Intermediate Mass Fragment Emission Research Articles

Development of a Monte-Carlo Evaporation Code for Multiple-Fragment Emission

An extended Hauser-Feshbach approach has been employed in a multi-step Monte-Carlo evaporation code designed to study the de-excitation of highly excited compound nuclei. The code is intended to account for emission of light particles ($\gamma$, n, $^{1,2,3}$H, $^{3-6}$He) and intermediate mass fragments in their ground and excited states (particle-bound or unbound). As a study case, we consider the decay of the compound nucleus $^{120}$Te$^*$ at excitation energy 100, 200 and 300 MeV. First chance decay widths are compared with treatments based on the Weisskopf and the s-wave approximation. Preliminary calculations are compared with experimental isotopic yields of intermediate mass fragments emitted in E/A = 50 MeV $^{4}$He + $^{116,124}$Sn $\rightarrow$ $^{120,128}$Te$^*$ reactions.

Isospin equilibration phenomenon and mid-rapidity IMF emission in the 48Ca +27 Al 40 MeV/nucleon collision

It is discussed the dynamics of Isospin equilibration in reactions involving the production of Intermediate Mass Fragment (IMF) in the mid-rapidity region. The investigation proceeds through the measurements of the reduced value of the total dipolar signal obtained from the measured velocities and charges of all fragments produced in the collision 48 Ca +27 Al at 40 MeV/nucleon. Preliminary experimental results, along with the comparison with CoMD-III model calculations for different density functionals related to the symmetry energy, are presented.

Emission characteristics of intermediate mass fragments by the $${^{84}_{36}{\hbox {Kr}}}$$ projectile in nuclear emulsion detector at 1 GeV per nucleon

This article is focused on the intermediate mass fragments emission induced by the Kr projectile at relativistic energy in nuclear emulsion. The study of correlation between intermediate mass fragments and the total charge confined in fragments for $$Z\ge $$ 2 with different targets (such as H, CNO and AgBr) shows that there is increase in mean multiplicity of intermediate mass fragments with increase in target mass. The observed anti-correlation between the mean multiplicities of particles emitted from the target and the bound system charge $$(Z_{\mathrm{b}})$$ reflects the relation of the $$Z_{\mathrm{b}}$$ to the centrality of the collision.

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.

Cluster correlation and fragment emission in C12+C12 at 95 MeV/nucleon

The impact of cluster correlations has been studied in the intermediate mass fragment (IMF) emission in $^{12}\mathrm{C}+^{12}\mathrm{C}$ at 95 MeV/nucleon, using antisymmetrized molecular dynamics (AMD) model simulations. In AMD, the cluster correlation is introduced as a process to form light clusters with $A\ensuremath{\le}4$ in the final states of a collision induced by the nucleon-nucleon residual interaction. Correlations between light clusters are also considered to form light nuclei with $A\ensuremath{\le}9$. This version of AMD, combined with GEMINI to calculate the decay of primary fragments, reproduces the experimental energy spectra of IMFs well overall with reasonable reproduction of light charged particles when we carefully analyze the excitation energies of primary fragments produced by AMD and their secondary decays. The results indicate that the cluster correlation plays a crucial role for producing fragments at relatively low excitation energies in the intermediate-energy heavy-ion collisions.

Clustering effects and decay analysis of the light-mass N=Z and N≠Z composite systems formed in heavy ion collisions

We investigate the clustering effects in light mass $N=Z$ and $N\ensuremath{\ne}Z$ composite systems $^{20}\mathrm{Ne}^{*}, ^{28}\mathrm{Si}^{*}, ^{40}\mathrm{Ca}^{*}$ and $^{21,22}\mathrm{Ne}^{*}, ^{39}\mathrm{K}^{*}$, respectively, formed in low-energy heavy ion reactions at different excitation energies, within the collective clusterization approach of the dynamical cluster-decay model (DCM) of Gupta and collaborators based on quantum-mechanical fragmentation theory (QMFT). Considering quadrupole deformed and compact orientated nuclei, a comparative decay analysis of these systems has been undertaken for the emission of different intermediate mass fragments (IMFs) or clusters, specifically the IMFs having $Z=3$, 4, and 5 (or $Z=7$, 6, and 5 complimentary fragments from the $^{20}\mathrm{Ne}^{*}$ and $^{21,22}\mathrm{Ne}^{*}$ composite systems) which are having the experimental data available for their $Z$ distribution. Quite interestingly, the QMFT supports clustering in $N=Z$ ($^{20}\mathrm{Ne}^{*}$ and $^{28}\mathrm{Si}^{*}$) and $N\ensuremath{\ne}Z$ ($^{21}\mathrm{Ne}^{*}$ and $^{22}\mathrm{Ne}^{*}$) nuclear systems at excitation energies corresponding to their respective decay threshold or resonant-state energies for the $4\ensuremath{\alpha}, ^{16}\mathrm{O}$ cluster and non-$\ensuremath{\alpha}$ cluster $^{14}\mathrm{C}$ (more so in $^{22}\mathrm{Ne}^{*}\phantom{\rule{4pt}{0ex}}N\ensuremath{\ne}Z$ composite system), supported by the Ikeda diagrams, taking into account the proper pairing strength in the temperature-dependent liquid drop energies. Within the DCM, we notice that at higher excitation energies in addition to $x\ensuremath{\alpha}$-type (where $x$ is an integer) clusters from $N=Z$ composite systems and $xn\ensuremath{-}x\ensuremath{\alpha}$-type clusters from $N\ensuremath{\ne}Z$ composite systems, $np\ensuremath{-}x\ensuremath{\alpha}$-type clusters are relatively quite dominant, with larger preformation probability due to the decreased pairing strength at higher temperatures in the liquid drop energies. Also, the study reveals the presence of competing reaction mechanisms of compound nucleus (fusion-fission, FF) and of noncompound nucleus origin (deep inelastic orbiting, DIO) in the decay of very-light-mass composite systems $^{20,21,22}\mathrm{Ne}^{*}$ and $^{28}\mathrm{Si}^{*}$ at different excitation energies. The DIO contribution in the IMF cross section ${\ensuremath{\sigma}}_{\text{IMF}}$ is extracted for these composite systems, ${\ensuremath{\sigma}}_{\text{IMF}}$ is given as the sum of FF cross section ${\ensuremath{\sigma}}_{\text{FF}}$ and DIO cross section ${\ensuremath{\sigma}}_{\text{DIO}}$. The DCM calculated FF cross sections ${\ensuremath{\sigma}}_{\text{FF}}^{\text{DCM}}$ are in good agreement with the available experimental data.

Ranking and validation of the spallation models for description of intermediate mass fragment emission from p + Ag collisions at 480 MeV incident proton beam energy

Double-differential cross-sections $d^{2}\sigma/d\Omega dE$ for isotopically identified intermediate mass fragments ( 6Li up to 27Mg from nuclear reactions induced by 480 MeV protons impinging on a silver target were analyzed in the frame of a two-step model. The first step of the reaction was described by the intranuclear cascade model INCL4.6 and the second one by four different models (ABLA07,GEM2, GEMINI++, and SMM). The experimental spectra reveal the presence of low-energy, isotropic as well as high-energy, forward-peaked contributions. The INCL4.6 model offers a possibility to describe the latter contribution for light intermediate mass fragments by coalescence of the emitted nucleons. The qualitative agreement of the model predictions with the data was observed but the high-energy tails of the spectra were significantly overestimated. The shape of the isotropic part of the spectra was reproduced by all four models. The GEM2 model strongly underestimated the value of the cross-sections for heavier IMF whereas the SMM and ABLA07 models generally overestimated the data. The best quantitative description of the data was offered by GEMINI++, however, a discrepancy between the data and the model cross-sections still remained for almost all reaction products, especially at forward angles. It indicates that non-equilibrium processes are present which cannot be reproduced by the applied models. The goodness of the data description was judged quantitatively using two statistical deviation factors, the H-factor and the M-factor, as a tool for ranking and validation of the theoretical models.

Intermediate mass fragment emission in 32S + 51V, 109Ag, and 238U collisions at E = 31.6 MeV A

Intermediate mass fragment emission for reactions of $^{32}\text{S} +\,^{51}\text{V},\,^{109}$Ag, and $^{238}\text{U}$ has been studied. Double differential cross sections were analysed in terms of the generalised moving source model yielding charge distributions. Isotope ratios show strong fragment mass dependencies. The data were successfully reproduced by the coalescence model as well as by statistical multifragmentation model calculations. Quantum molecular dynamics model calculations were not so successful.

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.

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.

Comparison between cross sections, saddle point and scission point barriers for the32S+24Mg reaction

One of the principal characteristics of nuclear multifragmentation is the emis- sion of complex fragments of intermediate mass. The statistical multifragmentation model has been used for many years to describe the distribution of these fragments. An extension of the statistical multifragmentation model to include partial widths and life- times for emission, interprets the fragmentation process as the near simultaneous limit of a series of sequential binary decays. In this extension, the expression describing inter- mediate mass fragment emission is almost identical to that of light particle emission. At lower temperatures, similar expressions have been shown to furnish a good description of very light intermediate mass fragment emission. However, this is usually not considered a good approximation to the emission of heavier fragments. These emissions seem to be determined by the characteristics of the system at the saddle-point and its subsequent dynamical evolution rather than by the scission point. Here, we compare the barriers and decay widths of these different formulations of intermediate fragment emission and analyze the extent to which they remain distinguishable at high excitation energy.

Detection and identification of large fragments from the partitioning of the 197Au + 197Au system at 23A MeV

First results of a study of partitioning of the nonfusing 197Au + 197Au system at an energy of 23A MeV are reported. A variety of light, medium and heavy fragments were detected and identified by using the 4π multidetector system CHIMERA (Charged Heavy Ion Mass and Energy Resolving Array). A new method of identification of low-energy fragments stopped in the silicon detectors, based on a precise time-of-flight analysis, is proposed. The method has been applied in a preliminary analysis of the 197Au + 197Au partitioning processes leading to the neck emission of intermediate mass fragments and sequential breakup of projectile-like or target-like fragments.

Correlations between isospin dynamics and Intermediate Mass Fragments emission time scales: a probe for the symmetry energy in asymmetric nuclear matter

We show new data from the 64Ni+124Sn and 58Ni+112Sn reactions studied in direct kinematics with the CHIMERA detector at INFN-LNS and compared with the reverse kinematics reactions at the same incident beam energy (35 A MeV). Analyzing the data with the method of relative velocity correlations, fragments coming from statistical decay of an excited projectile-like (PLF) or target-like (TLF) fragments are discriminated from the ones coming from dynamical emission in the early stages of the reaction. By comparing data of the reverse kinematics experiment with a stochastic mean field (SMF) + GEMINI calculations our results show that observables from neck fragmentation mechanism add valuable constraints on the density dependence of symmetry energy. An indication is found for a moderately stiff symmetry energy potential term of EOS.

From Sequential Processes to Multifragmentation in Proton Reactions with Gold

The distribution of relative angles between the intermediate mass fragments has been measured and analyzed for thermal multifragmentation in p + Au collisions at 2.1, 3.6 and 8.1 GeV. The analysis has been done on an event by event basis. The multibody Coulomb trajectory calculations of all charged particles have been performed starting with the initial break-up conditions given by the combined model with the revised intranuclear cascade (INC) followed by the statistical multifragmentation model. The measured correlation function was compared with the calculated one to find the actual time scale of the intermediate mass fragment (IMF) emission. It found transition from sequential evaporation for p(2.1 GeV) + Au to simultaneous multibody decay of a hot and expanded nuclear system in case of p(8.1 GeV) + Au.

Isospin diffusion and equilibration forSn+Sncollisions atE/A=35MeV

Equilibration and equilibration rates have been measured by colliding Sn nuclei with different isospin asymmetries at beam energies of E/A=35 MeV. Using the yields of mirror nuclei of 7Li and 7Be, we have studied the diffusion of isospin asymmetry by combining data from asymmetric 112Sn+124Sn and 124Sn+112Sn collisions with that from symmetric 112Sn+112Sn and 124Sn+124Sn collisions. We use these measurements to probe isospin equilibration in central collisions where nucleon-nucleon collisions are strongly blocked by the Pauli exclusion principal. The results are consistent with transport theoretical calculations that predict a degree of transparency in these collisions, but inconsistent with the emission of intermediate mass fragments by a single chemically equilibrated source. Comparisons with ImQMD calculations are consistent with results obtained at higher incident energies that provide constraints on the density dependence of the symmetry energy.

Variation of nonequilibrium processes in thep+Nisystem with beam energy

The energy and angular dependence of double differential cross sections dsigma/dOmega dE were measured for p, d, t, 3,4He, 6,7Li, 7,9Be, and 10,11B produced in collisions of 0.175 GeV protons with Ni target. The analysis of measured dfferential cross sections allowed to extract total production cross sections for ejectiles listed above. The shape of the spectra and angular distributions indicate the presence of other nonequilibrium processes besides the emission of nucleons from the intranuclear cascade, and besides the evaporation of various particles from remnants of intranuclear cascade. These nonequilibrium processes consist of coalescence of nucleons into light charged particles during the intranuclear cascade, of the fireball emission which contributes to the cross sections of protons and deuterons, and of the break-up of the target nucleus which leads to the emission of intermediate mass fragments. All such processes were found earlier at beam energies 1.2, 1.9, and 2.5 GeV for Ni as well as for Au targets, however, significant differences in properties of these processes at high and low beam energy are observed in the present study.

Measurement of distance parameter using semiclassical model in various heavy ion interactions

The technique of solid state nuclear track detection was employed to collect data of elastic scattering as well as inelastic reaction channel. The elastic data of the reactions U 238 + Bi 209 , U 238 + Au 197 , Pb 208 + Au 197 and Au 197 + Au 197 were used to calculate the experimental elastic scattering cross-sections for various angular bins. The ratio of these cross-sections to Rutherford/Mott cross-sections were plotted with respect to distance d ( θ ) . Distance parameter d 0 in all four reactions was obtained from the discontinuity of the slope following the method suggested in the semiclassical model. Partial reaction cross-sections were also obtained from the inelastic binary as well as multiprong events in the heavy ion interactions of Pb 208 + Au 197 and Au 197 + Au 197 both at 11.67 MeV/u beam energy. The higher values of partial cross-sections of two prong inelastic events as compared with σ 3 and σ 4 have been attributed to the emission of intermediate mass fragments in both the reactions.

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.

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

Dynamical cluster-decay model for hot and rotating light-mass nuclear systems applied to the low-energyS32+Mg24→Ni56∗reaction

The dynamical cluster-decay model (DCM) is developed further for the decay of hot and rotating compound nuclei (CN) formed in light heavy-ion reactions. The model is worked out in terms of only one parameter, namely the neck-length parameter, which is related to the total kinetic energy TKE($T$) or effective $Q$ value ${\mathit{Q}}_{\mathrm{eff}}(T)$ at temperature $T$ of the hot CN and is defined in terms of the CN binding energy and ground-state binding energies of the emitted fragments. The emission of both the light particles (LP), with $A\ensuremath{\le}4,Z\ensuremath{\le}2$, as well as the complex intermediate mass fragments (IMF), with $4lAl20,Zg2$, is considered as the dynamical collective mass motion of preformed clusters through the barrier. Within the same dynamical model treatment, the LPs are shown to have different characteristics compared to those of the IMFs. The systematic variations of the LP emission cross section ${\ensuremath{\sigma}}_{\mathrm{LP}}$ and IMF emission cross section ${\ensuremath{\sigma}}_{\text{IMF}}$ calculated from the present DCM match exactly the statistical fission model predictions. A nonstatistical dynamical description is developed for the first time for emission of light particles from hot and rotating CN. The model is applied to the decay of ${^{56}\mathrm{Ni}}^{\ensuremath{\ast}}$ formed in the $^{32}\mathrm{S}+^{24}\mathrm{Mg}$ reaction at two incident energies ${E}_{\mathrm{c}\mathrm{.}\mathrm{m}\mathrm{.}}=51.6$ and 60.5 MeV. Both the IMFs and average $\stackrel{\underline{}}{\mathit{TKE}}$ spectra are found to compare resonably well with the experimental data, favoring asymmetric mass distributions. The LPs' emission cross section is shown to depend strongly on the type of emitted particles and their multiplicities.