Production Of Intermediate Mass Fragments Research Articles

Role of mass asymmetry on the peak energy of intermediate mass fragments production and its influence towards isospin effects

Using the isospin-dependent quantum molecular dynamics model, we study the peak intermediate mass fragments production and corresponding center-of-mass energy in various mass asymmetric reactions. The study is spanned over different mass asymmetries for total system mass 92, 162 and 240. We propose the peak energy as a powerful probe to constrain the low-density behavior of symmetry energy. This peak energy is found to remove the dual dependence of isospin effects in symmetry energy and cross-section.

Effects of cluster correlations on fragment emission in C12+C12 at 50 MeV/nucleon

The effects of cluster correlations have been studied in the $^{12}\mathrm{C}+^{12}\mathrm{C}$ reaction at 50 MeV/nucleon, using three antisymmetrized molecular dynamics (AMD) models, the AMD (AMD/D) without any additional cluster correlations, AMD/D-COALS with nucleon correlations based on a coalescence prescription for light cluster formations with $A\ensuremath{\le}4$, and AMD-cluster with an extended cluster correlation in two-nucleon collision processes and a special treatment for intermediate fragment formation with $A\ensuremath{\le}9$. The angular distributions and energy spectra of fragments have been simulated and compared with the available experimental data. It is found that the cluster correlations take a crucial role to describe the productions of light charged particles (LCPs) and intermediate mass fragments (IMFs), and the AMD-cluster studied here provides a consistent overall reproduction of the experimental data. It is also shown that the significant effects of the secondary decay processes are involved for the fragment production besides the dynamical productions in the AMD stage. Detailed LCP and IMF production mechanisms involved in the intermediate energy heavy ion collisions are discussed.

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.

Intermediate Mass Fragments production at low energy: reaction mechanism and isospin influence

In this work, we present a recent study concerning the break-up of the Projectile-Like (PLF) into two fragments, following more violent deep-inelastic collisions in the reactions 78Kr + 40Ca and 86Kr + 48Ca at 10 A MeV, realized at INFN-Laboratori Nazionali del Sud by using the 4π multi detector CHIMERA. For these systems, we have already analyzed the fusion-evaporation and fission-like processes. A selection method has been developed, in order to discriminate PLF break up events from those due to other mechanisms which populate the same region of the phase- space. A preference for PLF aligned break-up, along the direction of the PLF-TLF separation axis with the light fragment emitted in the backward part, has been put in light, suggesting the presence of dynamical effects. Also a comparison between the neutron-rich 86Kr + 48Ca and neutron-poor 78Kr + 40Ca systems is presented, to put in evidence the isospin role in these processes.

A dynamical description of the 136Xe + p spallation at 1000 MeV/nucleon * * Supported by the National Natural Science Foundation of China (11405278)

We propose a dynamical description of the 136Xe + p spallation at 1000 MeV/nucleon with the aim of probing the mechanism which rules the production of intermediate-mass fragments (IMF). The isospin-dependent quantum molecular dynamics (IQMD) model is used to describe the dynamical process of spallation until hot fragments with excitation energy less than a certain value Estop are formed. The statistical code GEMINI is applied to simulate the light-particle evaporation from hot fragments. It is found that IMF production is well described by the model when Estop = 2 MeV/nucleon is used. Comparison of the calculated mean neutron-to-proton ratio and the experimental data indicates that Estop should be 3 MeV/nucleon.

Decay of the excited compound system Ni*56 formed through various channels using deformed Coulomb and deformed nuclear proximity potentials

The total cross section, the intermediate mass fragment (IMF) production cross section, and the cross section for the formation of light particles (LPs) for the decay of compound system $^{56}\mathrm{Ni}^{*}$ formed through the entrance channel $^{32}\mathrm{S}+^{24}\mathrm{Mg}$ have been evaluated by taking the scattering potential as the sum of deformed Coulomb and deformed nuclear proximity potentials, for various ${E}_{\mathrm{c}.\mathrm{m}.}$ values. The computed results have been compared with the available experimental data of total cross section corresponding to ${E}_{\mathrm{c}.\mathrm{m}.}=60.5$ and 51.6 MeV for the entrance channel $^{32}\mathrm{S}+^{24}\mathrm{Mg}$, which were found to be in good agreement. The experimental values for the LP production cross section and IMF cross section for the channel $^{32}\mathrm{S}+^{24}\mathrm{Mg}$ were also found to agree with our calculations. Hence we have extended our studies and have thus computed the total cross section, IMF cross section, and LP cross section for the decay of $^{56}\mathrm{Ni}^{*}$ formed through the other three entrance channels $^{36}\mathrm{Ar}+^{20}\mathrm{Ne},^{40}\mathrm{Ca}+^{16}\mathrm{O}$, and $^{28}\mathrm{Si}+^{28}\mathrm{Si}$ with different ${E}_{\mathrm{c}.\mathrm{m}.}$ values. Hence, we hope that our predictions on the evaluations of the IMF cross sections and the LP cross sections for the decay of $^{56}\mathrm{Ni}^{*}$ formed through these three channels can be used for further experimental studies.

Chemical potential and symmetry energy for intermediate-mass fragment production in heavy ion reactions near the Fermi energy

Ratios of differential chemical potential values relative to the temperature, $({\ensuremath{\mu}}_{n}\ensuremath{-}{\ensuremath{\mu}}_{p})/T$, extracted from isotope yields of 13 reaction systems at 40 MeV/nucleon are compared to those of a quantum statistical model to determine the temperature and symmetry energy values of the fragmenting system. The experimental $({\ensuremath{\mu}}_{n}\ensuremath{-}{\ensuremath{\mu}}_{p})/T$ values are extracted based on the modified Fisher model. Using the density value of $\ensuremath{\rho}/{\ensuremath{\rho}}_{0}=0.56$ from the previous analysis, the temperature and symmetry energy values of $T=4.6\ifmmode\pm\else\textpm\fi{}0.4$ MeV and ${a}_{\mathrm{sym}}=23.6\ifmmode\pm\else\textpm\fi{}2.1$ MeV are extracted in a framework of a quantum statistical model. These values agree well with those of the previous work, in which a self-consistent method was utilized with antisymmetrized molecular dynamics simulations. The extracted temperature and symmetry energies are discussed together with other experimental values published in literature.

Decay of the excited compound system 48Cr* formed through 24Mg + 24Mg , 36Ar + 12C and 20Ne + 28Si reactions

The total cross section, the intermediate mass fragment (IMF) production cross section, and the cross section for the formation of light particle (LP) for the decay of 48Cr* formed through the entrance channel 24Mg + 24Mg , have been evaluated using the barrier penetration model, taking the scattering potential as the sum of the Coulomb and nuclear proximity potential, for various ECM values. The computed results have been compared with the available experimental data of the total cross section corresponding to E CM = 44.4 MeV for the entrance channel 24Mg + 24Mg , and were found to be in good agreement. The experimental values for the LP production cross section for the channel 24Mg + 24Mg were also seen to be agreeing with our calculations. Hence we have extended our studies and have thus computed the total cross section, IMF cross section and LP cross section for the decay of 48Cr* formed through the other two entrance channels 36Ar + 12C , and 20Ne + 28Si with different ECM values. It was found that the computed total cross sections for the entrance channel 36Ar + 12C with E CM = 47 MeV agree well with the corresponding experimental values. Hence, we hope that our predictions on the evaluations of the IMF cross sections and the light charged particle cross sections for the decay of 48Cr* , formed through the two entrance channels 36Ar + 12C , and 20Ne + 28Si , can be used for further experimental studies.

IMF production and symmetry energy in heavy ion collisions near Fermi energy

The symmetry energy at the time of the production of intermediate mass fragments(IMFs) is studied using experimentally observed IMF multiplicities combined with quantum statistical model calculations(QSM of Hahn and St cker). The ratios of difference in chemical potentials between neutrons and protons relative to the temperature,(μn-μp)/T,and the double ratio temperature,T,were extracted experimentally in the reactions of64,70Zn,64Ni+58,64Ni,112,124Sn,197Au,232Th at 40A MeV. The extracted(μn-μp)/TTscales linearly with δNN,where δNN is the asymmetry parameter,(N–Z)/A,of the emitting source and(μn-μp)/T=(11.1 1.4)δNN0.21 was derived. The experimentally extracted(μn-μp)/T and the double ratio temperatures are compared with those from the QSM calculations. The temperatures-,T,and densities,ρ,extracted from the(μn-μp)/T values agreed with those from the double ratio thermometer which used the yield ratios of d,t,h and α particles. However the two analyses of the differential chemical potential analysis and the initial temperature analysis end up almost identical relation between T and ρ. T=5.25±0.75 MeV is evaluated from the(μn-μp)/Tanalysis,but no density determination was possible. From the extracted T value,the symmetry energy coefficient Esym =14.6±3.5 MeV is determined for the emitting source of T=5.25±0.75 MeV.

Dependence and Influence of Projectile Energy and Target Mass on the Production of Light Charged Particles and Intermediate Mass Fragments in Proton Induced Reactions

Calculations were performed for proton induced spallation reactions over a wide range of atomic masses on the targets: 12C, 27Al, natNi, 108Ag and 197Au using an Intra-nuclear Cascade Model (INCL4.6) with coalescence which includes the emission of protons, light clusters (d–4He), and intermediate mass fragments (up to A=8) formed by the nucleons during the first stage of the reaction. The emission of particles from excited cascade residua are described using three different theoretical models SMM, ABLA07, and GEMINI++. A comparison of calculations with experimental double differential cross–sections d2σ/dΩdE for light charged particles and selected intermediate mass fragments was studied at proton beam energies from 1.2−2.5 GeV. Systematic deviations of the simulated cross sections from the experimental data were found for both light charged particles and intermediate mass fragments.

On the stability of fragments and thermalization achieved in neutron-rich heavy-ion collisions

Using the isospin-dependent quantum molecular dynamics model, we study the stability of the fragments and the thermalization achieved in the collision of a neutron- rich colliding pair at energies where the maximum production of intermediate mass frag- ments (E max c.m. ) occurs. In particular, we check the stability of fragments through the persis- tence coefficient and gain term and information about the thermalization and stopping is obtained via the relative momentum, anisotropy ratio, and rapidity distribution. Our find- ings show a weak dependence of these quantities on the neutron content of the colliding pair. c.m. (the energy at which maximal production of intermediate mass fragments occurs) and found that for a complete stopping of incoming nuclei very heavy systems are required. The mass dependence of various quantities is found to follow a power law dependence. With the availability of radioactive ion beams (RIBs), it has become possible to study neutron- rich heavy-ion collisions. The neutron content of a colliding pair is found to affect the multifragment decay (7-14), collective as well as elliptic flow (15-17). Recently, Puri and co-worker (12) studied the isospin effects on the energy of peak mass production. Both E max c.m. (the energy at which maximal production of intermediate mass fragments occurs) andNIMFmax (the maximum number of interme- diate mass fragments) are found to be insensitive to the isospin dependence of the nucleon-nucleon cross section but E max.m. showed sensitivity towards the symmetry energy. One is, therefore, interested in understanding how nuclear dynamics behaves at this peak energy. A limited number of studies

Influence of in-mediumNNcross sections, symmetry potential, and impact parameter on isospin observables

We explore the influence of in-medium nucleon-nucleon cross section, symmetry potential and impact parameter on isospin sensitive observables in intermediate-energy heavy-ion collisions with the ImQMD05 code, a modified version of Quantum Molecular Dynamics model. At incident velocities above the Fermi velocity, we find that the density dependence of symmetry potential plays a more important role on the double neutron to proton ratio $DR(n/p)$ and the isospin transport ratio $R_i$ than the in-medium nucleon-nucleon cross sections, provided that the latter are constrained to a fixed total NN collision rate. We also explore both $DR(n/p)$ and $R_i$ as a function of the impact parameter. Since the copious production of intermediate mass fragments is a distinguishing feature of intermediate-energy heavy-ion collisions, we examine the isospin transport ratios constructed from different groups of fragments. We find that the values of the isospin transport ratios for projectile rapidity fragments with $Z\ge20$ are greater than those constructed from the entire projectile rapidity source. We believe experimental investigations of this phenomenon can be performed. These may provide significant tests of fragmentation time scales predicted by ImQMD calculations.

Stability of the fragments and thermalization at the peak centre-of-mass energy

We simulated the central reactions of nearly symmetric and asymmetric systems, for energies at which maximum production of intermediate mass fragments (IMFs) occurred ( $E_{\rm c.m.}^{\rm peak}$ ). This study was carried out using hard EOS along with Cugnon cross-section employing MSTB method for clusterization. We studied the various properties of fragments. The stability of fragments was checked through persistence coefficient and gain term. The information about the thermalization and stopping in heavy-ion collisions was obtained via relative momentum, anisotropy ratio and rapidity distribution. We found that for a complete stopping of incoming nuclei very heavy systems are required. The mass dependence of various quantities (such as average and maximum central density, collision dynamics as well as the time zone for hot and dense nuclear matter) was also presented. In all cases (i.e., average and maximum central density, collision dynamics as well as the time zone for hot and dense nuclear matter) a power-law dependence was obtained.

Nuclear fragmentation reactions in extended media studied with Geant4 toolkit

It is well known from numerous experiments that nuclear multifragmentation is a dominating mechanism for production of intermediate-mass fragments in nucleus-nucleus collisions at energies above 100 A MeV. In this paper we investigate the validity and performance of the Fermi break-up model and the statistical multifragmentation model implemented as parts of the Geant4 toolkit. We study the impact of violent nuclear disintegration reactions on the depth-dose profiles and yields of secondary fragments for beams of light and medium-weight nuclei propagating in extended media. Implications for ion-beam cancer therapy and shielding from cosmic radiation are discussed.

Contributions of complete fusion and break-up–fusion to intermediate mass fragment production in the low energy interaction of 12C and 27Al

The measured spectra of a large number of intermediate mass fragments produced at a CM energy of about 110 MeV in the 27Al( 12C, x) reaction as well as in its inverse reaction, 12C( 27Al, x), are presented. The analysis of these data suggests that, at this energy, the main reaction mechanisms which contribute to the intermediate mass fragment emission are two-nucleus complete fusion and break-up–fusion reactions.

The dynamical cluster-decay model of preformed clusters for a hot and rotating 116Ba* nucleus produced in the low-energy 58Ni + 58Ni reaction

The dynamical cluster-decay model (DCM) is extended to a positive Q-value (Qout), heavy compound system 116Ba*, with complete angular momentum and charge dispersion effects included in it. The contributions due to both the light particles (LPs) and intermediate mass fragments (IMFs) are considered to give the total cross section. Interestingly, instead of the complete IMF spectrum observed for lighter systems such as 48Cr* and 56Ni*, here two small ‘windows of IMFs’ are predicted, one for light masses (2 ⩽ Z ⩽ 9) and another for the heavy mass end of symmetric and nearly symmetric fragments (14 ⩽ Z ⩽ 28), in agreement with the available data for the light mass ‘IMF window’ and its indications of possible extension to the heavier mass fragments. Within a non-statistical model description, the definition of phase space is found to be contained in the DCM definition of the ‘IMF window’ for the compound nucleus process. As in experiments, the calculated excitation functions are shown to put a limit on the minimum incident centre-of-mass energy required for the production of IMFs, and it will be of further interest to observe in experiments the predicted structures in the excitation functions of both the individual fragments, like for 12C decay, and the summed-up cross sections. Also, further measurements of the total kinetic energies of the fragments are called for.

Production of 3He and 6He fragments in the interaction of 12C with 93Nb at 400 MeV

Spectra of inclusive double differential cross sections of 3He and 6He fragments produced in the interaction of 12C with 93Nb at 400 MeV are measured and analyzed. As was found for the production of intermediate mass fragments in this and similar reactions involving light projectiles, the spectra can to a large extent be satisfactorily reproduced only by considering the contribution of projectile break-up as well as nucleon coalescence.

Time sequence and time scale of intermediate mass fragment emission

Semiperipheral collisions in the Sn124+Ni64 reaction at 35 MeV/nucleon were studied using the forward part of the Charged Heavy Ion Mass and Energy Resolving Array. Nearly completely determined ternary events involving projectilelike fragments (PLF), targetlike fragments (TLF), and intermediate mass fragments (IMF) were selected. A new method of studying the reaction mechanism, focusing on the analysis of the correlations between relative velocities in the IMF+PLF and IMF+TLF subsystems, is proposed. The relative velocity correlations provide information on the time sequence and time scale of the neck fragmentation processes leading to production of IMFs. It is shown that the majority of light IMFs are produced within 40–80 fm/c after the system starts to reseparate. Heavy IMFs are formed at times of about 120 fm/c or later and can be viewed as resulting from two-step (sequential) neck rupture processes.1 MoreReceived 2 December 2004DOI:https://doi.org/10.1103/PhysRevC.71.044602©2005 American Physical Society

HOW TO CALIBRATE THE TIME SCALE OF EMISSION OF INTERMEDIATE MASS FRAGMENTS

In order to obtain information on the time sequence and time scale of the production of intermediate mass fragments (IMF) in nucleus-nucleus collisions at intermediate energies it is proposed to analyze correlations between relative velocities of IMF's with respect to projectile-like fragments (PLF) and target-like fragments (TLF). Experimental data on the production of IMF's in the 124 Sn +64 Ni reaction, taken with the CHIMERA multidetector array at 35 MeV/nucleon, have been analyzed with the proposed method.

Size and asymmetry of the reaction entrance channel: Influence on the probability of neck production

The results of experiments performed to investigate the Ni + Al, Ni + Ni, Ni + Ag reactions at 30 MeV / nucleon are presented. From the study of dissipative midperipheral collisions, it has been possible to detect events in which intermediate mass fragments (IMF) production takes place. The decay of a quasi-projectile has been identified; its excitation energy leads to a multifragmentation totally described in terms of a statistical disassembly of a thermalized system ( T ≃ 4 MeV , E ∗ ≃ 4 MeV / nucleon ). Moreover, for the systems Ni + Ni, Ni + Ag, in the same nuclear reaction, a source with velocity intermediate between that of the quasi-projectile and that of the quasi-target, emitting IMF, is observed. The fragments produced by this source are more neutron rich than the average matter of the overall system, and have a charge distribution different, with respect to those statistically emitted from the quasi-projectile. The above features can be considered as a signature of the dynamical origin of the midvelocity emission. The results of this analysis show that IMF can be produced via different mechanisms simultaneously present within the same collision. Moreover, once fixed the characteristics of the quasi-projectile in the three considered reactions (in size, excitation energy and temperature), one observes that the probability of a partner IMF production via dynamical mechanism has a threshold (not present in the Ni + Al case) and increases with the size of the target nucleus.