Light Charged Particles Research Articles

Effects of T-odd asymmetry of the emission of light charged particles and photons during fission of heavy nuclei by polarized neutrons

The new physical effects of T-odd asymmetry of the emission of light charged particles (LCPs) during the ternary fission of some heavy nuclei by cold polarized neutrons have been experimentally studied. The coefficients of triple scalar and vector correlation of the pulses of light particles and fission fragments (TRI effect) and the fivefold correlation of the same vectors (ROT effect) have been measured. These effects are believed to be caused by the rotation of polarized fissioning system around its polarization direction. The treatment of the experimental data for LCPs in the framework of this hypothesis leads to a good agreement between the calculation results and experimental data. The calculated value of the angle of rotation of the fission axis in the ternary fission of the polarized fissioning 236U* compound nucleus was used to process the results of measuring the ROT effect for γ photons from binary-fission fragments of the same nucleus. A satisfactory description of these experimental data is obtained which serves a convincing confirmation of the rotation hypothesis.

Semiclassical description of TRI asymmetry in ternary fission induced by cold polarized neutrons

The possibility of semiclassically describing T-even TRI-type asymmetry in ternary fission induced by polarized neutrons is considered on the basis of employing Coriolis interaction that takes into account the coupling of a light charged particle to the collective rotation of a polarized fissile nucleus. It is shown that allowance for this interaction makes it possible to explain qualitatively the magnitudes of two asymmetry effects observed in light-charge-particle emission both within the semiclassical and within the quantum-mechanical approach. The difference in the relative magnitudes and signs of the effects between different target nuclei is associated with the interference contributions to the cross section from neighboring neutron resonances and therefore cannot be explained within the semiclassical approach.

Investigating the symmetry energy of nuclear equation of state with heavy-ion reactions

Heavy-ion collisions provide a unique opportunity to examine nuclear matter at temperatures, densities, and neutron-to-proton (N/Z) ratios away from that of ground state nuclei. A wide range of observables from heavy-ion collisions has been used to study and constrain the nuclear equation of state (EOS). Recently much work has been focused on the symmetry energy of the EOS. The transverse collective flow of light charged particles and intermediate mass fragments is sensitive to the fragmentation mechanism and – in particular – the density dependence of the nuclear symmetry energy. The present work demonstrates the impact of the N/Z of the system on the transverse flow of LCPs and IMFs from the reactions 70Zn + 70Zn, 64Zn +64Zn, and 64Ni + 64Ni at 35 Mev/nucleon. The IMF flow is consistent with a "stiff" equation of state as implemented in the AMD model.

Production of heavy clusters (up to A=10) by coalescence during the intranuclear cascade phase of spallation reactions

Spallation reactions are generally considered to proceed in two stages: a cascade stage followed by an evaporation stage. Light charged particle (lcp) spectra indicate that such particles are produced by both stages. The mechanism of production in the cascade stage is still not fully understood. Using the improved versions of the Liège Intra-Nuclear Cascade model and of the ABLA evaporation model, we have recently shown that lcp's are presumably produced by some kind of dynamical coalescence process, by which a fast particle of the cascade drags a few other nucleons. In the very recent years, precise measurements of the production of heavier clusters have been performed. We improved and generalized our production models for the heavier clusters, up to A=10 and we reached good agreement with experiment. These results strongly suggest that the dynamical coalescence mechanism applies to heavy clusters. The importance of these data for spallation neutron sources and accelerator-driven systems is underlined.

Activation cross sections of proton induced nuclear reactions on natural hafnium

In a systematic study of light charged particle induced nuclear reactions we investigated the excitation functions of proton induced reactions on natural hafnium targets. Experimental excitation functions of proton induced reactions up to 36 MeV on high purity natural hafnium were determined using the stacked foil activation technique. High resolution off-line gamma-ray spectrometry was applied to assess the activity of each foil. From the measured activity independent and/or cumulative elemental or isotopic cross section data for production of Ta, Hf and Lu radioisotopes were determined. The experimental data were compared to the data published earlier by other authors and to results of TALYS theoretical code taken from TENDL-2010 database. Thick target yields of the investigated radionuclides were calculated from the excitation function that was deduced as an analytical fit to our experimental cross section data points.

Derivation of Pekar's Polarons from a Microscopic Model of Quantum Crystal

A polaron is an electron interacting with a polar crystal, which is able to form a bound state by using the distortions of the crystal induced by its own density of charge. In this paper we derive Pekar's famous continuous model for polarons (in which the crystal is replaced by a simple effective Coulomb self-attraction) by studying the macroscopic limit of the reduced Hartree-Fock theory of the crystal. The macroscopic density of the polaron converges to that of Pekar's nonlinear model, with a possibly anisotropic dielectric matrix. The polaron also exhibits fast microscopic oscillations which contribute to the energy at the same order, but whose characteristic length is small compared to the scale of the polaron. These oscillations are described by a simple periodic eigenvalue equation. Our approach also covers multi-polarons composed of several electrons, repelling each other by Coulomb forces.

Study of Light Charged Particle Production Double Differential Cross Sections from Proton-Actinide Reactions at 360 MeV

Study of Light Charged Particle Production Double Differential Cross Sections from Proton-Actinide Reactions at 360 MeV

Target set-up for measurements with the use of a Si-ball detector

The thin n–p+ layer of Si n+–n–p+ wafers composing the light charged particles detectors can be easily damaged by scattered beam projectiles or heavy products of the nuclear reaction in target material. Such damages of the sensitive part of the wafer may in consequence shorten the detector life-time. This could be avoided by protecting the sensitive part of the detector with shielding that stops the heavy projectiles.Due to the compact configuration of the detectors in a small Si-ball counters placing the shielding in front of each wafer could be difficult and therefore a target-shielding assembly is proposed.

Evaporation and fission decay of 132Ce compound nuclei at Ex=122 MeV: some limitations of the statistical model

Light charged particle (LCP) emission in the evaporation residue (ER) and fusion fission (FF) channels have been studied for the 200 MeV 32S + 100Mo reaction, leading to 132Ce composite nuclei at E x =122 MeV. The main goal was to study the decay of 132 Ce on the basis of an extended set of observables, to get insights on the fission dynamics. The proton and alpha particle energy spectra, their multiplicities, ER-LCP angular correlations, ER and FF angular distributions, and ER and FF cross-sections were measured. The measured observables were compared with the Statistical Model (SM). Using standard parameters, the model was able to reproduce only the pre-scission multiplicities and the FF and ER cross-sections. The calculation was observed to strongly overestimate the proton and alpha particle multiplicities in the ER channel. Disagreements were also observed for the ER-LCP correlations, the LCP energy spectra and the ER angular distribution. By varying the SM input parameters over a wide range of values, it is shown that it is not possible to reproduce all the observables simultaneously with a unique set of parameters. The inadequacy of the model in reproducing the ER particle multiplicities is also observed analysing data from the literature for other systems in the A ≈ 150 and E x ≈ 100−200 MeV region. These results indicate serious limitations about the use of the SM in extracting information on fission dynamics.

A note on the examination of isospin effects in multi-dimensional Langevin fission dynamics

In [W. Ye, F. Wu, H.W. Yang, Phys. Lett. B 647 (2007) 118] prescission protons and α particles of high-isospin 206Pb were shown to be almost independent of the dissipation strength ks. Subsequently, in [P.N. Nadtochy, et al., Phys. Lett. B 685 (2010) 258] prescission light charged particles (LCPs) were shown to have approximately the same sensitivity as neutrons to ks for 206Pb and 204Hg nuclei. In this Letter we point out that the reason for the apparent contradictory conclusions is that the authors in the latter did not compute the changes in the absolute yields of prescission LCPs multiplicities with increasing ks and compare them with typical experimental uncertainties. It is shown that the expected changes are very small in the case of neutron-rich 206Pb and 204Hg systems, which are within experimental error bars. This indicates that, from the viewpoint of experiment, LCPs emission of 206Pb and 204Hg is insensitive to dissipation.

Rapidity distribution as a probe for elliptical flow at intermediate energies

Abstract The interplay between spectator and participant matter in heavy-ion collisions is investigated within the isospin-dependent quantum molecular dynamics (IQMD) model in terms of the rapidity distribution of light charged particles. The effect of different types and sizes of rapidity distributions is studied in elliptical flow. The elliptical-flow patterns show the important role of nearby spectator matter on the participant zone. This role is further explained on the basis of the passing time of the spectator and the expansion time of the participant zone. The transition from in-plane to out-of-plane emission is observed only when the mid-rapidity region is included into the rapidity bin. Otherwise no transition occurs. The transition energy is found to be highly sensitive to the size of the rapidity bin, while it is only weakly dependent on the type of the rapidity distribution. These theoretical findings are found to be in agreement with experimental results.

Calculation of the number of atoms displaced during the irradiation of monolayer graphene

The irradiation of monolayer graphene, combined with chemical functionalization, could be an effective method for modifying its electronic structure and for achieving specific physical properties adjusted to different applications. A difficulty arising during planning and studying the irradiation in this system is that many of the models of interaction of the radiation with the substance cannot be applied to the two-dimensional structure of graphene. In particular, the mathematical expressions available to calculate the number of atoms displaced during the bombardment with particles can be applied only to 3D isotropic solids. In the present work, an alternative analytic expression is presented for the irradiation of graphene with heavy ions or with protons and other light charged particles. The expression was obtained on the basis of the classic theory of dispersion, using a Coulomb potential for the light charged particles and one of Inverse Square for heavy ions. For medium values of the energy of the incident particle a decreasing dependence of the number of displaced atoms with energy is obtained. This behavior, related with the two-dimensional structure of the target, had been observed in other authors’ works using computational simulation.

Transverse collective flow and midrapidity emission of isotopically identified light charged particles

The transverse flow and relative midrapidity yield of isotopically identified light charged particles (LCPs) has been examined for the 35 MeV/nucleon ${}^{70}\mathrm{Zn}+{}^{70}\mathrm{Zn}$, ${}^{64}\mathrm{Zn}+{}^{64}\mathrm{Zn}$, and ${}^{64}\mathrm{Ni}+{}^{64}\mathrm{Ni}$ systems. A large enhancement of the midrapidity yield of the LCPs was observed relative to the yield near the projectile rapidity. In particular, this enhancement was increased for the more neutron-rich LCPs demonstrating a preference for the production of neutron-rich fragments in the midrapidity region. Additionally, the transverse flow of the LCPs was extracted, which provides insight into the average movement of the particles in the midrapidity region. Isotopic and isobaric effects were observed in the transverse flow of the fragments. In both cases, the transverse flow was shown to decrease with an increasing neutron content in the fragments. A clear inverse relationship between the transverse flow and the relative midrapidity yield is shown. The increased relative midrapidity emission produces a decreased transverse flow. The stochastic mean-field model was used for comparison to the experimental data. The results showed that the model was able to reproduce the general isotopic and isobaric trends for the midrapidity emission and transverse flow. The sensitivity of these observables to the density dependence of the symmetry energy was explored. The results indicate that the transverse flow and midrapidity emission of the LCPs are sensitive to the denisty dependence of the symmetry energy.

PRE-EQUILIBRIUM ALPHA-PARTICLE EMISSION AS A PROBE TO EXPLORE ALPHA CLUSTERING IN NUCLEI

Experimental data of the double-differential spectra of light particles emitted at pre-equilibrium stage of nuclear processes were obtained at Laboratori Nazionali di Legnaro for the heavy-ion reactions 130 and 250 MeV 16 O + 116 Sn . Light charged particles were measured in coincidence with evaporation residues in order to avoid unwanted competing mechanisms. The experimental data were collected in a wide angular range from 29 to 82 degrees in the laboratory system. Theoretical model was developed in order to describe simultaneously evaporative and pre-equilibrium emission of the light particles in heavy-ion reactions. Griffin exciton model was used for the description of the pre-equilibrium stage of the compound nucleus formation, while the equilibrium evaporation processes were analyzed in the framework of the statistical theory of heavy-ion reactions. Experimental data were compared with the results of the model calculations and new approach was suggested to take into account alpha cluster formation in the projectile nucleus by measuring and analyzing pre-equilibrium alpha-particle spectra.

ON THE EMISSION OF HEAVY CLUSTERS FROM MEDIUM MASS COMPOUND NUCLEUS

Fragments with atomic number 6≤ Z ≤28 were measured in singles and in coincidence with emitted light charged particles in the 78 Kr +40 Ca reactions at 5.5 AMeV incident energy by means of the 4π-INDRA array at GANIL. Kinematical features of the fragments are compatible with a binary fission from compound nucleus. The cross-sections σZ indicate the coexistence of macroscopic behaviour and structure effects. Fragment-particle coincidences show that fragments with Z ≤12 are excited below the separation energies of light particles.

Multifragmentation around the transition energy in intermediate-energy heavy-ion collisions

Fragmentation of light charged particles is studied for various systems at different incident energies between 50 and 1000 MeV/nucleon. We analyze fragment production at incident energies above, below and at transition energies using the isospin dependent quantum molecular dynamics(IQMD) model. The trends observed for the fragment production and rapidity distributions depend upon the incident energy, size of the fragments, composite mass of the reacting system as well as on the impact parameter of the reaction. The free nucleons and light charged particles show continous homogeneous changes irrespective of the transition energies indicating that there is no relation between the transition energy and production of the free as well as light charged particles.

The extension of the INCL model for simulation of shielding in space

Radiation hazard for space missions is mainly due to cosmic ray protons, helium nuclei and light ions, whose energy spectrum is maximum around 1 GeV per nucleon but remains non-negligible for energies up to 15 GeV per nucleon. Nuclear reactions induced by high energy protons are often described by intranuclear cascade plus evaporation models. The attention is focused here on the Liège Intranuclear Cascade model (INCL), which has been shown to reproduce fairly well a great deal of experimental data for nucleon-induced reactions in the 200 MeV to 2 GeV range, when coupled with the ABLA evaporation-fission code. In order to extend the model to other conditions relevant for space radiation, three improvements of INCL are under development. They are reported on here. First, the reaction model has been extended to nucleon–nucleus reactions at incident energies up to 15 GeV, mainly by the inclusion of additional pion production channels in nucleon–nucleon collisions during the cascade. Second, a coalescence mechanism for the emission of light charged particles has been implemented recently. Finally, the model has been modified in order to accommodate light ions as projectiles. First results are shown and compared with illustrative experimental data. Implications for issues concerning radiation protection in space are discussed.

On the elliptical flow and mass asymmetry of the colliding nuclei

A study of elliptical flow is carried out for different mass asymmetries of colliding nuclei using the reactions of Cr5024+Ru10244 (η=0.3), S3216+Sn12050 (η=0.5) and O168+Xe13654 (η=0.7). The present reactions are simulated at incident energies between 50 and 250 MeV/nucleon within the framework of isospin-dependent quantum molecular dynamics model. For the present analysis, total mass of colliding pairs is kept fixed and mass asymmetry is varied between 0.3 and 0.7. The elliptical flow shows a transition from in-plane to out-of-plane in the mid rapidity region with incident energy. The transition energy is found to increase with the mass asymmetry for light charged particles. A good agreement is obtained with experimental measurements.

Correlated n 1,3 S states for coulomb three-body systems

n1,3S (n = 1 − 4) states for atomic three-body systems are studied with the Angular Correlated Configuration Interaction method. A recently proposed angularly correlated basis set is used to construct, simultaneously and with a single diagonalization, ground and excited states wave functions which: (i) satisfy exactly Kato cusp conditions at the two-body coalescence points; (ii) involve only linear parameters; (iii) show a fast convergency rate for the energy; and (iv) form an orthogonal set. The efficiency of the method is illustrated by the study a variety of three-body atomic systems [mmm] with two negatively charged light particles, with diverse masses m and m, and a heavy positively charged nucleus m. The calculated ground 11S and excited n1,3S (n = 2 − 4) state energies are compared with those given in the literature, when available. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011

Dynamical Dipole mode in heavy-ion fusion reactions by using stable and radioactive beams

The existence of the dynamical dipole mode in the 192 Pb composite system was investigated through the study of its prompt γ decay employing the 40 Ca + 152 Sm and 48 Ca + 144 Sm reactions at E lab =11 and 10.1 MeV/u, respectively. The γ -rays and light charged particles were detected in coincidence with evaporation residues and fission fragments. First results of this experiment show that the dynamical dipole mode survives in collisions involving heavier mass reaction partners than those studied previously. As a fast cooling mechanism on the fusion path, the prompt dipole γ radiation could be of interest for the synthesis of super-heavy elements through ”hot” fusion reactions. Furthermore, by using radioactive beams and the prompt γ radiation as a probe we could get information on the symmetry energy at sub-saturation densities.