INVESTIGATION OF THE DYNAMICAL DIPOLE MODE IN THE 192 Pb 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 E lab = 11 and 10.1 MeV/nucleon, respectively. Both fusion– evaporation and ﬁssion events were studied simultaneously for the ﬁrst time. Preliminary results obtained with a part of the collected statistics show that the dynamical dipole mode survives in reactions involving heavier mass reaction partners than those investigated in our previous works. As it represents a fast cooling mechanism on the fusion path, it could be used to favor the synthesis of super heavy elements through “hot” fusion reactions.


Introduction
In N/Z asymmetric heavy-ion reactions, it is possible to excite a collective dipole oscillation that can develop along the symmetry axis of the dinuclear system [1][2][3].This oscillation, called "dynamical dipole mode" (DD throughout the text), decays emitting prompt dipole γ-rays, in addition to those coming from the Giant Dipole Resonance (GDR) thermally excited in the hot compound nucleus (CN).The DD radiation presents i) a lower centroid energy than that of a statistical GDR built in a spherical nucleus of similar mass due to the high deformation of the emitting source [2,3] ii) an anisotropic angular distribution with respect to the beam axis because the oscillation is confined in the reaction plane [4] and iii) a γ yield that is predicted to depend on both the beam energy and the reaction dynamics [3].Experimentally, the existence of the DD mode has been studied in deep inelastic and fusionevaporation heavy-ion collisions [5][6][7][8][9].In these measurements, an excess of γ-rays was observed in the GDR energy region for a charge asymmetric reaction, with respect to that of a more charge symmetric one forming the same CN at identical conditions [6][7][8] or with respect to statistical model calculations [9].This γ excess was attributed to the decay of the predicted DD.
The emission of DD γ-rays decreases the excitation energy and hence the initial temperature of the nucleus reaching the statistical phase.This cooling mechanism could be suitable to favour the production of super-heavy elements in "hot" fusion processes.However, TDHF calculations [2] showed that the prompt dipole γ yield decreases as the mass of colliding ions increases since the reactions with small nuclei are less damped than those involving more nucleons.In order to understand if this pre-equilibrium effect survives in heavier systems than those studied before and to test its usefulness in super-heavy element production, we decided to study the DD in the mass region of the 192 Pb CN.

Experimental results for 19Pb
The experiment was performed by using the 40 Ca( 48 Ca) pulsed beam provided by the Superconducting Cyclotron of the Laboratori Nazionali del Sud (LNS, Italy), impinging on a 1 mg/cm 2 thick 152 Sm( 144 Sm) target at E lab = 440(485) MeV.Both entrance channels populate the same CN, 192 Pb, through a quite different initial dipole moment ranging from 30.6 fm for the 40 Ca + 152 Sm charge asymmetric reaction to 5.3 fm for the 48 Ca + 144 Sm more charge symmetric one.The mass asymmetry of the two entrance channels is very similar, namely 0.22(0.18)for the 40 Ca + 152 Sm ( 48 Ca + 144 Sm) system.Furthermore, the formed CN has identical spin distribution: L max = 74 for fusion and L max = 36 for fusion-evaporation, according to PACE2 calculations [10] by using a level density parameter of a=A/8, A being the compound nucleus mass, and identical excitation energy of 236 MeV, evaluated by means of the empirical formula of [11].
The γ-rays and the light charged particles were detected by using the MEDEA experimental apparatus [12], made of 180 BaF 2 scintillators.The discrimination between γ-rays, light charged particles and neutrons was performed by combining a pulse shape analysis of the BaF 2 signal with a time of flight measurement between each scintillator and the radiofrequency signal of the Cyclotron.The fusion-evaporation residues were detected by four position sensitive Parallel Plate Avalanche Counters (PPACs) placed symmetrically around the beam direction at 70 cm from the target at θ = 7 • and subtending 7 • in θ.The fission events were selected by detecting the two kinematically coincident fission fragments with position sensitive PPACs, centered at θ = 52.5 • symmetrically around the beam axis at 16 cm from the target covering 22 • in both θ and ϕ and allowing the study of γ-ray -fragment angular correlations.Down-scaled single events together with coincidence events between at least one fired BaF 2 scintillator and a PPAC (two PPACs) for evaporation (fission) events were collected during the experiment.The coincidence request eliminated any cosmic ray contamination of the γ-ray spectra.By using the above trigger there are no normalization factors in the γ-ray spectra as the double differential γ multiplicity is obtained from the ratio of the number of coincidences between γ-rays and evaporation residues (fission fragments) and the number of single events of evaporation (fission).Preliminary results of the experiment, concerning a partial statistics are shown in [13].

γ-ray spectra and angular distributions
The average excitation energy and the average mass of the composite system after preequilibrium particle emission were evaluated by studying the energy spectra of the light charged particles (p, α) detected in coincidence with evaporation residues.These spectra for the BaF2 rings placed from θ = EPJ Web of Conferences 03069-p.251.5 • to θ = 159.7 • were analyzed in the framework of a two moving source scenario : a slow source describing the statistical evaporation from the hot CN and an intermediate-velocity (between the CN and the projectile velocity) source related to the pre-equilibrium particles emitted by the composite system before thermalization.The preliminary analysis confirmed us that the two considered reactions lead to the formation of a CN with the same average mass at the same average excitation energy.Therefore, as in our previous works [7,8], all the parameters except the dipole moment were kept identical in the two reactions, so that any difference in their γ-ray spectra and angular distributions can be attributed to the difference in the entrance channel charge asymmetry.By comparing the centerof-mass double differential γ-ray spectra of the two reactions for fusion-evaporation and fission events an excess of γ-rays in the more charge asymmetric reaction was observed, concentrated in the energy range E γ =8-14 MeV as can be seen in the left-hand side of Fig. 1 where the difference between the spectra of the two systems is showed for evaporation (fission) events in the top (bottom).This excess is related to the DD decay and can be reproduced by means of a lorentzian curve folded by the experimental apparatus response function [14] (line in the figure) with a centroid energy E DD = 10.5 MeV and a width Γ DD = 3.5 MeV, for both exit channels.It is interesting to note that E DD is lower than the ground state GDR centroid energy E GDR ∼ 13.5 MeV for a mass in the region of 192.This result confirms the high deformation of the emitting source, in agreement with expectations [2,3] and with our previous works [7,8].By integrating over energy the difference between the γ-ray spectra of the two reactions for the BaF 2 rings situated at polar angles from θ = 51.5 • to θ = 128.5 • and by taking into account the response function of the experimental set up [14] we obtain for the DD yield: (8 ± 1)*10 −5 sr −1 for evaporation events and (10 ± 3)*10 −5 sr −1 for fission events, with the quoted errors being statistical.A 3% error in the BaF 2 scintillator efficiency gives a ± 0.3*10 −5 sr −1 error in the above values of the DD multiplicity, smaller than the statistical error.Although such γ excess constitutes one of the signatures of the DD radiation, the angular distribution is also an important observable since it gives information about the reaction dynamics and the DD lifetime.This is related to (a) the rotation angular velocity of the dinuclear system during the prompt dipole emission and (b) the instant at which this emission occurs [4].We display in the right-hand side INPC 2013 of Fig. 1 the center-of-mass angular distribution with respect to the beam direction of the γ-rays detected in coincidence with evaporation residues for the 40,48 Ca+ 152,144 Sm reactions (top) and for their difference (bottom).The double differential γ-ray multiplicity was integrated over energy from 9 to 16 MeV, after the subtraction of (nn)-bremsstrahlung component, and was corrected by the experimental setup efficiency.The lines in both panels of the above figure describe the angular distribution of the emitted γ-rays given by the Legendre polynomial expansion , where a 2 is the anisotropy coefficient and Q 2 is an attenuation factor for the finite γ-ray counter, which, for the present geometry, is 0.98 [15].The coefficient M 0 and a 2 were obtained from a best fit to the data.The charge asymmetric reaction (squares) displays a more anisotropic angular distribution around 90 • than the charge symmetric one (circles).Since we have the same CN, with the same excitation energy and spin distribution, such a difference is related to entrance channel effects.
As a consequence of the above, the experimental angular distribution of the difference (squares in the bottom of the figure) is very anisotropic with a maximum at 90 • .The data can be reproduced with a 2 = -1 (solid line), that is compatible with an emission from a dipole oscillation along an axis that has performed a small rotation with respect to the beam axis.The dashed line, obtained with a 2 = -0.25,corresponds to a more isotropic angular distribution and is also showed in the figure for comparison.Although we are not able to evaluate the rotation angle of the DD axis around the beam direction when the DD oscillation is completely damped, due to the large statistical errors, we can confine the γ-emission time scale at the very beginning of the reaction.This result is in agreement with our previous results [8] for evaporation events corresponding to small impact parameters.
The present experimental findings on the prompt dipole radiation in 40 Ca+ 152 Sm reaction were compared with preliminary calculations performed within the BNV transport model framework and based on a collective bremsstrahlung approach of the entrance channel reaction dynamics [3].These calculations give centroid energy, width and angular distribution of the DD in good agreement with those of the experiment.However, the theoretical γ yield overestimates the data, an aspect that should be further investigated.

Figure 1 .
Figure 1.(Left-hand side) Difference between the charge asymmetric and charge symmetric reaction center-ofmass γ-ray spectra for fusion-evaporation (top) and fission (bottom) events.The solid lines in both panels are described in the text.(Right-hand side) Center-of mass angular distribution of the γ-rays for the two reactions (top) and of their difference (bottom) in the energy interval 9≤E γ ≤16 MeV corrected by the experimental setup efficiency.The lines are described in the text.