Pre-equilibrium Emission Research Articles

Probing the momentum dependence of the symmetry potential by the freen/pratio of pre-equilibrium emission

Based on an isospin and momentum-dependent transport model, we studied the effect of momentum-dependent symmetry potential on the free neutron to proton ratio of preequilibrium nucleon emission. It is found that for $^{132}Sn+^{124}Sn$ reaction at 400 MeV/nucleon incident beam energy, free n/p ratio of preequilibrium nucleon emission mainly probe the momentum-dependence of symmetry potential at nucleon momentum around 400 $\sim$ 600 MeV/c. Whereas for 200 MeV/nucleon incident beam energy, this observable mainly probe the momentum-dependence of symmetry potential at nucleon momentum around 200 $\sim$ 400 MeV/c. To probe the symmetry energy/potential using free n/p ratio, not all the details of the momentum-dependence of the symmetry potential are important, the values of symmetry potential at only certain momentum range are crucial for an observable. It is important to input reasonable density- and momentum-dependence of the symmetry potential according to the magnitude of incident beam energy of heavy-ion collisions. The present experimental data on the symmetry potential are not enough for probing the density-dependent symmetry energy. More experimental data (such as nucleon and nuclei's scattering experiments at different nucleonic momenta and densities) on the symmetry potential are therefore needed to pin down the density-dependent symmetry energy.

Systematic study of pre-equilibrium emission at low energies inC12- andO16-induced reactions

Background: The role of pre-equilibrium emission within the heavy-ion fusion process has not been fully characterized. An accurate description of this process is important for understanding the formation of the compound nucleus in fusion reactions.Purpose: We develop a systematic description, based on experimental measurements, of the strength of the pre-equilibrium process in heavy-ion fusion reactions.Method: With a view to study pre-equilibrium emission process, the excitation functions for some neutron emission channels occurring in the fusion of $^{12}\mathrm{C}$ with $^{128}\mathrm{Te}$ and $^{169}\mathrm{Tm}$, and of $^{16}\mathrm{O}$ with $^{159}\mathrm{Tb},^{169}\mathrm{Tm}$, and $^{181}\mathrm{Ta}$, respectively, have been measured at incident energies from near the Coulomb barrier to $\ensuremath{\approx}7$ MeV/nucleon. The off-line $\ensuremath{\gamma}$-ray spectrometry--based activation technique has been used for the measurements of excitation functions. The measured excitation functions have been compared with theoretical predictions based on pure statistical model code pace4 and Geometry Dependent Hybrid (GDH)-based code alice-91. The strength of pre-equilibrium emission has also determined from comparison of the experimental excitation functions and the pace4 calculations.Results: The measured excitation functions are satisfactorily reproduced by the pace4 calculations in the energy region up to the peak position. However, at relatively higher energies, the enhancement of experimental cross sections in the tail portion of excitation functions as compared to the theoretical predictions of code pace4 has been observed. The observed deviation may be attributed to the pre-equilibrium emission of particles during the thermalization of the compound nucleus. Further, alice-91 calculations which include PE emission satisfactorily reproduce the experimental data even at higher energies, indicating the significant contribution of pre-equilibrium emissions.Conclusions: Analysis of data clearly indicates that pre-equilibrium emission is an important reaction mechanism even at low projectile energies where the compound nucleus reaction mechanism dominates, and pre-equilibrium fraction `${P}_{FR}$ strongly depends on excitation energy available for surface nucleons in composite systems above the Coulomb barrier and the mass of the composite system.

Pre-equilibrium emission and its possible relation toα-clustering in nuclei

The study of nuclear states built on clusters bound by valence neutrons in their molecular configurations is a field of large interest. Clustering becomes particularly important at the dripline, where weakly bound systems prevail. For light nuclei, at an excitation energy close to the particle separation value, there are experimental evidences of such structure effects, but this is still not the case for heavier nuclear systems. Several theoretical efforts have been done in the development of pre-formation alpha-clustering models, but there is still a lack of experimental data capable to give a direct feedback. The search of alpha-cluster evidences in medium-mass systems is therefore a new challenge which can give new hints in this field of research.

Symmetry energy dependence of light fragment production in heavy-ion collisions

We study the pre-equilibrium light cluster emission in low to intermediate energy heavy ion collisions in transport simulations as a means to obtain information about the density and momentum dependence of the nuclear symmetry potential, i.e. about the asy-stiffness and the neutron-proton effective mass splitting of the symmetry energy. We find that the study of n/p and t/He-3 ratios as a function of the energy of the emitted particles may allow to disentangle these two behaviors The t/He-3 ratios appear to carry similar information as the n/p ratios, making this a promising option for experimental investigations. More n-rich systems enhance the sensitivity to the symmetry energy, while double ratios between n-rich and n-poor systems tend to reduce it. A preliminary comparison to experimental data for the Xe-136, Xe-124 + (124, 112) Sn systems is also shown.

Onset of quenching of the giant dipole resonance at high excitation energies

The evolution of the giant dipole resonance (GDR) properties in nuclei of mass $A=120$ to 132 has been investigated in an excitation energy range between 150 and 270 MeV through the study of complete and nearly complete fusion reactions using $^{116}\mathrm{Sn}$ beams at $17A$ and $23A$ MeV from the cyclotron of the Laboratorio Nazionale del Sud impinging on $^{12}\mathrm{C}$ and $^{24}\mathrm{Mg}$ targets. $\ensuremath{\gamma}$ rays and light charged particles were detected using the multi-element detector array MEDEA in coincidence with evaporation residues detected by using mass and charge identification spectrometry with telescope (MACISTE). Light-charged-particle energy spectra were analyzed within the framework of a multiple-source-emission scenario by using a fitting procedure to determine the amount of pre-equilibrium emission and deduce the excitation energies reached in the compound nuclei. A detailed analysis of the $\ensuremath{\gamma}$-ray spectra and their comparison with statistical model calculations is presented. Evidence of a quenching of the GDR gamma yield was found at 270 MeV excitation energy. The quenching effect becomes progressively more important with increasing excitation energy, as observed when the comparison is extended to data from the reaction $^{36}\mathrm{Ar}+^{96}\mathrm{Mo}$ at $37A$ MeV where hot nuclei were populated up to 430 MeV excitation energy. A coherent scenario emerges indicating the existence of a limiting excitation energy for the collective motion of about ${E}^{*}/A=2.1$ MeV for systems of mass $A=105$ to 111 while a slightly lower value was observed for nuclei of mass $A\ensuremath{\sim}132$. The existence of a possible link between GDR disappearance and the liquid-gas phase transition is discussed.

Theoretical Predictions of Excitation Functions of Neutron-Induced Reactions on 6Li, 9Be, 12C and 23Na Nuclei at Low Energies

In the present work, the excitation functions of neutron reactions 6Li(n,p)6He, 9Be(n,α)6He, 12C(n,p)12B, 12C(n,α)9Be, 23Na(n,p)23Ne, 23Na(n,2n)22Na and 23Na(n,α)20F produced by the incident neutrons were studied for the investigation of fusion structural materials. Tel et al. formulae (for sodium isotopes), the codes ALICE–ASH and CEM03.01 (for all nuclei) were employed for the obtaining of cross-sections. In addition, hybrid model and geometry-dependent hybrid model with pre-equilibrium emissions, and Weisskopf Ewing model with the equilibrium emissions, and the cascade-exciton model with cascade interactions were used to calculate the excitations curves. The obtained results and Tendl-2011 library data were compared with the experimental literature values. Finally it is to mention that the cross-section data obtained may be useful for the safety design of future fusion reactors.

Consistent Analysis of the Nuclear Reaction Mechanisms Involved in Deuteron-induced Activation at Low and Medium Energies

A complete analysis of the nuclear reaction mechanisms involved within natFe(d, x)55Co reaction, i.e. the breakup, stripping, pick-up, pre-equilibrium emission, and evaporation from fully equilibrated compound nucleus, is presented in order to highlight the importance of direct mechanisms that are not appropriately considered in the evaluation procedure. The overall agreement between the measured data and model calculations confirms the correctness of nuclear mechanism description taken into account stressing out the omissions of the evaluation procedures.

New Evaluation of n+63,65Cu Nuclear Cross Section Data up to 200 MeV Neutron Energy

This work presents newly evaluated general purpose nuclear data files for the n+63Cu and n+65Cu reactions with upper neutron incident energy 200 MeV. Special attention was devoted to the accuracy and consistency of the evaluated data. The TALYS code was used for the nuclear model simulations in the energy range from 1 keV to 200 MeV. To improve the pre-equilibrium particle emission the Geometry-Dependent Hybrid model (GDH) was included in TALYS as a new option. The nuclear model parameters were properly adjusted to achieve the best description of the available measured data. New resonance data based on recent measurements and their covariances were included in the files. A set of covariance data for all nuclear reactions was also prepared making use of the Unified Monte Carlo Approach.

Preequilibrium Emission of Light Fragments in Spallation Reactions

The ability to describe production of light fragments (LF) is important for many applications, such as cosmic-ray-induced single event upsets (SEUs), radiation protection, and cancer therapy with proton and heavy-ion beams. The cascade-exciton model (CEM) and the Los Alamos version of the quark-gluon string model (LAQGSM) event generators in the LANL transport code MCNP6, describe quite well the spectra of fragments with sizes up to 4He across a broad range of target masses and incident energies (up to ∼ 5 GeV for CEM and up to ∼ 1 TeV/A for LAQGSM). However, they do not predict the high-energy tails of LF spectra heavier than 4He well. Most LF with energies above several tens of MeV are emitted during the precompound stage of a reaction. The current versions of our event generators do not account for precompound emission of LF larger than 4He. The aim of our work is to generalize the precompound model to include such processes, leading to increased predictive power of LF production. Extending the model in this way provides preliminary results that have much better agreement with experimental data.

Improved Simulation of the Pre-equilibrium Triton Emission in Nuclear Reactions Induced by Nucleons

A new approach is proposed for the calculation of non-equilibrium triton energy distributions in nuclear reactions induced by nucleons of intermediate energies. It combines models describing the nucleon pick-up, the coalescence and the triton knock-out processes. Emission and absorption rates for excited particles are represented by the pre-equilibrium hybrid model. The model of Sato, Iwamoto, Harada is used to describe the nucleon pick-up and the coalescence of nucleons from exciton configurations starting from (2p,1h) states. The contribution of the direct nucleon pick-up is described phenomenologically. Multiple pre-equilibrium emission of tritons is accounted for. The calculated triton energy distributions are compared with available experimental data.

Symmetry Energy Dependence of Light Fragment Production in Heavy Ion Collisions

We study the pre-equilibrium light cluster emission in low to intermediate energy heavy ion collisions in transport simulations as a means to obtain information about the density and momentum dependence of the nuclear symmetry potential, i.e. about the asy-stiffness and the neutron-proton effective mass splitting of the symmetry energy. We find that the study of n/p and t/He-3 ratios as a function of the energy of the emitted particles may allow to disentangle these two behaviors The t/He-3 ratios appear to carry similar information as the n/p ratios, making this a promising option for experimental investigations. More n-rich systems enhance the sensitivity to the symmetry energy, while double ratios between n-rich and n-poor systems tend to reduce it. A preliminary comparison to experimental data for the Xe-136, Xe-124 + (124, 112) Sn systems is also shown.

Production of Energetic Light Fragments in Spallation Reactions

Different reaction mechanisms contribute to the production of light fragments (LF) from nuclear reactions. Available models cannot accurately predict emission of LF from arbitrary reactions. However, the emission of LF is important for many applications, such as cosmic-ray-induced single event upsets, radiation protection, and cancer therapy with proton and heavy-ion beams, to name just a few. The cascade-exciton model (CEM) and the Los Alamos version of the quark-gluon string model (LAQGSM), as implemented in the CEM03.03 and LAQGSM03.03 event generators used in the Los Alamos transport code MCNP6, describe quite well the spectra of fragments with sizes up to 4He across a broad range of target masses and incident energies. However, they do not predict high-energy tails for LF heavier than 4He. The standard versions of CEM and LAQGSM do not account for preequilibrium emission of LF larger than 4He. The aim of our work is to extend the preequilibrium model to include such processes. We do this by including the emission of fragments heavier than 4He at the preequilibrium stage, and using an improved version of the Fermi Break-up model, providing improved agreement with various experimental data.

Weisskopf–Ewing and Hauser–Feshbach calculations of photonuclear cross sections used for electromagnetic dissociation

The Weisskopf–Ewing (WE) and Hauser–Feshbach (HF) theory are statistical methods, which are often used to calculate photonuclear cross sections for compound nucleus reactions. In our past work, WE methodology was presented and photonuclear reaction cross sections for nucleon emission were calculated using WE theory. Here, our previous results, which neglect pre-equilibrium emissions and do not include multiple particle emission, are compared to those calculated with HF theory and experimental data. For the reactions considered herein, it is found that the WE theory and HF method are in reasonable agreement below the two neutron separation energy assuming an energy dependent branching ratio for intermediate and heavy nuclei. In addition, qualitative confidence of WE theory for electromagnetic dissociation (EMD) cross section calculations was found.

Thick target neutron yield from 145 MeV 19F+27Al system

The double differential neutron energy distribution has been measured for the 19F+27Al system at 145MeV projectile energy. The time of flight technique was used to measure the energy while pulse shape discrimination has been used to separate the neutrons from photons. The results are compared with the statistical nuclear reaction model codes PACE and EMPIRE. The PACE code appears to predict the slope and the end point energy of the experimental spectra fairly well but over predicts the values. The slope obtained from the EMPIRE calculations appears to be harder while the values being closer to the experimental results. The yield from the Hauser–Feshbach based compound nucleus model calculations agree reasonably well with the experimental results at the backward angles but not in the forward directions. The energy integrated angular distribution from 145MeV projectiles show an enhanced emission in the forward angles compared to the similar results from 110MeV projectiles. This analysis suggests some contribution from the pre-equilibrium emissions from the system at the higher projectile energy.

Extension of a nuclear reaction calculation code CCONE toward higher incident energies — multiple preequilibrium emission, and spectrum in laboratory system

A nuclear reaction calculation code CCONE, which was developed for nuclear data evaluation for JENDL/AC-2008 and JENDL-4, has been upgraded to improve the prediction accuracy for calculated cross sections at nucleon incident energies higher than 20 MeV. Multiple particle emission, in which nucleons and complex particles up to α-particle are involved, from pre-equilibrium reaction process was implemented based on the sequential-decay calculations for all produced exciton states within the framework of the two-component exciton model. The effect of velocity-change of particle-emitting nuclei on the multiple emission in preequilibrium and compound processes, which was not included in the previous evaluations, was taken into account to obtain spectra in the laboratory system using an average velocity approximation for each composite/compound nucleus. Calculated nucleon emission spectra at nucleon incident energies from 20 to 200 MeV were compared with experimental and evaluated data for the proton- and neutron-induced reactions on 27Al. The present results are in good agreement with experimental data. It was found that their predictions were better than those of JENDL/HE-2007 especially for low emission energies at high incident energies.

Isotopic fission fragment distributions as a deep probe to fusion-fission dynamics

During the fission process, the atomic nucleus deforms and elongates up to the two fragments inception and their final separation at the scission deformation. The evolution of the nucleus energy with deformation defines a potential energy landscape in the multidimensional deformation space. It is determined by the macroscopic properties of the nucleus, and is also strongly influenced by the single-particle structure of the nucleus, which modifies the macroscopic energy minima. The fission fragment distribution is a direct consequence of the deformation path the nucleus has encountered, and therefore is the most genuine experimental observation of the potential energy landscape of the deforming nucleus. Very asymmetric fusion-fission reactions at energy close to the Coulomb barrier, produce well-defined conditions of the compound nucleus formation, where processes such as quasi-fission, pre-equilibrium emission and incomplete fusion are negligible. In the same time, the excitation energy is sufficient to reduce significantly structural effects, and mostly the macroscopic part of the potential is responsible for the formation of the fission fragments. We use inverse kinematics combined with a spectrometer to select and identify the fission fragments produced in 238U+12C at a bombarding energy close to and well-above the Coulomb barrier. For the first time, the isotopic yields are measured over the complete atomic-number distribution, between Z=30 and Z=63. In the experimental set-up, it is also possible to identify transfer-induced reactions, which lead to low-energy fission

New isospin effects in central heavy-ion collisions at Fermi energies

Isospin e ffects on multifragmentation properties were studied thanks to nuclear collisions between di fferent isotopes of xenon beams and tin targets. It is shown that, in central collisions leading to multifragmentation, the mean number of fragments and their mean kinetic energy increase with the neutron-richness of the total system. Comparisons with a stochastic transport model allow to attribute the multiplicity increase to the multifragmentation stage, before secondary decay. The total charge bound in fragments is proposed as an alternate variable to quantify preequilibrium emission and to investigate symmetry energy e ffects.

Experimental and theoretical study of the (n,2n) reaction on174,176Hf isotopes

Cross sections for the ${}^{174}$Hf($n,2n$)${}^{173}$Hf and ${}^{176}$Hf($n,\phantom{\rule{-0.16em}{0ex}}2n$)${}^{175}$Hf reactions have been measured on the 5.5-MV Van de Graaff tandem accelerator of National Centre for Scientific Research ``Demokritos'' in Athens, in the neutron energy region from 8.8 to 11.0 MeV, using the activation technique. An experimental method to account for the contamination of the ${}^{176}$Hf($n,\phantom{\rule{-0.16em}{0ex}}2n$) reaction by the ${}^{174}$Hf($n,\phantom{\rule{-0.16em}{0ex}}\ensuremath{\gamma}$) reaction activated by the parasitic neutrons of the beam is presented. Statistical model calculations have also been performed using the nuclear-reaction codes empire-ii and talys. The results and effects of the nuclear input parameters as well as pre-equilibrium emission are discussed in detail.

Energy dependence of pre-equilibrium emission for the (p,xn) reactions in niobium

Proton induced reactions have been studied in the energy region from ≈7.0 to 12.5 MeV for niobium, using the stacked foil activation technique followed by gamma ray spectroscopy. The excitation functions for the production of 93mMo, 93mNb and 89gZr have been determined. The experimental data have been compared with theoretical calculations based on compound reactions in statistical equilibrium as well as on pre-equilibrium reactions.

STUDY OF ISOMERIC CROSS-SECTION RATIO AND PRE-EQUILIBRIUM FRACTION IN PROTON AND ALPHA PARTICLE INDUCED NUCLEAR REACTIONS ON 197Au

Excitation functions for the reactions 197 Au(p,n) 197g Hg and 197 Au(p,n) 197m Hg have been measured over the energy ranges from threshold up to 20 MeV using stacked foil activation technique. The isomeric cross-section ratio σm/(σm+σg) for the formation of 197g, m Hg was determined. The excitation functions and isomeric cross-section ratios were calculated for the reactions 197 Au(p,n) 197g,m Hg , 197 Au(p,2n) 196 Hg , 197 Au(p,3n) 195g,m Hg , 197 Au(p,4n) 194 Hg , 197 Au(p,5n) 193g,m Hg , 197 Au (α, n )200 Tl , 197 Au (α, 2n )199 Tl , 197 Au (α, 3n )198g,m Tl , 197 Au (α, 4n )197 Tl and 197 Au (α, 5n )196g,m Tl from reaction threshold up to 40 MeV for proton induced reactions and 80 MeV for alpha induced reactions. The results were compared with the theoretical cross-sections calculated by means of the code EMPIRE-II and with the available literature data, which makes use of the Hauser Feshbach model for the compound nuclear calculations and the exciton model for the pre-equilibrium emission part. The isomeric cross-section ratio is found to depend strongly on the relative spins of the isomeric and ground state, energy difference between the levels, presence of intermediate states and some dependence on decay modes and on the onset of pre-equilibrium emission.