Intermediate Mass Fragments Research Articles

Evolution of fragment production at the onset of Multifragmentation

The onset of Multifragmentation phenomenon is investigated at low excitation energies. A detailed study on the origin of Intermediate Mass Fragment (IMF, Z>=3) produced in central collisions in the 58Ni+40Ca reaction at 25 AMeV is presented. The experimental campaign was performed with CHIMERA multi-detector at INFN Laboratori Nazionali del Sud in Catania (Italy). The multiple identification techniques of the 4p apparatus, together with low detection thresholds, enable the performance of a careful selection of Fusion-evaporation residues, Multifragmentation sources and their decay products. Comparisons with dynamical approach based on Boltzmann-Langevin-One-Body (BLOB) model predictions coupled with sequential emission code, were used as useful tools to depict and understand the characteristics of fragments emitted from an equilibrated compound nucleus or formed simultaneously in the multifragmenting source. A comparison with the preliminary results obtained for fragment production in central collisions of the same system, 58Ni+40Ca, at higher incident energies Ebeam=35AMeV, allows to study and characterize the evolution of multifragmentation phenomenon at the lower end of Fermi energies.

Statistical approach of nuclear multifragmentation with a realistic nuclear equation of state

In this work, Canonical Thermodynamical model for nuclear multifragmentation has been updated with realistic nuclear equation of state. Mass distribution, intermediate mass fragment multiplicity as well as isospin sensitive observables have been investigated with semi-microscopic approach of determining nuclear binding and excitation energies. Production of neutron rich isotopes as well as isoscaling and isobaric yield ratio parameters have been significantly modified due to inclusion of this realistic nuclear equation of state.

Reaction dynamics and in-medium nucleon-nucleon cross section with C12+H1 at 95 MeV/nucleon

Reaction dynamics and effective in-medium nucleon-nucleon $(NN)$ cross section are studied, using the antisymmetrized molecular dynamics (AMD) model with one of the simplest heavy ion collisions, $^{12}\mathrm{C}+^{1}\mathrm{H}$. The roles of different stochastic processes are studied, which are installed in the original AMD. One is the diffusion process as a part of the quantum fluctuation originated from the Fermi motion during the time evolution of the wave packets in the mean field (AMD/D). The second is a coalescence treatment for light clusters with $A\ensuremath{\le}8$ (AMD/D-COALS). The third is the collision process, installed in AMD with different form of the in-medium $NN$ cross sections. These different processes are examined, using the experimental angular distributions and energy spectra of light charged particles and intermediate mass fragments. The effective in-medium $NN$ cross sections are evaluated from the simulated AMD events and compared with the experimentally extracted values. A good agreement is obtained between the AMD values from the $^{12}\mathrm{C}$ $+$ $^{1}\mathrm{H}$ reaction at 95 MeV/nucleon and the experimental results for nuclear matter.

Determining the nuclear temperature dependence on source neutron-proton asymmetry in heavy-ion reactions at intermediate energy* *Supported by the National Natural Science Foundation of China (12275186, 11705242, 12175156, 11805138, 11905120) and the Fundamental Research Funds For the Central Universities in China (YJ201954, YJ201820)

In this article, we investigate the dependence of nuclear temperature on emitting source neutron-proton () asymmetry with light charged particles (LCPs) and intermediate mass fragments (IMFs) generated from intermediate-velocity sources in thirteen reaction systems with different asymmetries, on , and , on , , , and at 40 MeV/nucleon. The apparent temperature values of LCPs and IMFs from different systems are deduced from the measured yields using two helium-related and eight carbon-related double isotope ratio thermometers, respectively. Then, the sequential decay effect on the experimental apparent temperature deduction with the double isotope ratio thermometers is quantitatively corrected explicitly with the aid of the quantum statistical model. The present treatment is an improvement compared to our previous studies in which an indirect method was adopted to qualitatively consider the sequential decay effect. A negligible asymmetry dependence of the real temperature after the correction is quantitatively addressed in heavy-ion reactions at the present intermediate energy, where a change of 0.1 units in source asymmetry corresponds to an absolute change in temperature of an order of 0.03 to 0.29 MeV on average for LCPs and IMFs. This conclusion is in close agreement with that inferred qualitatively via the indirect method in our previous studies.

Fusion enhancement within a collective clusterization approach applied to the isotopic chain of neutron-rich light-mass compound nuclei

The dynamics of neutron-rich light-mass compound nuclei $^{24,25,26,27}\mathrm{Mg}^{*}$ formed in $^{12,13,14,15}\mathrm{C}+^{12}\mathrm{C}$ fusion reactions, respectively, is explored at different ${E}_{\mathrm{c}.\mathrm{m}.}$ values within the dynamical cluster decay model (DCM). DCM is based on the quantum mechanical fragmentation theory, which treats the binary fragmentation of emitting compound nucleus as light particles (LPs), intermediate mass fragments (IMFs), as well as symmetric mass fragments (SMFs), on equal footing, which is not the case with other fission and statistical models. All calculations are made for spherical considerations and angular momenta taken up to the $\ensuremath{\ell}$-critical value for the respective reactions. A fusion enhancement is observed as we go from compound nuclei (CN) having an even number of neutrons ($^{24,26}\mathrm{Mg}^{*}$) to CN with the odd number of neutrons ($^{25,27}\mathrm{Mg}^{*}$). Interestingly, we find that the only parameter of the DCM, the neck-length parameter $\mathrm{\ensuremath{\Delta}}R$ related to the barrier lowering, is in linear relation with ${\ensuremath{\sigma}}_{\mathrm{Fusion}}$ at chosen incident laboratory energy. The fusion enhancement is found to be larger for odd neutron number nuclei, which indicates the influence of unpaired neutrons on fusion cross sections. Within the formalism of DCM, the phenomenon of fusion enhancement finds its base in the process of collective clusterization, which is quite evident from the larger values of summed-up preformation probability for the reactions having odd mass projectiles or forming odd mass CN. The DCM calculated fusion cross sections are in good agreement with the available experimental data for the given reactions.

Characteristic time scale of cluster production at the Fermi energy

The study of heavy-ion collisions in the Fermi energy domain (20 MeV/nucleon < E/A < 100 MeV/nucleon) is a fundamental research topic in modern nuclear physics. In the case of semi-peripheral collisions, the Fermi energy regime is characterized by the formation of a transient, neck-like structure that connects a projectile-like fragment (PLF) with a target-like fragment (TLF). The neck structure represents a precursor of the fireball which is a typical overlap of participating nucleons at relativistic energies (E/A> 200 MeV/nucleon). It undergoes an expansion phase in a short time scale of the order of 100 fm/c with the formation of a low-density region of nuclear matter, therefore favoring the clusterization of intermediate mass fragments (IMFs) of atomic number Z (typically) less than 20. Particular emphasis is given to some relevant results obtained by the CHIMERA collaboration in the last decade, regarding the time scale of the production mechanisms of the intermediate mass fragments in neck fragmentation and their neutron enrichment.

Isospin effects on intermediate mass fragments at intermediate energy-heavy ion collisions

Isospin effects on intermediate mass fragments at intermediate energy-heavy ion collisions

Dissipation of energy and higher-order fluctuations of the largest fragment charge in projectile fragmentation

Abstract The higher-order fluctuations of the largest fragment charge were found to exhibit the signatures of a second-order phase transition in experimental data and simulations of the statistical multifragmentation model recently (Pietrzak et al 2020 Phys. Lett. B 809 135763). This work is devoted to study the dissipation of energy and higher-order fluctuations of the largest fragment charge in projectile fragmentation. The isospin-dependent quantum molecular dynamics (IQMD) model is used to study the non-equilibrium thermalization and fragmentation. The statistical code GEMINI is applied to simulate the second decay of the pre-fragments. To reveal how the incident energy dissipates into the excitation energy of the projectile-like system, the time evolution of the density, collective velocity, and random kinetic energy are displayed. It is found that the competition between the heat conduction and heat flux affects the excitation energy of the projectile-like system, which is the key variable in the fragmenting process. Displaying the mean multiplicity of the intermediate mass fragments and the higher-order fluctuations of the largest fragment charge, it is found that the decay mechanism of the projectile-like system transfers from the multi-fragmentation at mid-peripheral collision into the nucleon-evaporation at peripheral collision. The pseudo-critical point of this transition can be indicated by the zero of third order fluctuation together with the minimum of fourth order fluctuation. The calculations of the pseudo-critical point by the IQMD + GEMINI model agree with that extracted from the experimental data. Finally, the influences of the pseudo-critical point by the second decay, the mass number of the colliding system, and the incident energy are studied.

Angular momentum effects on the decay modes of hot compound nuclei formed in 86Kr+134,138Ba reactions

The dependence of decay modes of hot compound nuclei formed in [Formula: see text]Kr[Formula: see text]Ba reactions on the angular momentum has been analyzed using the dynamical cluster-decay model. This has been done by analyzing the change of fragmentation potential, preformation probability, penetrability and cross-section as a function of the fragment mass number at various values of the angular momentum. The evaporation residue cross-section calculated using dynamical cluster-decay model has been compared with the measured one for the channels: [Formula: see text], [Formula: see text], [Formula: see text] for the decay of [Formula: see text]U[Formula: see text] and [Formula: see text] for [Formula: see text]U[Formula: see text]. The summed up fragment cross-sections for the light particles, intermediate mass fragments and fission fragments as a function of angular momentum have been studied which gives an idea for the change of reaction mechanism with angular momentum. The charge distribution of the fission fragment cross-section shows an odd–even staggering, i.e., the probability of even-Z fission fragments is relatively higher than others. The mass distributions obtained within the dynamical cluster-decay model for the decay of [Formula: see text]U[Formula: see text] are in conformity with mass distributions determined by means of the double kinetic-energy technique in literature. The position of the most probable channel has been found to be fixed to a constant atomic number instead of mass number.

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.

Decay aspects of the compound nuclei formed via CF and ICF path in 12C[formula omitted]Cr reaction

The reaction dynamics of 12C+52Cr reaction induced via a cluster-structured projectile (12C) is examined theoretically using the dynamical cluster decay model (DCM) with respect to the recent experimental data available at wide span of center-of-mass energies (Ec.m.=41.84-71.20 MeV). In view of the cluster-structured projectile, a systematic analysis is carried out for both complete fusion (CF) and incomplete fusion (ICF) channel. The calculations are done by including the quadrupole (β2) deformations and optimum (θiopt.) orientation criteria of the decaying fragments. With an aim to have comprehensive analysis of the decay of compound nuclei formed via CF and ICF processes, the role of center-of-mass energy (Ec.m.) and angular momentum (ℓ) is explored in terms of various parameters of DCM such as fragmentation potential, preformation probability, penetrability and barrier modification. Further, the role of diffuseness parameter is also studied for 12C+52Cr reaction. For the given reaction, the contribution of light particles (LPs), and energetically favored intermediate mass fragments (IMFs) constitutes the decay cross sections. Therefore, the contribution of emitted LPs and IMFs is estimated for the decay of both compound nuclei formed via CF and ICF channels. Furthermore, the comparative analysis of different proximity potentials is also worked out for ICF. Finally, the effect of Ec.m. on the contribution of ICF cross sections is studied and compared with the experimental data.

Isospin effects of projectile fragmentation in a Boltzmann-Langevin approach * *Supported by the National Natural Science Foundation of China (11635003, 11025524, 11161130520), National Basic Research Program of China (2010CB832903) and the European Commissions 7th Framework Programme (FP7-PEOPLE-2010-IRSES) under Grant Agreement Project (269131)

The isospin effects of projectile fragmentation at intermediate energies are investigated using an isospin-dependent Boltzmann-Langevin model. The collisions of mass-symmetric reactions including 58Fe, 58Ni + 58Fe, and 58Ni at intermediate energies, in the 30 to 100 MeV/A range, are studied for different symmetry energies. Yield ratios of the isotopic, isobaric, and isotonic pairs of fragments from the intermediate-mass region using three symmetry energies are extracted as functions of the N/Z ratio of the composite systems in the entrance channel and the incident energies. It is found that the yield ratios are sensitive to symmetry energies, especially for neutron-rich systems, and the calculations using soft symmetry energy are closer to the experimental data. The isospin effect is stronger for the soft symmetry energy, owing to the competition of the repulsive Coulomb force and the symmetry energy attractive force on the proton. For the first time, the splits are presented, revealing a transition from the isospin equilibrium at lower energies to translucency at intermediate energies. The results show a degree of transparency in that intermediate mass fragments undergo a transition from dependence on the composite systems in the entrance channel to reliance on the projectile and target nuclei.

Interplay of Coulomb and symmetry potential in peak fragment production in asymmetric collisions

Role of the nuclear symmetry potential and Coulomb potential is explored on the peak energy of intermediate mass fragments ([Formula: see text]) and on peak multiplicity ([Formula: see text]) and their dependence on mass asymmetry of the reaction is also investigated. The calculations are done using Isospin-dependent Quantum Molecular Dynamics (IQMD) model. We also showed that the momentum-dependent interactions have uniform effects of [Formula: see text] and these effects are independent of mass asymmetry of the reaction. Further, we see that isospin effects that enter through the Coulomb and symmetry potential show much significant role as one increases the mass asymmetry of reaction. Mass asymmetric reactions thus serve a sensitive tool to investigate the nuclear symmetry energy effects.

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.

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

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

Nuclear temperature and its dependence on the source neutron-proton asymmetry deduced using the Albergo thermometer

Albergo thermometers with double isotope, isotone, and isobar yield ratio pairs with one proton and/or neutron difference are investigated. Without any extra sequential decay correction, a real temperature value of $4.9\ifmmode\pm\else\textpm\fi{}0.5$ MeV is deduced from the yields of the experimentally reconstructed primary hot intermediate mass fragments (IMFs) from $^{64}\mathrm{Zn}+^{112}\mathrm{Sn}$ collisions at 40 MeV/nucleon using the Albergo thermometer for the first time. An experimental sequential decay correction from the apparent temperatures to the real ones for 12 other reaction systems with different neutron-proton ($N/Z$) asymmetries in the same experiment, $^{70}\mathrm{Zn}$, $^{64}\mathrm{Ni}$ on $^{112,124}\mathrm{Sn}$, $^{58,64}\mathrm{Ni}$, $^{197}\mathrm{Au}$, and $^{232}\mathrm{Th}$ at 40 MeV/nucleon, is performed using an empirical correction factor approach of Tsang et al. [Phys. Rev. Lett. 78, 3836 (1997)] with the deduced 4.9-MeV temperature value. The dependence of nuclear temperature on the source $N/Z$ asymmetry is further investigated using these deduced real source temperature values from the present 13 systems. It is found that the deduced real source temperatures at the present source $N/Z$ range show a rather weak dependence on the source $N/Z$ asymmetry. By comparison between our previous results and those from other independent experiments, a consistent description for the $N/Z$ asymmetry dependence of nuclear temperature is addressed.

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.

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.

Elucidating the differences in oxidation of high-performance \u03b1- and \u03b2- diisobutylene biofuels via Synchrotron photoionization mass spectrometry

Biofuels are a promising ecologically viable and renewable alternative to petroleum fuels, with the potential to reduce net greenhouse gas emissions. However, biomass sourced fuels are often produced as blends of hydrocarbons and their oxygenates. Such blending complicates the implementation of these fuels in combustion applications. Variations in a biofuel’s composition will dictate combustion properties such as auto ignition temperature, reaction delay time, and reaction pathways. A handful of novel drop-in replacement biofuels for conventional transportation fuels have recently been down selected from a list of over 10,000 potential candidates as part of the U.S. Department of Energy’s (DOE) Co-Optimization of Fuels and Engines (Co-Optima) initiative. Diisobutylene (DIB) is one such high-performing hydrocarbon which can readily be produced from the dehydration and dimerization of isobutanol, produced from the fermentation of biomass-derived sugars. The two most common isomers realized, from this process, are 2,4,4-trimethyl-1-pentene (α-DIB) and 2,4,4-trimethyl-2-pentene (β-DIB). Due to a difference in olefinic bond location, the α- and β- isomer exhibit dramatically different ignition temperatures at constant pressure and equivalence ratio. This may be attributed to different fragmentation pathways enabled by allylic versus vinylic carbons. For optimal implementation of these biofuel candidates, explicit identification of the intermediates formed during the combustion of each of the isomers is needed. To investigate the combustion pathways of these molecules, tunable vacuum ultraviolet (VUV) light (in the range 8.1–11.0 eV) available at the Lawrence Berkeley National Laboratory’s Advanced Light Source (ALS) has been used in conjunction with a jet stirred reactor (JSR) and time-of-flight mass spectrometry to probe intermediates formed. Relative intensity curves for intermediate mass fragments produced during this process were obtained. Several important unique intermediates were identified at the lowest observable combustion temperature with static pressure of 93,325 Pa and for 1.5 s residence time. As this relatively short residence time is just after ignition, this study is targeted at the fuels’ ignition events. Ignition characteristics for both isomers were found to be strongly dependent on the kinetics of C4 and C7 fragment production and decomposition, with the tert-butyl radical as a key intermediate species. However, the ignition of α-DIB exhibited larger concentrations of C4 compounds over C7, while the reverse was true for β-DIB. These identified species will allow for enhanced engineering modeling of fuel blending and engine design.

Fragment emission and critical behavior in light and heavy charged systems * *Supported Council of Scientific and Industrial Research (CSIR), Govt. of India (03 (1388)/16/EMR-II)

We study the emission of fragments in central collisions of light and heavily charged systems of 40Ar+45Sc and 84Kr+197Au, respectively, using the Quantum Molecular Dynamics (QMD) model as the primary model. The fragments are identified using an energy based clusterization algorithm, i.e., the Simulated Annealing Clusterization Algorithm (SACA). The charge distributions of intermediate mass fragments [3≤ ≤12] are fitted with power-law ( ) and exponential ( ) fits in order to extract the parameters τ and whose minimum values are also sometimes linked with the onset of fragmentation or the critical point for a liquid-gas phase transition. Other parameters such as the normalized second moment , , average size of the second largest cluster , phase separation parameter ( ), bimodal parameter (P), information entropy (H), and Zipf's law are also analyzed to find the exact energy of the onset of fragmentation. Our detailed analysis predicts that an energy point exists between 20-23.1 MeV/nucleon, which is very close to the experimentally observed value of 23.9 MeV/nucleon for the 40Ar+45Sc reaction. We also find that the critical energy deduced using Zipf's law is higher than those predicted from other critical exponents. Moreover, no minimum is found for τ values of the highly charged system of 84Kr+197Au, in agreement with experimental findings and various theoretical calculations. We observe that the QMD + SACA model calculations are in agreement with the experimental observations. This agreement supports our results regarding the energy point of the liquid-gas phase transition and the onset of fragmentation.