In this paper, preliminary results for the reactions 40,48Ca + 40,48Ca at 35MeV/u in the framework of the FAZIASYM experiment are discussed. FAZIASYM experiment was performed at the LNS cyclotron with 4 blocks of the FAZIA apparatus. We will focus on the quasi-projectile ﬁssion events. After a brief overview on the various analysis steps, we will show some results on the isospin content of both ﬁssion-like fragments, comparing the cases of the two extreme n-rich and n-deﬁcient systems also by exploiting a recently introduced order variable

Alba, R.

Bonnet, E.

Camaiani, A.

Casini, G.

Piantelli, S.

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DOI 10.1393/ncc/i2018-18172-xColloquia: IWM-EC 2018IL NUOVO CIMENTO 41 C (2018) 172Isotopic composition of quasi-projectile fission fragments for thesystems 40,48Ca + 40,48Ca at 35 AMeVA. Camaiani(1)(2)(∗) G. Casini(2), S. Piantelli(2), E. Bonnet(3), R. Alba(4),S. Barlini(1)(2), M. Bini(1)(2), B. Borderie(5), R. Bougault(6), M. Bruno(7)(8),A. Buccola(1)(2), A. Chbihi(9), M. Cicerchia(10)(11), M. Cinausero(10),D. Dell’Aquila(12), J. A. Duenas(13), Q. Fable(6), D. Fabris(14),L. Francalanza(12), J. D. Frankland(9), C. Frosin(1)(2), F. Gramegna(10),D. Gruyer(6), M. Henri(6), A. Kordyasz(15), T. Kozik(16), N. Le Neindre(6),I. Lombardo(17), O. Lopez(6), C. Maiolino(4), T. Marchi(10), L. Morelli(9),A. Olmi(1)(2), P. Ottanelli(1)(2), G. Pasquali(1)(2), G. Pastore(1)(2),M. Parlog(6)(18), G. Poggi(1)(2), D. Santonocito(4), A. A. Stefanini(1)(2),S. Valdré(2), E. Vient(6), G. Verde(17) and M. Vigilante(12)(1) Dipartimento di Fisica, Universitá di Firenze - Firenze, Italy(2) INFN, Sezione di Firenze - Firenze, Italy(3) SUBATECH UMR 6457, IMT Atlantique, Universitè de Nantes, CNRS-IN2P3 - 44300,Nantes, France(4) INFN, Laboratori Nazionali del Sud - Catania, Italy(5) IPN, CNRS/IN2P3, Université Paris-Sud, Université Paris-Saclay - F-91406 Orsay Cedex,France(6) Normandie Université, ENSICAEN, UNICAEN, CNRS/IN2P3, LPC Caen - F-14000 Caen,France(7) Dipartimento di Fisica, Università di Bologna - Bologna, Italy(8) INFN, Sezione di Bologna - Bologna, Italy(9) GANIL, CEA/DRFCNRS/IN2P3 - Bvd. Henri Becquerel, 14076 Caen, France(10) INFN, Laboratori Nazionali di Legnaro - Legnaro(PD), Italy(11) Dipartimento di Fisica, Università di Padova - Padova, Italy(12) Dipartimento di Fisica and Sezione INFN, Università di Napoli - I-80126 Napoli, Italy(13) Departamento de Fisica Aplicada, FCCEE Universidad de Huelva - 21071 Huelva, Spain(14) INFN, Sezione di Padova - Padova, Italy(15) Heavy Ion Laboratory, Warsaw University - Warsaw, Poland(16) Jagiellonian University - Cracow, Poland(17) INFN, Sezione di Catania, Catania, Italy(18) “Horia Hulubei” National Institute for R&D in Physics and Nuclear Engineering (IFIN-HH) - P.O. BOX MG-6, Bucharest Măgurele, Romaniareceived 3 December 2018(∗) E-mail: alberto.camaiani@fi.infn.itCreative Commons Attribution 4.0 License (http://creativecommons.org/licenses/by/4.0) 12 A. CAMAIANI et al.Summary. — In this paper, preliminary results for the reactions 40,48Ca + 40,48Caat 35 MeV/u in the framework of the FAZIASYM experiment are discussed. FAZIA-SYM experiment was performed at the LNS cyclotron with 4 blocks of the FAZIAapparatus. We will focus on the quasi-projectile fission events. After a brief overviewon the various analysis steps, we will show some results on the isospin content of bothfission-like fragments, comparing the cases of the two extreme n-rich and n-deficientsystems also by exploiting a recently introduced order variable.1. – IntroductionIn the past years the FAZIA Collaboration developed and built a new modular detec-tor aimed at investigating nuclear reactions in the Fermi energy domain (25–50 MeV/u):after a long R&D phase, some physics campaigns at the INFN Laboratori Nazionali delSud (LNS) were carried out. In this proceeding we are going to talk about preliminaryresults from the second experiment of the first campaign, devised to explore the reactions40,48Ca+40,48Ca at 35 MeV/u.The so-called FAZIASYM experiment was proposed in order to complete a studyon the symmetry energy term of the nuclear Equation of State (EoS) started using theINDRA + VAMOS setup at GANIL [1,2]. A limit of the latter measurement, due to thepresence of the spectrometer, is the lack of coincidence among quasi-projectile (QP) andlight charged particles (LCP) or intermediate mass fragments (IMF) at forward angles.Such a limit could be overcome by a modular detector like FAZIA. However, in thispaper we mainly focus on a subject that recently became of interest to our collaboration,following previous ideas and analyses with FAZIA [3,4] and also supported by some recentliterature [5, 6], i.e., the investigation of the isospin content of both fission fragmentscoming from the QP, detected in coincidence.2. – Experimental apparatusThe experimental apparatus consists of 4 blocks of the FAZIA detector, located inthe Ciclope Scattering Chamber of LNS; a complete description of the features of thetelescopes, in particular for what concerns the Z,A identification can be found else-where [7-10]. A block of FAZIA consists of 16 Si-Si-CsI telescopes fully equipped withdigital electronics; each layer has a thickness of 300 μm, 500 μm and 10 cm, respectively.The fragments can be isotopically identified exploiting the Si-Si and Si-CsI ΔE-E corre-lations up to Z ≈ 25 and the Pulse Shape Analysis (PSA) techniques (the latter for theparticles stopped in the first Silicon layer) up to Z ≈ 20 with low identification thresholdsas reported in [10]; LCPs are mainly identified in charge and mass in the CsI detectorby means of conventional fast-slow PSA.The four blocks were placed 80 cm far from the target, covering the polar angles from2◦ up to 8◦ with respect to the beam axis. Each block covers a surface of about 9×9 cm2and an active solid angle of ∼0.01 sr.Since the angular acceptance of our experimental apparatus is small, it is extremelyimportant to discuss any result by comparing it with the distributions obtained viaISOTOPIC COMPOSITION OF QUASI-PROJECTILE FISSION FRAGMENTS ETC. 3realistic simulations of the considered reactions. As conventionally done in this researchfield, the reactions are simulated in two steps. The first one is the nuclear collision itselfand the second is the de-excitation process which brings the initially excited fragmentstowards the ground or low-lying states. For the description of the dynamical phase weadopted the AMD model [11], choosing a stiff parametrization of the symmetry energyterm (L = 108) and stopping the calculations at 500 fm/c. As afterburner we have chosenthe statistical model GEMINI++ [12]; for sake of brevity in the following we label theAMD + GEMINI simulated data simply as “AMD”. The AMD events have then beenfiltered via a software replica of the apparatus, including the efficiencies, the resolutionsand the identification thresholds of the various detectors. We refer to these filtered dataas “AMD geo”, while “AMD 4π” means a sample of events that does not consider theapparatus efficiency at all.3. – Event selection: Fission of the quasi-projectileThe selection of the QP fission-like events has been done via software gates: in thefollowing we will discuss these criteria using the n-rich system as an example. The sameselections have been applied to the n-deficient one.To disentangle peripheral and central events we exploit the correlation between thetotal detected charge (ZTOT ) and the flow angle (θflow), in events where the number ofdetected fragments (with Z ≥ 3) is greater than one. As shown in literature, while themost strongly dissipative collisions produce a flat θflow distribution, the more peripheralcollisions are mainly located at low angles [13]. In our case due to the low multiplicityof the events, because of the limited geometrical efficiency, θflow is dominated by thedirection of the biggest fragment.The results are shown in the left panel of fig. 1: a clear accumulation of events ispresent at small angles. We have verified by means of the simulation that such anaccumulation corresponds to peripheral events in which the QP has been detected. Thuswe define peripheral events (black rectangle in fig. 1) as those with 2◦ < θflow < 20◦ and12 ≤ ZTOT ≤ 20 (which represents a reasonable domain for the QP charge).The selection of the QP fission can be done assuming a two step process: the QPformed in semi-peripheral collisions, as selected before, can undergo a further break-up intwo fragments. Such fragments can thus be selected with proper conditions. In particularwe can exploit the correlation between the System Center of Mass (SCM) relative angle [deg]flowθ0 50 100TOTZ0102030110210310410Peripheral events [mm/ns]relv0 50 100relCMθ050100150110210QP fissionFig. 1. – Left panel: correlation between the flow angle and the total detected charge. Rightpanel: correlation between the SCM relative angle of the two biggest fragments and their relativevelocity. Both correlations are for the n-rich system.4 A. CAMAIANI et al.of the two biggest fragments (θCMrel ), and their relative velocity (vrel). Indeed, if the twodetected fragments are produced in the QP fission, θCMrel must be much smaller than180◦, meaning that both are forward emitted with respect to the SCM, and vrel has tobe compatible with the Viola systematics [14].A class of events that meets these criteria can be found in the right panel of fig. 1(red rectangle), where the correlation θCMrel vs. vrel is shown for the peripheral eventsselected as described above with two detected fragments.4. – ResultsFirst information can be extracted from the average neutron number per charge unit(〈N〉/Z) of the fission fragments as a function of their charge. The results are shown infig. 2, both for the 48Ca+48Ca and 40Ca+40Ca systems, where the first and the secondbiggest fragments of each event, assumed to be the remnant of the QP fission, are shown,labelled as “BIG” and “small”, respectively, according to the legend.The different neutron richness of the systems causes a net shift, approximately of 0.1,between the n-rich and the n-deficient system, but the behavior is almost the same. Inboth systems the values clearly oscillate below Ne ions and then they flatten; such atrend is quite similar to that observed in other systems [15,16]Recently Jedele et al. [5] associated the isospin content of the QP fission fragmentsto the timescale of the rupture of the excited (and deformed) QP after the interactionwith the target nucleus introducing an ordering variable (studied system 70Zn+70Znat 35 MeV/u). Such a variable is the angle (labelled as α) between the center-of-massvelocity of the original fissioning QP (reconstructed as the CM of the fission pair) and therelative velocity of the two fragments after the split (α = 0 means that the two fragmentsare aligned along the direction of their center of mass, with the light one toward the targetside). The faster the fission mechanism, the smaller the α angle [17, 18]. In ref. [5] aregular trend of the fragment isospin content is shown as a function of the α angle fora given fission pair: the isospin content tends to exponentially decrease (increase) withincreasing α for the smaller (bigger) fragment of the pair. After deducing the decayconstant for this trend the authors of [5] could extract the fission time scale.Following [5, 6] we started a similar analysis for the QP fission fragments. First ofall we needed to understand the effect of our limited angular acceptance. For such apurpose we investigated the cos(α) distributions, shown in fig. 3. Our distributions doZ0 2 4 6 8 10 12 14 16 18<N> / Z11.051.11.151.2Ca: small48Ca+48Ca: BIG48Ca+48Ca: small40Ca+40Ca: BIG40Ca+40Fig. 2. – Average neutron number per charge unit of the fission fragments as a function of theircharge. Statistical errors are smaller than the marker size.ISOTOPIC COMPOSITION OF QUASI-PROJECTILE FISSION FRAGMENTS ETC. 5)αcos(1− 0.5− 0 0.5 1) [arb. units]αdN/dcos(00.10.20.30.4Ca48Ca+48)αcos(1− 0.5− 0 0.5 1Ca40Ca+40)αcos(1− 0.5− 0 0.5 1πAMD 4Ca48Ca+48)αcos(1− 0.5− 0 0.5 1AMD geoCa48Ca+48Fig. 3. – Experimental and simulated distributions of cos(α). All the histograms are normalizedto unity.not manifest a shape similar to that often observed by other experiments [5, 6, 16] witha pronounced peak at cos(α) = 1. However, an aligned emission seems favored, with aslight preference for the smaller fragment emitted towards midvelocity.By means of the reaction simulations we verified that the geometry significantly affectsthe shape of the measured cos(α) distribution. This is shown for the n-rich system in thethird and fourth panel of fig. 3; the labels refer to the convention introduced in sect. 2.As we can see the effect of the geometry is mainly to deplete the distribution aroundcos(α) ∼ 0, as also observed in other experiments with limited angular coverage [19].In fig. 4 the isospin content (here quoted as Δ = 〈N−ZA 〉 for an easier comparisonwith [5, 6]) is reported for many fission pairs as a function of the α angle. Due to thelow statistics collected for the n-deficient system, here we limit ourselves to show onlythe data of the n-rich one. Data are organized in the following way: each row representsclasses of events with fixed sum of the “small” and the “BIG” fragment (ZQP ), decreasingfrom ZQP = 20 (top) to ZQP = 12 (bottom); in some sense moving down a column we aregoing from peripheral events to more central ones. Moreover, moving horizontally, fromleft to right, the event asymmetry decreases (left are the most asymmetric break-ups).The Δ values of the “small” and the “BIG” fragment of the pair are represented as blueand magenta dots, respectively.00.10.2=20QPZ=17BIG=3  ZsmallZ00.10.2=18QPZ=15BIG=3  ZsmallZΔ00.10.2=16QPZ=13BIG=3  ZsmallZ00.10.2=14QPZ=11BIG=3  ZsmallZ0 50 100 15000.10.2=12QPZ=9BIG=3  ZsmallZ=16BIG=4  ZsmallZ=14BIG=4  ZsmallZ=12BIG=4  ZsmallZ=10BIG=4  ZsmallZ0 50 100 150=8BIG=4  ZsmallZ=15BIG=5  ZsmallZ=13BIG=5  ZsmallZ=11BIG=5  ZsmallZ=9BIG=5  ZsmallZ0 50 100 150=7BIG=5  ZsmallZ=14BIG=6  ZsmallZ=12BIG=6  ZsmallZ=10BIG=6  ZsmallZ=8BIG=6  ZsmallZ [deg]α0 50 100 150=13BIG=7  ZsmallZ=11BIG=7  ZsmallZ=9BIG=7  ZsmallZ0 50 100 150=12BIG=8  ZsmallZ=10BIG=8  ZsmallZ0 50 100 150=11BIG=9  ZsmallZ0 50 100 150Fig. 4. – Δ as a function of the α angle for the n-rich system. Blue dots: Δ value for the smallfragment. Magenta dots: Δ value for the BIG fragment. See the text for the panels displacement.6 A. CAMAIANI et al.00.050.10.150.2=15BIG=3  ZsmallZEXP [deg]α0 50 100 150Δ00.050.10.150.2=10BIG=8  ZsmallZπAMD 4 [deg]α0 50 100 150AMD geo [deg]α0 50 100 150Fig. 5. – Isospin content as a function of the α angle for the fission pairs Zsmall = 3, ZBIG = 15and Zsmall = 8, ZBIG = 10. The panels are organized as follow: in the first column theexperimental plots, in the second the 4π original AMD simulation, in the third column theAMD results after the geometrical filter.Some observations can be made looking at fig. 4. First, an equilibration of the isospincontent as the symmetry increases is clear, independently of the ZQP value: indeed, theΔ gap between the “BIG” and the “small” fragment decreases moving toward symmetricevents. Second, in every panel, a clear decrease/increase of Δ as a function of α, asreported for instance in [5, 6], does not appear. A weak decreasing trend of Δ withincreasing of α for Zsmall seems visible for the pairs involving Zsmall = 3 and 4, butnot with the evidence found in [5, 6]. Mostly for asymmetric splits, a slight and broadΔ increase around 90◦ seems apparent. This can be an artifact due to the the limitedcoverage of the blocks and will be thoroughly verified in our next analysis with the helpof the AMD simulation.A first indication in this sense can be seen in fig. 5. Here the filtered events (AMDgeo) present a broad irregular bump at 60–120◦, not present in the original simulation(AMD 4π). However, the geometry effects do not change too much the main trends ofthe figure: the Δ gap between the “small” and “BIG” fragment is preserved for all thefission directions (no clear tendency to isospin equilibration when α increases). Indeed,an important observation is that not even the AMD 4π simulation predicts an evolutionof Δ as a function of α, at variance with the experimental results of [5, 6], and it isqualitatively in agreement with our findings of fig. 4. The lack of an evolution of Δ vs. αcould signal some differences in the time scale of the break-up process according to thescenario proposed in [5, 6]. These subjects will be carefully investigated to understandthe reasons of the apparent different behavior of the splitting of Zn QP [5, 6] and CaQP. This might be related to the different size of the two systems. The work is still inprogress.5. – Conclusion and prospectivesWe have presented some preliminary experimental results concerning the analysis ofthe systems 48,40Ca+48,40Ca at 35 MeV/u on data collected with the FAZIA apparatusat the INFN Laboratori Nazionali del Sud (Italy). Motivated by the recent results on thequasi-projectile fission obtained in our previous FAZIA experiment [3, 4] and in similarsystems [5, 6], we started a study of the isospin dynamics for the QP fission events.ISOTOPIC COMPOSITION OF QUASI-PROJECTILE FISSION FRAGMENTS ETC. 7The main preliminary results are the following: as expected, the neutron richness ofthe original systems reflects in the neutron richness of the fission fragments. For thesefragments the mean N/Z difference between the 48Ca+48Ca and 40Ca+40Ca is 0.1, andshows a behavior quite similar to that observed in other experiments [15,16]. The founddependence of the isospin content as a function of the fission step alignment presentsdifferences with respect to published results for 70Zn+70Zn at similar bombarding ener-gies. This interesting result, which seems supported by our transport model simulationswill be further investigated. We are also planning some dedicated experiments with areduced FAZIA setup coupled to other detectors such as OSCAR [20] in order to detectalso the QT to obtain an estimate of the event centrality.∗ ∗ ∗Thanks are due to INFN-CNAF and GARR for providing the computing resourcesto perform Monte Carlo simulations.REFERENCES[1] Fable Q., Universitè de Caen Normandie, PhD Thesis (2018).[2] Wigg P. C. et al., EPJ Web of Conferences, 31 (2012) 00015.[3] Pastore G., Univeristà degli studi di Firenze, PhD Thesis (2017).[4] Piantelli S. et al., these proceedings.[5] Jedele A. et al., Phys. Rev. Lett., 118 (2017) 062501.[6] Rodriguez Manso A. et al., Phys. Rev., 95 (2017) 044604.[7] The FAZIA Collaboration and Bougault R. et al., Eur. Phys. J. A, 50 (2014) 47.[8] Carboni S. et al., Nucl. Instrum. Methods A, 664 (2012) 251.[9] Le Neindre N. et al., Nucl. Instrum. Methods A, 701 (2013) 145.[10] Pastore G., Nucl. Instrum. Methods A, 860 (2017) 42.[11] Ono A. et al., Phys. Rev. C, 53 (1996) 2958.[12] Charity R. J., Phys. Rev. C, 82 (2010) 014610.[13] Frankland J. D. et al., Nucl. Phys. A, 689 (2001) 905.[14] Viola V. E. et al., Phys. Rev. C, 31 (1985) 1550.[15] Barlini S. et al.., Phys. Rev. C, 87 (2013) 054607.[16] De Filippo E. et al., Phys. Rev. C, 86 (2012) 014610.[17] Casini G. et al., Phys. Rev. Lett., 71 (1993) 2567.[18] Stefanini A. A. et al., Z. Phys. A, 251 (1995) 167.[19] McIntosh A. B. et al., Phys. Rev. C, 81 (2010) 034603.[20] Dell’Aquila D. et al., Nucl. Instrum. Methods A, 877 (2018) 227.

Isotopic composition of quasi-projectile ﬁssion fragments for the systems 40,48Ca + 40,48Ca at 35 AMeV

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Isotopic composition of quasi-projectile ﬁssion fragments for the systems 40,48Ca + 40,48Ca at 35 AMeV

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