Nuclear reactions between medium-mass nuclei at low bombarding energy are characterized by the competition between binary and evaporation processes in the compound nucleus decay. A study of the inﬂuence of the neutron richness of the entrance channel on the decay paths of the compound nuclei formed in the 78Kr + 40Ca and 86Kr + 48Ca at 10MeV/A is presented. The experiment has been performed at Laboratori Nazionali del Sud by using the CHIMERA 4π multidetector for charged particles. The kinematical characteristics of the two reactions support the conclusion of a production of the IMFs via long-lived system and the global features suggest a diﬀerent competition among the various reaction channels. The cross sections of the mechanisms responsible for the fragments production have been extracted. Besides the results relative to the n-poor system are compared to those obtained at GANIL, performed at 5.5AMeV, in order to study the energy inﬂuence on the mechanisms

Gnoffo, B.

La Commara, M.

Pirrone, S.

Politi, G.

Wieleczko, J. P.

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DOI 10.1393/ncc/i2016-16403-xColloquia: IWM-EC 2016IL NUOVO CIMENTO 39 C (2016) 403N/Z effect on reaction mechanisms cross sections in the 78Kr + 40Caand 86Kr + 48Ca collisions at 10 A MeVB. Gnoffo(1)(2), S. Pirrone(1), G. Politi(1)(3), M. La Commara(4)(5),J. P. Wieleczko(6), E. De Filippo(1), P. Russotto(1), M. Trimarchi(7),M. Vigilante(4)(5), G. Ademard(8), L. Auditore(7), C. Beck(9),I. Bercenau(10), E. Bonnet(6), B. Borderie(8), G. Cardella(1), A. Chibihi(6),M. Colonna(11), S. De Luca(7), D. Dell’Aquila(4)(5), A. D’Onofrio(5)(12),J.D. Frankland(6), G. Lanzalone(11)(13), P. Lautesse (14), D. Lebhertz(6),N. Le Neidre(15), I. Lombardo(4)(5), N. S. Martorana(3)(11),K. Mazurek(6),S. Norella(7), A. Pagano(1), E.V. Pagano(3)(11), M. Papa(1), E. Piasecki(16),F. Porto(3), L. Quattrocchi(1)(3), F. Rizzo(3)(11), G. Spadaccini(4)(5),A. Trifirò(7) and G. Verde(1)(8)(1) INFN Sezione di Catania - Catania, Italy(2) Centro Siciliano Fisica Nucleare e Struttura della Materia - Catania, Italy(3) Dipartimento di Fisica e Astronomia, Università di Catania - Catania, Italy(4) Dipartimento di Fisica, Università Federico II - Napoli, Italy(5) INFN Sezione di Napoli - Napoli, Italy(6) GANIL - Caen, France(7) Dipartimento di Fisica, Università di Messina and INFN, Gruppo collegato di MessinaMessina, Italy(8) IN2P3-IPN - Orsay, France(9) IN2P3-IPHC - Strasbourg, France(10) IPNE - Bucarest, Romania(11) INFN, Laboratori Nazionali del Sud - Catania, Italy(12) Dipartimento di Matematica e Fisica, Seconda Università di Napoli - Caserta, Italy(13) “Kore” Università - Enna, Italy(14) IN2P3-IPN - Lyon, France(15) IN2P3-LPC - Caen, France(16) Faculty of Physics and Heavy Ion Laboratory, University of Warsaw - Warsaw, Polandreceived 10 January 2017Creative Commons Attribution 4.0 License (http://creativecommons.org/licenses/by/4.0) 12 B. GNOFFO et al.Summary. — Nuclear reactions between medium-mass nuclei at low bombard-ing energy are characterized by the competition between binary and evaporationprocesses in the compound nucleus decay. A study of the influence of the neutronrichness of the entrance channel on the decay paths of the compound nuclei formedin the 78Kr + 40Ca and 86Kr + 48Ca at 10 MeV/A is presented. The experiment hasbeen performed at Laboratori Nazionali del Sud by using the CHIMERA 4π multi-detector for charged particles. The kinematical characteristics of the two reactionssupport the conclusion of a production of the IMFs via long-lived system and theglobal features suggest a different competition among the various reaction channels.The cross sections of the mechanisms responsible for the fragments production havebeen extracted. Besides the results relative to the n-poor system are compared tothose obtained at GANIL, performed at 5.5 AMeV, in order to study the energyinfluence on the mechanisms.1. – IntroductionHeavy-ion collisions are a powerful tool in order to have information about nuclearstructure properties and to investigate the characteristics of reaction dynamics. In fact,different incident energies and impact parameters lead the interacting systems to variousconfigurations that result in the observation of a wide variety of reaction channels.The collisions in the, so-called, low-energy domain (E/A ≤ 15MeV) are character-ized by the competition between fusion process, where the compound nucleus decaysor through evaporation of light particles or through fission, and binary processes. Thecompound nucleus disexcitation modes produce particles in a wide mass range, in par-ticular the production of fragments with atomic numbers 3 ≤ Z ≤ 20, denoted as theIntermediate Mass Fragments, IMFs, is very interesting because of many features thatare not well understood yet.The N/Z ratio, strongly correlated to the isospin degree of freedom, has importanteffects on the characteristics of the fragments production and it is expected to play acrucial role in the competition among the different decay channels.In this work, the study of the influence of the neutron enrichment on the decay modesof the compound nuclei formed in the reactions 78Kr + 40Ca (n-poor) and 86Kr + 48Ca(n-rich) at 10 A MeV, will be discussed. In fact these reactions, lead to the formationof two systems with N/Z ratios, 1.11 and 1.39, respectively, providing the maximumdifference available with stable beams. The experiment called ISODEC [1-3], where thereactions mentioned before have been realized, has been performed at Laboratori Nazion-ali del Sud in Catania, by using the beams delivered by the Superconductive Cyclotronand the detection and identification capability of the 4π multidetector CHIMERA [4,5].Moreover, some effects of the incident energy will be shown through the comparisonbetween the results relative to the reaction 78Kr + 40Ca performed at 10 A MeV and at5.5 AMeV, respectively.N/Z EFFECT ON REACTION MECHANISMS CROSS SECTIONS ETC. 3Z0 2 4 6 8 10 12 14 16 18 20> (cm/ns)c.m<v00.511.522.533.54° = 5.2labθ° = 10.75labθ° = 21labθFig. 1. – Values of the average velocities, in the c.m. frame, vs. Z at 3 different laboratoryangles for 78Kr + 40Ca at 10A MeV.2. – Experimental resultsThe CHIMERA apparatus is a powerful device for studying the heavy-ion collisionsnot only for the good identification in mass and/or charge of the reaction products and thecomplete event reconstruction but also for the precise information about the kinematicalfeatures that allow the discrimination among the various reaction channels.In fact the analysis of the velocity spectra suggests a high relaxation of the degreeof freedom [6-8]; plotting (fig. 1 [3]) at different laboratory angles, the average values,obtained in the center-of-mass frame, nearly independent of the emission angles, vs. theatomic number it is possible to observe a quasi linear decrease with the increase of theatomic number.Moreover, as it is shown in fig. 2 [3], the angular distributions of the IMFs in thecenter-of-mass frame follow a 1/ sin θ behavior confirming a production via a sourcewhich has reached an equilibration after the fusion of the target and projectile, thus witha lifetime longer than its rotational period, that implies a loss of memory of the entrancechannel direction.Therefore we can conclude that the major sources of emission of the IMFs are theasymmetric and very asymmetric fission-like mechanisms; another proof is that the emis-sion velocity can be explained in terms of pure Coulomb repulsion between the coupleof fragments, as it is demonstrated by the comparison (fig. 3 [3]) between the average°c.mθ0 10 20 30 40 50 60 70 80 90 100 (a.u.)c.m.Ω/dσd1−10110210310Z=10Z=12Z=14°c.mθ0 10 20 30 40 50 60 70 80 90 100 (a.u.)c.m.Ω/dσd1−10110210310Z=18Z=15Z=13Fig. 2. – For the 78Kr + 40Ca reaction: angular distributions of IMFs, in the center-of-massframe compared to 1/ sin θc.m. (solid line).4 B. GNOFFO et al.Z0 2 4 6 8 10 12 14 16 18 20> (cm/ns)c.m<v00.511.522.533.54Hinde valuesExperimental average valuesFig. 3. – For the 78Kr + 40Ca reaction: comparison of the mean values of the velocity, in thecenter-of-mass frame, averaged over all angles, with the values of Hinde’s systematic.values of the velocity in the center-of-mass frame and the values obtained through theViola systematic [9], with the correction by Hinde for the asymmetric fission [10], thatprovides the most probable value of the energy released in a fission process.The results shown before have been obtained for the n-poor system but similar con-sideration can be made for the n-rich one; however, despite the production mechanismis the same for the two reactions, there is a big difference in the probability of the IMFsemission.In fact if we look at the production cross sections of the IMF, as a function the atomicnumber, up to Z = 15, in fig. 4 [3], we can observe a strong influence by the isospin.The fission process would seem to be more probable for the n-poor system, as proved bythe higher values of the production cross sections of the IMFs. Moreover, in agreementwith other examples in the literature, the preferential production of fragments with aneven value of the atomic number, responsible of the presence of an even-odd effect, thestaggering [11, 12], in the charge distribution, is more evident for the n-poor system,in particular for fragments with a value of the atomic number less than 10; this effectcould be strongly connected to the pairing forces [13]. The staggering is also observedin the plot of IMF’s yields vs. the neutron number, shown in fig. 5, but in this case itis more pronounced for the n-rich system [11]. One should observe that in the literaturethere are few examples of this kind of study, as it requires an apparatus allowing a goodisotopic discrimination; the CHIMERA device provides, with high resolution, the isotopicFig. 4. – Production cross section for fragments with different atomic numbers for the tworeactions 78Kr + 40Ca and 86Kr + 48Ca. The error bars are inside the graphical symbols.N/Z EFFECT ON REACTION MECHANISMS CROSS SECTIONS ETC. 5Fig. 5. – IMFs yields versus the neutron number, at θ  11◦, in black for 78Kr + 40Ca and inred for 86Kr + 48Ca. The error bars are smaller than the symbols.composition of the fragments with Z ≤ 9. The produced IMFs seem to be emitted inan excited state as suggested by a preliminary study of the relative velocity of the IMFand one α particle emitted in coincidence, projected in the reference frame of the fissionFig. 6. – v⊥ vs. v‖ plot for the n-poor system, in the frame of the light fragment and in itscomplementary partner’s frame, on the left for incident energy E = 10A MeV and on the rightfor E = 5.5 AMeV.6 B. GNOFFO et al.fragment. This frame is constituted by the axis in the direction of the velocity of thefragment in the center-of-mass frame and by the plane perpendicular to this axis. Thev⊥ vs. v‖ plots, in the case of very asymmetric fission, where the emitted IMF is theCarbon (a panel), asymmetric fission, where the emitted IMF is the Silicon (b panel) andsymmetric fission (c panel), in the frame of the IMF and in its complementary partnerframe, are shown in the left part of fig. 6. By the comparison between the results relativeto the ISODEC experiment and those relative to the experiment realized at GANIL,with the INDRA device, where the 78Kr + 40Ca reaction was studied at 5.5 A MeV (theright part of the picture) [6] one observes that in the case of very asymmetric fission, athigher energy the v⊥ vs. v‖ plot seems to present a Coulomb ring centered at the originalso when the relative velocity are plotted in the frame of the IMF, suggesting that at10 A MeV, also this fragment is emitted in an excited state.3. – ConclusionsSome results of the analysis of the experiment ISODEC have been presented, inparticular the influence of the isospin degree of freedom on the aspects characterizing thefission process. The two different systems studied, 78Kr + 40Ca and 86Kr + 48Ca, exhibitan odd-even effect, the staggering, presented in both, charge distributions and the IMFsyields vs. the neutron number, anyway the oscillation amplitude is more pronounced inthe first case for the n-poor system and in the second one for the n-rich system.Also the energy of the entrance channel influences the emission processes, in fact apreliminary study of the relative velocity of the α particles emitted in the same eventwith an IMF and the heavy partner of the latter, would suggest an α particle’s emissionalso by the IMF in a very asymmetric fission process, at higher energy.For better understanding the staggering effect it is necessary to study the coincidencebetween the light particles and IMfs and the other heavier fragments. Moreover thecompetition among the different reaction channels could be different between the twostudied systems, therefore the cross sections will be calculated, allowing also a bettercomparison with statistical [7] and dynamical [14] models and between the two differentincident energies.REFERENCES[1] Pirrone S. et al., J. Phys. Conf., 515 (2014) 012018.[2] Politi G. et al., JPS: Conf. Proc., 6 (2015) 030082.[3] Gnoffo B. et al., Nuovo Cimento C, 39 (2016) 275.[4] Pagano A. et al., Nucl. Phys. News, 22 (2012) 25.[5] Politi G. et al., IEEE NSS: Conf. Rec., 2005 (N28-5).[6] Ademard G. et al., Phys. Rev. C, 83 (2011) 054619.[7] Charity R. J. et al., Nuc. Phys. A, 476 (1988) 516.[8] Jing K. X. et al., Nuc. Phys. A, 645 (1999) 203.[9] Viola V. E. et al., Phys. Rev. C, 31 (1985) 1550.[10] Hinde D. J. et al., Nuc. Phys. A, 472 (1987) 318.[11] Lombardo I. et al., Phys. Rev. C, 84 (2011) 024613.[12] Casini G. et al., Phys. Rev. C, 86 (2012) 011602.[13] D’ Agostino M. et al., Nuc. Phys. A, 861 (2011) 47.[14] Kalandarov S. A. et al., Phys. Rev. C, 82 (2010) 044603.

N/Z eﬀect on reaction mechanisms cross sections in the 78Kr + 40Ca and 86Kr + 48Ca collisions at 10A MeV

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N/Z eﬀect on reaction mechanisms cross sections in the 78Kr + 40Ca and 86Kr + 48Ca collisions at 10A MeV

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