: eaf-2010, rosfond-2010, and jeff-3.3 within the experimental uncertainties.

Heavy-isotope production in Xe136+Pb208 collisions at Ec.m.=514 MeV

Employing the quantal diffusion mechanism for multi-nucleon transfer, the double differential cross-sections are calculated for production of primary projectile-like and target-like fragments in collisions of ${}^{136}\text{Xe}+{}^{208}\text{Pb}$ system at $E_\text{c.m.} =514$ MeV. Including de-excitation due to neutron emission, the cross-section for production of ${}^{210}\text{Po}$, ${}^{222}\text{Rn}$ and ${}^{224}\text{Ra}$ isotopes are estimated and compared with data.

Dynamical ternary decays of excited projectile-like fragments

We study neutron-proton equilibration in dynamically deformed nuclear systems by investigating the correlations between the three largest fragments produced in collisions of $^{70}\mathrm{Zn}+^{70}\mathrm{Zn}$ at 35 MeV per nucleon. The rotation angle is interpreted as a measure of the duration of contact between the regions of the excited projectile-like fragment as it decays. In this work we address questions of whether there is a simultaneous or a double rupture scenario, sizing of the fragments, and alignment of the configuration.

The Xe136+Pt198 reaction: A test of models of multi-nucleon transfer reactions

The yields of 42 projectile-like fragments (PLFs) and fission fragments and 36 target-like fragments (TLFs) were measured using off-line $\ensuremath{\gamma}$-ray spectroscopy in a thin target experiment involving the $^{136}\mathrm{Xe}+^{198}\mathrm{Pt}$ reaction. The center of target beam energy was 760.5 MeV $({\mathrm{E}}_{\mathrm{c}.\mathrm{m}.}=450$ MeV). The reported yields are compared with those from previous measurements for this reaction and with predictions of the GRAZING, di-nuclear systems (DNS), and improved quantum molecular dynamics (ImQMD) models. The yields of the TLFs and PLFs are, in general, substantially smaller than those previously observed at a beam energy of 1085 MeV. Neither the GRAZING nor the DNS model correctly describes the measured TLF and PLF yields in this lower- energy reaction but the ImQMD model describes these yields adequately.

The Synthesis of New Neutron-Rich Heavy Nuclei

All the known isotopes of the elements Fm through Og are neutron-deficient relative to -stability. In this contribution, I discuss two methods of producing more n-rich heavy nuclei, i.e., the use of radioactive nuclear beams (RNBs) and the use of multi-nucleon transfer (MNT) reactions. In the former case, I discuss recent studies of the interaction of 39,46K with 181Ta and their implications for the synthesis of more n-rich isotopes of Bh and Hs. In the case of MNT reactions, I discuss recent results for the reaction of 136Xe with 208Pb, 204Hg + 198Pt and 136Xe + 198Pt. I compare measured distributions of the target-like fragments (TLFs) and projectile-like fragments (PLFs) with current models of MNT reactions.

Systematic Study of Neutron-Rich Rare Isotope Production in Peripheral Heavy-Ion Collisions at Beam Energies 15–25 MeV/nucleon

The production cross sections of projectile-like fragments from collisions of 86Kr projectiles with 64,58Ni and 124,112Sn targets at 15 and 25 MeV/nucleon are studied systematically with emphasis on the neutron-rich isotopes. Our recent experimental data are compared with calculations for the above collisions employing a hybrid approach. The dynamical stage of the projectile-target interaction was described with either the phenomenological deep-inelastic transfer (DIT) model or with the the microscopic constrained molecular dynamics model (CoMD). Subsequently, for the de-excitation of the projectile-like fragments, the statistical multifragmentation model (SMM) or the binary-decay code GEMINI were employed. An overall good agreement with the experimental results was obtained. We point out that our current understanding of the reaction mechanism at beam energies below the Fermi energy suggests that such nuclear reactions, involving peripheral nucleon exchange, can be exploited as a novel route to access extremely neutron-rich rare isotopes toward the r-process path and the hard-to-reach neutron drip-line. For this purpose, we believe that the use of re-accelerated neutron-rich radioactive beams may offer unique and exciting opportunities toward unexplored regions of the nuclear landscape.

Measurement of incomplete fusion cross sections in Li6,7+U238 reactions

Background: Incomplete fusion (ICF) in reactions involving weakly bound projectiles is understood to play an important role in enhancing the ratio of asymmetric to symmetric fission at $E\ensuremath{\le}{V}_{b}$, widening the fission fragment folding angle distribution at $E\ensuremath{\le}{V}_{b}$ and suppressing the complete fusion (CF) cross section at $Eg{V}_{b}$. Experimental cross sections for individual ICF channels are necessary to obtain quantitative estimates of the above effects.Purpose: Measurement of the cross sections for individual transfer-induced or ICF-fission channels in $^{6,7}\mathrm{Li}+^{238}\mathrm{U}$ reactions to explain quantitatively the difference in the ratio of asymmetric to symmetric fission for total fission and CF fission.Methods: Triple coincidence measurement of two fission fragments and one light charged projectilelike fragment (PLF) is carried out using two multiwire proportional counter (MWPC) detectors and an array of CSI(Tl) scintillation detectors at energies near the Coulomb barrier. By calculating the efficiency of the detectors by Monte Carlo simulation and fission probability by GEF code, the angle-integrated ICF cross sections corresponding to p, d, t, and $\ensuremath{\alpha}$ emission are obtained. Mass distributions for total fission are calculated by adding the distributions for CF fission and all ICF fissions with weight factors proportional to the measured cross sections.Results: ICF cross sections corresponding to $\ensuremath{\alpha}$ emission are found to be the highest in both the reactions. The relative contribution of ICF to total fusion at the lowest measured energy is found to be $\ensuremath{\approx}70%$ and it decreases with increasing projectile energy, consistent with the systematics of several reactions involving the same weakly bound projectiles. The simulated mass distributions for total fission are found to reproduce the experimental mass distributions.Conclusions: Deuteron (triton) capture is observed to be the major ICF channel in the reaction involving $^{6}\mathrm{Li}\phantom{\rule{0.28em}{0ex}}(^{7}\mathrm{Li})$ projectile. The ratio of the sum of ICF to total fusion cross section for the present systems are consistent with the systematics. The difference in the ratio of asymmetric to symmetric fission between total fission and CF fission is explained quantitatively.

Yield ratios and directed flows of light particles from proton-rich nuclei-induced collisions

The neutron-to-proton and \({}^3{\hbox {H}}\)-to-\({}^3{\hbox {He}}\) yield ratios, and the directed flows of particles dependent on a reduced rapidity, the transverse momentum per nucleon, and a reduced impact parameter are investigated for \({}^{28}{\hbox {S}} + {}^{28}{\hbox {Si}}\) and \({}^{32}{\hbox {S}} + {}^{28}{\hbox {Si}}\) systems at 50 and 400 MeV/u using an isospin-dependent quantum molecular dynamics model. The results show that these yield ratios of projectile-like fragments are approximately equal to the constituent neutron-to-proton ratio of the projectile. There are clear differences of the directed flows for isospin-related fragments neutron and proton, \({}^3{\hbox {H}}\) and \({}^3{\hbox {He}}\) from \({}^{28}{\hbox {S}} + {}^{28}{\hbox {Si}}\) collisions. The differences in directed flows for neutrons and protons and \({}^3{\hbox {H}}\)–\({}^3{\hbox {He}}\) from a proton-rich nucleus \({}^{28}{\hbox {S}}-\) induced collisions are noticeably larger than those from a stable nucleus \({}^{32}{\hbox {S}}-\) induced reactions under medium impact parameters. Thus, the yield ratios and differences in directed flows for the neutrons and protons and \({}^3{\hbox {H}}\)–\({}^3{\hbox {He}}\) under medium impact parameters are proposed as possible observable items for studying isospin physics.

Nuclear dynamics in multinucleon transfer reactions near Coulomb barrier energies

The mechanism of multinucleon transfer reactions has been investigated within the dinuclear system model, in which the sequential nucleon transfer is described by solving a set of microscopically derived master equations. The transfer dynamics in the reaction of $${^{136}}{\mathrm{Xe}}+{^{208}}{\mathrm{Pb}}$$ near Coulomb barrier energies is thoroughly analyzed. It is found that the total kinetic energies of primary fragments are dissipated from the relative motion energy of two touching nuclei and exhibit a symmetric distribution along the fragment mass. The angular distribution of the projectile-like fragments moves forward with increasing beam energy. However, the target-like fragments exhibit an opposite trend. The shell effect is pronounced due to the fragment yields in multinucleon transfer reactions.

Isospin effect in peripheral heavy-ion collisions at Fermi energies**Supported by the National Natural Science Foundation of China (11722546, 11675226) and the Talent Program of South China University of Technology (K5180470)

The isospin effect in peripheral heavy-ion collisions was thoroughly investigated within the framework of the Lanzhou quantum molecular dynamics (LQMD) transport model. A coalescence approach was used to recognize the primary fragments formed in nucleus-nucleus collisions. The secondary decay process of these fragments was described using the statistical code GEMINI. The production mechanism and isospin effect of the projectile-like and target-like fragments were analyzed using the combined approach. It was found that the isospin migration from the high-isospin density to the low-density matter occurred in the neutron-rich nuclear reactions, i.e., 48Ca+208Pb, 86Kr+48Ca/208Pb/124Sn, 136Xe+208Pb, 124Sn+124Sn, and 136Xe+136Xe. A hard symmetry energy was available for creating the neutron-rich fragments, particularly in the medium-mass region. The isospin effect of the neutron-to-proton (n/p) ratio of the complex fragments was reduced when the secondary decay process was included. However, a soft symmetry energy enhanced the n/p ratio of the light particles, particularly at kinetic energies greater than 15 MeV/nucleon.

Mass distributions of fission fragments from nuclei populated by multinucleon transfer or incomplete fusion channels in Li6,7+U238 reactions

The multinucleon transfer reaction or incomplete fusion reaction (ICF) is a powerful tool to study fission of exotic nuclei that cannot be formed by stable heavy-ion fusion reactions. In the present work, mass distributions of fission fragments (FFs) from fissioning nuclei $^{241,242,243,244}\mathrm{Pu}$ and $^{240,241}\mathrm{Np}$ populated in multinucleon transfer or ICF reactions on $^{6,7}\mathrm{Li}+^{238}\mathrm{U}$ systems have been studied. Among these, $^{244}\mathrm{Pu},\phantom{\rule{0.16em}{0ex}}^{243}\mathrm{Pu}$, and $^{241}\mathrm{Np}$ are formed by the capture of unstable nuclei $^{6}\mathrm{He},\phantom{\rule{0.16em}{0ex}}^{5}\mathrm{He}$, and $t$, respectively, by the target $^{238}\mathrm{U}$. Identification of fissioning nuclei and determination of excitation energies have been performed by finding the details of the outgoing projectile-like fragments detected in coincidence with both the fission fragments on an event-by-event basis. The measurements of FF mass distributions and FF folding angle distributions of different ICF channels confirm that these channels are the prime factors for the modifications in the experimental ratio of asymmetric to symmetric fission yields and the width of folding angle distributions for inclusive fission reported earlier on the same reactions. Comparison among the ratio of asymmetric to symmetric fission yields from $^{241,242,243,244}\mathrm{Pu}$ and $^{240,241}\mathrm{Np}$ nuclei formed in the present reactions, available literature data, and the theoretical calculations using gef code shows that the shell correction for symmetric fission channels plays an important role in describing the experimental mass distribution.

Resonant breakup of Be8 in Sn112(Li7,Be8→2α) reaction

Background: Reactions involving weakly bound projectiles with $\ensuremath{\alpha}+x$ cluster structure are known to produce a large number of $\ensuremath{\alpha}$ particles, where transfer-induced breakup is one of the possible origins. Detailed investigation on each of these channels is desirable to understand the underlying reaction mechanism.Purpose: Our purpose is to measure the $+1p$ transfer-induced breakup in the $^{112}\mathrm{Sn}(^{7}\mathrm{Li},^{8}\mathrm{Be}\ensuremath{\rightarrow}2\ensuremath{\alpha})$ reaction to understand the possible modes of $^{8}\mathrm{Be}$ breakup and their contribution to inclusive $\ensuremath{\alpha}$ production.Methods: $\ensuremath{\alpha}\ensuremath{-}\ensuremath{\alpha}$ coincidence measurements have been carried out for the above reaction at ${E}_{\mathrm{beam}}=30$ MeV. Projectile-like fragments were detected using five sets of Si-strip-detector telescopes and five sets of single Si-detector telescopes. Optical model analysis of elastic scattering data, coupled reaction channels (CRC), and continuum discretized coupled channels (CDCC) calculations were performed to understand the measured cross sections.Results: The experimental cross sections for $+1p$ transfer-induced breakup in ($^{7}\mathrm{Li},^{8}\mathrm{Be}\ensuremath{\rightarrow}2\ensuremath{\alpha}$) reaction through different resonance states of $^{8}\mathrm{Be}$ have been obtained. Relative energy distribution and Monte Carlo simulation confirm the observation of breakup of $^{8}\mathrm{Be}$ from its ${4}^{+}$ (11.35-MeV) resonant state for the first time along with its well-known ${0}^{+}$ (92-keV) and ${2}^{+}$ (3.12-MeV) resonances. Simultaneous description of elastic scattering, transfer, and breakup cross sections have been made using CDCC plus CRC formalism.Conclusions: Production of $\ensuremath{\alpha}$ particles in $+1p$ transfer-triggered breakup is found to proceed mainly through three resonance (${0}^{+},\phantom{\rule{0.16em}{0ex}}{2}^{+}$, and ${4}^{+}$) states of $^{8}\mathrm{Be}$.

Momentum peak shift and width of longitudinal momentum distribution of projectilelike fragments produced at E=290MeV/nucleon

Longitudinal momentum (${P}_{\mathrm{L}}$) distributions of projectilelike fragments produced at $E=290\phantom{\rule{0.16em}{0ex}}\mathrm{MeV}/\mathrm{nucleon}$ are investigated. ${P}_{\mathrm{L}}$ distributions of fragments produced by Ar and Kr beams with a wide variety of targets (C, Al, Nb, Tb, and Au) were measured using the fragment separator at HIMAC. ${P}_{\mathrm{L}}$ distributions observed for fragments with a wide range of mass losses $\mathrm{\ensuremath{\Delta}}A$ (1--30 for Ar beam and 1--64 for Kr beam), show a slightly, but definitely asymmetric nature. The peak shift and width were obtained from the observed ${P}_{\mathrm{L}}$ distributions. No significant target dependence was found in either the peak shift or width. For the practical application, the variation in momentum peak shift with fragment mass (${A}_{\mathrm{F}}$) was represented by a parabolic function. The width on the high-${P}_{\mathrm{L}}$ side (${\ensuremath{\sigma}}_{\mathrm{High}}$) is well reproduced by the Goldhaber formula, which is obtained from the contribution of the Fermi momentum. The behavior of the reduced width, ${\ensuremath{\sigma}}_{0}$, obtained from ${\ensuremath{\sigma}}_{\mathrm{High}}$ via the Goldhaber formulation, is consistent with the mass-dependent Fermi momentum of a nucleon. The width on the low-${P}_{\mathrm{L}}$ side (${\ensuremath{\sigma}}_{\mathrm{Low}}$) is markedly larger than ${\ensuremath{\sigma}}_{\mathrm{High}}$ and exhibits a clear ${A}_{\mathrm{F}}$ dependence.

Effect of isospin diffusion on the production of neutron-rich nuclei in multinucleon transfer reactions

The isospin dissipation dynamics in multinucleon transfer reactions has been investigated within the dinuclear system model. Production cross sections of neutron-rich isotopes around projectile-like and target-like fragments are estimated in collisions of $^{58,64}\mathrm{Ni}+^{208}\mathrm{Pb}$ and $^{78,86,91}\mathrm{Kr}+^{198}\mathrm{Pt}$ near Coulomb barrier energies. The isospin diffusion in the nucleon transfer process is coupled to the dissipation of relative motion energy and angular momentum of colliding system. The available data of projectile-like fragments via multinucleon transfer reactions are nicely reproduced. It is found that the light projectile-like fragments are produced in the neutron-rich region because of the isospin equilibrium in two colliding nuclei. However, the heavy target-like fragments tend to be formed on the neutron-poor side above the $\ensuremath{\beta}$-stability line. The neutron-rich projectiles move the maximal yields of heavy nuclei to the neutron-rich domain and are available for producing the heavy exotic isotopes, in particular around the neutron shell closure of $N=126$.

Experimental study of the transfer-induced fission fragment angular distribution in the Li6+U238 reaction

Transfer-induced fission fragment angular distribution measurements have been carried out in $^{6}\mathrm{Li}+^{238}\mathrm{U}$ reaction at beam energies of 36 and 40 MeV in coincidence with projectile-like fragments. The fission fragment anisotropies for $\ensuremath{\alpha}$- and deuteron-transfer-induced fission reactions are obtained with respect to the recoil direction. The anisotropy values for transfer-induced fissions are observed to be either similar or lower than the inclusive measurement. In order to quantitatively understand the present experimental data on transfer-induced fission, anisotropy values have been compared with the preequilibrium fission model considering ground-state spin of the projectile and/or target. It is found that the model predicts the inclusive and exclusive anisotropy data reasonably well.

Production mechanism of new neutron-rich heavy nuclei in the [formula omitted] reaction

The multinucleon transfer reaction of Xe136+198Pt at Elab=7.98 MeV/nucleon is investigated by using the improved quantum molecular dynamics model. The quasielastic, deep-inelastic, and quasifission collision mechanisms are studied via analyzing the angular distributions of fragments and the energy dissipation processes during the collisions. The measured isotope production cross sections of projectile-like fragments are reasonably well reproduced by the calculation of the ImQMD model together with the GEMINI code. The isotope production cross sections for the target-like fragments and double differential cross sections of 199Pt, 203Pt, and 208Pt are calculated. It is shown that about 50 new neutron-rich heavy nuclei can be produced via deep-inelastic collision mechanism, where the production cross sections are from 10−3 to 10−6 mb. The corresponding emission angle and the kinetic energy for these new neutron-rich nuclei locate at 40∘–60∘ and 100–200 MeV, respectively.

α -conjugate neck structures in the collisions of 35 MeV/nucleon Ca40 with Ca40

The de-excitation of alpha-conjugate nuclei produced in reactions of 35 MeV/nucleon 40Ca with 40Ca has been investigated. Particular emphasis is placed on examining the dynamics of collisions leading to projectile-like fragment exit channels. A general exploration of the reaction systematics reveals the binary dissipative character of the collisions and a hierarchy effect similar to that seen for heavier systems. Investigation of the subset of events characterized by a total alpha-conjugate mass (alpha particles plus alpha-conjugate fragments) equal to 40 and atomic number equal to 20 reveals a dominance of alpha-conjugate exit channels. The hierarchy effect for these channels leads to the production of alpha-clustered neck structures with potentially exotic geometries and properties.

Modeling multi-nucleon transfer in symmetric collisions of massive nuclei

Symmetric collisions of massive nuclei, such as 238U + 248Cm, have been proposed as ways to make new n-rich heavy nuclei via multi-nucleon transfer (MNT) reactions. We have measured the yields of several projectile-like and target-like fragments from the reaction of 1360 MeV 204Hg + 198Pt. We find that current models for this symmetric collision (GRAZING, DNS, ImQMD) significantly underestimate the yields of these transfer products, even for small transfers.