Dynamical Dipole Mode Research Articles

Dynamical dipole excitation in the fission of a Ca40 + Sm152 composite system

The excitation of the dynamical dipole mode was explored in the formation of a heavy composite system with mass A≃ 190, by investigating its fission channel. The composite system was produced through the charge asymmetric reaction, Ca40 + Sm152, and the nearly charge symmetric one, Ca48 +Sm144, at Elab = 11 and 10.1 MeV/nucleon, respectively. High-energy γ rays and light charged particles were detected in coincidence with the two fission fragments by means of the MEDEA multidetector array coupled to two parallel plate avalanche counters. The kinetic energy spectra of the light charged particles measured at different angles were used to infer the average excitation energy, the average mass, and the average charge of the produced nuclei whereas the time of flight and the emission angle of the fragments were employed to reconstruct the fission dynamics. The study of the γ-ray spectra and angular distributions for the selected fission events, allowed us to establish (i) the excitation of the giant dipole resonance in the composite system of both reactions and (ii) the excitation of the dynamical dipole mode in the dinucleus of the charge asymmetric reaction by isolating its prompt γ radiation through the difference technique. The present results on the dynamical dipole mode were compared with the experimental findings for the evaporation channel of the Ca40 + Sm152 reaction and moreover with calculations based on a collective bremsstrahlung analysis of the reaction dynamics. Interesting hints connecting the dynamical dipole γ radiation with the superheavy element quest are given.10 MoreReceived 16 February 2022Accepted 6 June 2022DOI:https://doi.org/10.1103/PhysRevC.105.064611©2022 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasFissionLow & intermediate energy heavy-ion reactionsNuclear reactionsResonance reactionsNuclear Physics

Pre-equilibrium GDR in a heavy composite system

The excitation of the Dynamical Dipole mode along the fusion path was investigated for the first time in the formation of a heavy composite system in the A∼190 mass region, in fusion-evaporation and fission events. The composite system was formed at identical conditions of excitation energy and spin from two reactions with different charge asymmetry: the charge asymmetric 40Ca + 152Sm and the nearly charge symmetric 48Ca + 144Sm at Elab=11 and 10.1 MeV/nucleon, respectively. In this paper, we report the results on fusion evaporation events and the preliminary analysis on fission channel.

Dynamical Dipole mode in heavy-ion fusion reactions

In this work we give evidence for the first time of the Dynamical Dipole mode in a heavy system in the A∼190 mass region, in both fusion–evaporation and fission events. The 40,48Ca + 152,144Sm reactions at Elab=11(10.1) MeV/nucleon were employed. Our results for evaporation and fission events (preliminary) show that the dynamical dipole mode survives in reactions involving heavier nuclei than those studied previously.

Dynamical Dipole mode in heavy-ion fusion reactions

The excitation of the Dynamical Dipole mode along the fusion path was investigated for the first time in the formation of a heavy compound nucleus in the A~190 mass region. The compound nucleus was formed at identical conditions of excitation energy and spin from two entrance channels having different charge asymmetry: the charge asymmetric 40 Ca + 152 Sm and the nearly charge symmetric 48 Ca + 144 Sm at Elab =11 and 10.1 MeV/nucleon, respectively. Both fusion–evaporation and fission events were studied simultaneously for the first time. The Dynamical Dipole mode excitation in the charge asymmetric channel was evidenced, in a model-independent way, by comparing the γ -ray multiplicity spectra and angular distributions of the two entrance channels with each other.

Evidence of dynamical dipole excitation in the fusion-evaporation of theCa40+Sm152heavy system

The excitation of the dynamical dipole mode along the fusion path was investigated for the first time in the formation of a heavy compound nucleus in the A∼190 mass region. The compound nucleus was formed at identical conditions of excitation energy and spin from two entrance channels: the charge-asymmetric Ca40+Sm152 and the nearly charge-symmetric Ca48+Sm144 at Elab=11 and 10.1 MeV/nucleon, respectively. High-energy γ rays and light charged particles were measured in coincidence with evaporation residues by means of the MEDEA multidetector array (Laboratori Nazionali del Sud, Italy) coupled to four parallel plate avalanche counters. The charged particle multiplicity spectra and angular distributions were used to pin down the average excitation energy, the average mass, and the average charge of the compound nucleus. The γ-ray multiplicity spectrum and angular distribution related to the nearly charge-symmetric channel were employed to obtain new data on the giant dipole resonance in the compound nucleus. The dynamical dipole mode excitation in the charge-asymmetric channel was evidenced, in a model-independent way, by comparing the γ-ray multiplicity spectra and angular distributions of the two entrance channels with each other. Calculations of the dynamical dipole mode in the Ca40+Sm152 channel, based on a collective bremsstrahlung analysis of the reaction dynamics, are presented. Possible interesting implications in the superheavy-element quest are discussed.2 MoreReceived 19 January 2016DOI:https://doi.org/10.1103/PhysRevC.93.044619©2016 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasCollective levelsLow & intermediate energy heavy-ion reactionsTechniquesNuclear reactionsNuclear Physics

Dynamical Dipole mode in the40,48Ca +152,144Sm fusion reactions at 11 MeV/nucleon

The excitation of the dynamical dipole mode along the fusion path was investigated in the formation of a heavy compound nucleus in the A=190 mass region. To form the compound nucleus, the 40 Ca + 152 Sm and 48 Ca + 144 Sm reactions were employed at Elab =11 and 10.1 MeV/nucleon, respectively. Both fusion–evaporation and fission events were studied simultaneously for the first time. Our results for evaporation and fission events (preliminary) show that the dynamical dipole mode survives in reactions involving heavier nuclei than those studied previously.

Dynamical Dipole mode in heavy-ion fusion reactions in the 192Pb mass region

The dynamical dipole mode was investigated in the mass region of the 192Pb compound nucleus, by using the 40Ca + 152Sm and 48Ca + 144Sm reactions at Elab=11 and 10.1 MeV/nucleon, respectively. Both fusion-evaporation and fission events were studied simultaneously for the first time. Our results for evaporation and fission events (preliminary) show that the dynamical dipole mode survives in reactions involving heavier nuclei than those studied previously.

Investigation of the Dynamical Dipole mode in the192Pb mass region

The dynamical dipole mode was investigated in the mass region of the 192 Pb compound nucleus, by using the 40 Ca + 152 Sm and 48 Ca + 144 Sm reactions at Elab=11 and 10.1 MeV/nucleon, respectively. Both fusion-evaporation and fission events were studied simultaneously for thefirst time. Our results show that the dynamical dipole mode survives in reactions involving heavier nuclei than those studied previously, however, its yield is lower than that expected within BNV calculations.

Symmetry-energy dependence of the dynamical dipole mode in the Boltzmann-Uehling-Uhlenbeck model

Using an isospin-dependent Boltzmann-Uehling-Uhlenbeck (IBUU) transport model, we have studied the connection between the symmetry energy and features of the dynamical dipole mode in fusion reactions with charge-asymmetric entrance channel. The yield and angular distribution of the prompt photon emission are extracted by a bremsstrahlung approach. The experimental data of $^{36}$Ar+$^{96}$Zr at 16 MeV/nucleon and $^{32}$S + $^{100}$Mo at 9.3 MeV/nucleon are compared with IBUU model calculations, and the soft symmetry energy is found to describe the data reasonably well.

Search for the Dynamical Dipole mode in the 192Pb mass region

The prompt γ radiation originating from the dynamical dipole mode decay was investigated in the 192Pb composite system employing the 40Ca + 152Sm and 48Ca + 144Sm reactions at Elab=11 and 10.1 MeV/nucleon, respectively. The γ-ray energy spectra at various polar angles were obtained for fusion-evaporation and fission events by detecting the high energy γ rays with the MEDEA experimental apparatus in coincidence with evaporation residues and fission fragments. Preliminary 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.

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.

DYNAMICAL DIPOLE MODE IN FUSION HEAVY-ION REACTIONS

The prompt γ-ray emission, associated with the dynamical dipole mode decay, was investigated in the 32,36 S + 100,96 Mo and 36,40 Ar + 96,92 Zr fusion-evaporation reactions in the energy range E lab= 6 - 16 MeV / nucleon . The above reaction pairs populate, through entrance channels having different charge asymmetries, a compound nucleus in the 132 Ce mass region at excitation energies of 117, 174 and 284 MeV with identical spin distribution. By studying the differential γ-ray multiplicity spectra of the considered systems, the features of the dynamical dipole mode as a function of the beam energy were extracted while the γ-ray angular distributions were used to prove its pre-equilibrium character. The experimental findings were compared with theoretical predictions performed within a BNV transport model and based on a collective bremsstrahlung analysis of the entrance channel reaction dynamics. As a fast cooling mechanism on the fusion path, the prompt dipole radiation could be of interest for the synthesis of super heavy elements through hot fusion reactions providing a way to cool down the hot fusion paths, so ending up with a larger survival probability. To shed light on this hypothesis, the dynamical dipole mode investigation was extended to a heavier compound nucleus, 192 Pb , by means of the 40 Ca + 152 Sm and 48 Ca + 144 Sm reactions at E lab= 11 and 10.1 MeV/nucleon, respectively. Preliminary results of this measurement, concerning both fusion-evaporation and fission events are presented.

Dynamical dipole mode in heavy-ion fusion reactions

The dynamical dipole mode, excited in charge asymmetric heavy-ion collisions, was investigated in the mass region of the 192Pb compound nucleus, formed by using the 40,48Ca + 152,144Sm reactions at ∼11 MeV/nucleon. Preliminary results of this measurement, concerning both fusion-evaporation and fission events are presented. As a fast cooling mechanism on the fusion path, the dynamical dipole mode could be useful for the synthesis of super heavy elements through “hot” fusion reactions.

Dynamical dipole mode in fusion reactions at 16 MeV/nucleon and beam energy dependence

High-energy {gamma} rays and light charged particles from the {sup 36}Ar+{sup 96}Zr and {sup 40}Ar+{sup 92}Zr reactions at E{sub lab}=16 and 15.1 MeV/nucleon, respectively, were measured in coincidence with evaporation residues by means of the MEDEA multidetector array coupled to four parallel plate avalanche counters. The aim of this experiment was to investigate the prompt {gamma} radiation, emitted in the decay of the dynamical dipole mode, in the {approx}16 MeV/nucleon energy range and to map its beam energy dependence, comparing the present results with our previous ones obtained at lower energies. The studied reactions populate, through entrance channels having different charge asymmetries, a compound nucleus in the region of Ce under the same conditions of excitation energy and spin. Light charged particle energy spectra were used to pin down the average excitation energy and the average mass of the system. By studying the {gamma}-ray spectra of the charge symmetric reaction {sup 40}Ar+{sup 92}Zr, the statistical giant dipole resonance (GDR) parameters and angular distribution were extracted, and a comparison of the linearized 90 deg. {gamma}-ray spectra of the two reactions revealed a 12% extra yield in the GDR energy region for the more charge asymmetric system. The center-of-mass angular distribution datamore » of this extra {gamma} yield, compatible with a dipole oscillating along the symmetry axis of the dinuclear system, support its dynamical nature. The experimental findings are compared with theoretical predictions performed within a Boltzmann-Nordheim-Vlasov transport model and based on a collective bremsstrahlung analysis of the entrance channel reaction dynamics. An interesting sensitivity to the symmetry term of the equation of state and to in-medium effects on nucleon-nucleon (nn) cross sections is finally discussed.« less

Dynamical dipole mode in fusion reactions with exotic nuclear beams

We report the properties of the prompt dipole radiation, produced via a collective bremsstrahlung mechanism, in fusion reactions with exotic beams. We show that the $\ensuremath{\gamma}$ yield is sensitive to the density dependence of the symmetry energy below/around saturation. Moreover, we find that the angular distribution of the emitted photons from such fast collective mode can represent a sensitive probe of its excitation mechanism and of fusion dynamics in the entrance channel.

Soft-to-hard mode transition in the dynamical dipole mode induced in dissipative heavy-ion collisions

The collective dipole dynamics evolving in intermediate dinuclear systems with exotic shape and charge distributions formed in charge asymmetric fusion entrance channels are systematically studied by solving a microscopic transport Boltzmann-Nordheim-Vlasov (BNV) equation and analyzing the obtained dipole signal. The BNV analysis shows a typical transition in the dipole oscillation frequency related to a transition from soft-to-hard mode in the dynamical dipole mode. The relative pre-equilibrium {gamma}-ray emission probability is also evaluated by using the classical bremsstrahlung approach. In particular, by using this approach an analytical formula that gives the time-dependent {gamma} rate during the equilibration of the collective mode versus the statistical GDR is extrapolated. The results are in agreement with other models and the few existing experimental data.

The dynamical dipole mode in dissipative heavy-ion collisions

We study the effect of a direct giant dipole resonance (GDR) excitation in intermediate dinuclear systems with exotic shape and charge distributions formed in charge asymmetric fusion entrance channels. A related enhancement of the GDR gamma yield in the evaporation cascade of the fused nucleus is expected. The dynamical origin of such GDR extra strength will show up in a characteristic anisotropy of the dipole gamma-emission. A fully microscopic analysis of the fusion dynamics is performed with quantitative predictions of the GDR photon yield based on a dynamics-statistics coupling model. In particular we focus our attention on the energy and mass dependence of the effect. We suggest a series of new experiments, in particular some optimal entrance channel conditions. We stress the importance of using the new available radioactive beams.