Isospin transport in 84Kr+112,124Sn reactions at Fermi energies
S.Piantelli, G.Casini, A.Olmi, S.Barlini, M.Bini, S.Carboni, P.R.Maurenzig, G.Pasquali, G.Poggi, A.A.Stefanini, R.Bougault, N.LeNeindre, O.Lopez, M.Parlog, E.Vient, E.Bonnet, A.Chbihi, J.D.Frankland, D.Gruyer, E.Rosato, G.Spadaccini, M.Vigilante, B.Borderie, M.F.Rivet, M.Bruno, L.Morelli, M.Cinausero, M.Degerlier, F.Gramegna, T.Marchi, R.Alba, C.Maiolino, D.Santonocito, T.Kozik, T.Twarog
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Owned by the authors, published by EDP Sciences, 2018
Isospin transport in Kr+ , Sn reactions at Fermi energies
S.Piantelli , a , G.Casini , A.Olmi , S.Barlini , M.Bini , , S.Carboni , , P.R.Maurenzig , ,G.Pasquali , , G.Poggi , , A.A.Stefanini , , R.Bougault , N.LeNeindre , O.Lopez , M.Parlog ,E.Vient , E.Bonnet , A.Chbihi , J.D.Frankland , D.Gruyer , E.Rosato , G.Spadaccini ,M.Vigilante , B.Borderie , M.F.Rivet , M.Bruno , L.Morelli , M.Cinausero , M.Degerlier ,F.Gramegna , T.Marchi , R.Alba , C.Maiolino , D.Santonocito , T.Kozik , et T.Twarog INFN -Sezione di Firenze, Italy Dip. di Fisica, Univ. di Firenze, Firenze, Italy LPC, IN2P3-CNRS, ENSICAEN and Univ. Caen, Caen, France GANIL, CEA/DSM-CNRS/IN2P3, Caen, France INFN -Sezione di Napoli, Italyand Dip.di Fisica, Univ. di Napoli Federico II,Napoli, Italy IPN- Orsay, Orsay, France INFN -Sezione di Bologna, Italyand Dip.di Fisica, Univ. di Bologna, Bologna, Italy INFN -LNL, Italy INFN -LNS, Italy Jagellonian University, Instituteof Nuclear Physics, Krakow, Poland
Résumé.
Isospin transport phenomena in dissipative heavy ion collisions have been in-vestigated at Fermi energies with a beam of Kr at 35AMeV. A comparison of the h N i / Z of light and medium products forward-emitted in the centre of mass frame when the beamimpinges on two di ff erent targets, the n-poor Sn and the n-rich
Sn, is presented. Datawere collected by means of a three-layer telescope with very good performances in termsof mass identification (full isotopic resolution up to Z ∼
20 for ions punching through thefirst detector layer) built by the FAZIA Collaboration and located just beyond the grazingangle for both reactions. The h N i / Z of the products detected when the n-rich target isused is always higher than that associated to the n-poor one ; since the detector was ableto measure only fragments coming from the QuasiProjectile decay and / or neck emission,the observed behaviour can be ascribed to the isospin di ff usion process, driven by theisospin gradient between QuasiProjectile and QuasiTarget. Moreover, for light fragmentsthe h N i / Z as a function of the lab velocity of the fragment is observed to increase whenwe move from the QuasiProjectile velocity to the centre of mass (neck zone). This e ff ectcan be interpreted as an evidence of isospin drift driven by the density gradient betweenthe QuasiProjectile zone (at normal density) and the more diluted neck zone. The availability of detectors able to isotopically resolve the ejectiles of a heavy ion reaction atFermi energies (30-50AMeV) allows to investigate the isospin degree of freedom and its evolution a. e-mail: silvia.piantelli@fi.infn.it
PJ Web of Conferences during the collision. Many experimental e ff orts were devoted to the study of the isospin di ff usionprocess, by means of reactions involving partners with di ff erent N / Z (see e.g. [1–4]) ; in fact theisospin di ff usion is driven by the isospin gradient between projectile and target. Other studies (e.g.[5–7]), sometimes involving symmetric reactions, were devoted to the investigation of the isospindrift i.e. the isospin enrichment of the low density neck zone with respect to the QuasiProjectile (QP)and QuasiTarget (QT) regions that are at normal density ; this process is supposed to be driven by thedensity gradient between the two regions. As it is explained in many theoretical works [8, 9], the studyof these kind of phenomena is extremely important, because it can give information on the symmetryenergy term in the nuclear equation of state. Recently also the FAZIA Collaboration [10] has started toinvestigate these processes [11], thanks to the good capabilities in terms of isotopic resolution of thedeveloped three-layer telescopes (full isotopic resolution up to Z ∼ −
23 for ions punching throughthe first detector layer). In this work we present some evidences of isospin di ff usion and isospin driftobserved by the FAZIA Collaboration for two reactions with the same beam, Kr at 35AMeV, and twodi ff erent targets, the “n-poor” Sn and the “n-rich”
Sn. The experiment was performed at LNS ofINFN in Catania (Italy) by means of the beam delivered by the CS Superconducting Cyclotron. Datawere collected by means of a telescope located just beyond the grazing angle for both reactions (4 . ◦ for the n-poor target and 4 . ◦ for the n-rich one) ; the detector covered polar angles between 4 . ◦ and6 ◦ . The basic FAZIA detector consists of a three-layer telescope, with two reverse mounted n-TDSilicon detectors (thicknesses : 300 µ m and 500 µ m, respectively) manufactured by FBK, followed bya 10cm long CsI(Tl), manufactured by Amcrys and read out by a photodiode. The Silicon detectorshave doping uniformity better than 3% FWHM, thickness uniformity within 1 µ m and they have beenobtained from wafers cut 7 ◦ o ff the h i axis in order to minimize the channeling e ff ect. The te-lescope is fully equipped with digital electronics. The identification procedure uses the standard ∆ E - E technique for particles punching through the first Si layer, obtaining full isotopic resolution in aregion up to now accessible only by means of spectrometers (full isotopic identification up to Z ∼ µ m (the threshold value in-creases with the ion charge). An example of the identification capabilities of the detector can be foundin figure 1, where the particle identification spectrum obtained by means of the ∆ E - E technique forions punching through the first Si layer is presented. In particular, in the inset the zone between S andK is expanded and the isotopic resolution capability can be appreciated. More details on the detectorperformances and on the algorithms used by the Collaboration to treat the signals collected by thetelescope can be found in [12] and references cited therein. The results presented in this work referonly to particles punching through the first Si layer.Since in this experiment we had only one telescope, only inclusive measurements could be done,without any selection on the event type. In any case, due to the geometrical position of the detector,some kind of selection is automatically done. In fact, as it is evident from the Z - lab velocity correla-tion reported in figure 2, only fragments forward-emitted in the centre of mass can be detected by ourtelescope. Thanks to their position in the correlation of figure 2 and relying on the existing literaturewe can identify them as QP residues (for Z & Z ∼ − Z ≤
20. The obtained average isotopic distribution as a
NPC 2013
PI5 10 15 20 25 30 35 y i e l d s
16 16.5 17 17.5 18 18.5 19 19.50100200300
S Cl Ar K F igure Particle identification spectrum for ions punching through the first Si layer. In the inset the S - K regionis expanded (mm/ns) lab v0 50 100 Z beam v CM v Sn Kr+ F igure Correlation between the charge and the lab velocity for ions detected by the FAZIA telescope for then-rich reaction. The dashed line corresponds to the thresholds for the punch through of the first Si layer. Thearrows correspond, respectively, to the center of mass velocity ( v CM ) and to the beam velocity ( v beam ). From [11] function of the lab-velocity for both investigated systems is plotted in figure 3, where open pointscorrespond to the neutron rich case and full points to the neutron poor one. Each panel refers to adi ff erent element. For all ions and all lab-velocity values, the h N i / Z associated to the neutron richsystem is higher than that found when the target is the neutron-poor Sn. For both reactions thesame projectile ( Kr) was used and the kinematics are very similar, as proved by the similarity of thegrazing angles. Since the telescope can detect only fragments coming from the QP decay or from theforward (in the centre of mass frame) neck emission, the observed di ff erence is an evidence of isospindi ff usion, driven by the isospin gradient between target and projectile.Another observation emerging from this picture is the fact that while the h N i / Z for heavy products(bottom part) is independent of their lab-velocity, this is not true for light fragments (top part). Infact in this last case we observe a clear increase of the average isospin value when moving fromvelocities close to QP towards the centre of mass region. This last e ff ect may be explained as anevidence of isopin drift : light fragments coming from neck emission (i.e. velocities close to thecentre of mass) show a neutron enrichment with respect to those evaporated from the excited QP.According to theoretical models (e.g. [9]), this neutron enrichment is driven by the density gradientbetween the QP zone, where nuclear matter is at normal density, and the more diluted neck region.On the contrary, heavier fragments, which are supposed to come from the QP fission, do not show anyvelocity dependence of their average isospin content, because they come from a unique source (thefissioning QP). PJ Web of Conferences
40 60 80 100 < N > / Z Z=4 Sn Kr+ Sn Kr+ (mm/ns) lab v40 60 80 100 < N > / Z
40 60 80 1001.11.21.3
Z=7 (mm/ns) lab v40 60 80 100
Z=20 F igure h N i / Z as a function of the lab velocity. Each panel refers to a di ff erent element ; open points correspondto the reaction Kr + Sn, while full points correspond to Kr + Sn.
The good capabilities of the FAZIA telescope have allowed us to extend the isotopic identificationin a region which was up to now accessible only by means of spectrometers. As a consequence, furtherresults on the isospin transport topic will be possible when, in the next future, more FAZIA telescopeswill be available, allowing us to perform coincidences among the di ff erent ejectiles. In this way someselection on the centrality and on the event class will be possible in an extended solid angle withrespect to what is accessible by means of spectrometers. Références [1] M.B.Tsang et al., Phys. Rev. Lett. , 062701 (2004)[2] T.X.Liu et al., Phys. Rev. C , 034603 (2007)[3] E.Galichet et al., Phys. Rev. C , 335 (2005)[9] M.Di Toro et al., J. Phys. G , 083101 (2010)[10] http : // fazia2.in2p3.fr / spip[11] S.Barlini et al., Phys. Rev. C , 054607 (2013)[12] S. Carboni et al., Nucl. Instrum. Methods A664