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For the 68Zn + 112Sn reaction the Coulomb parameter Z1Z2 is equal to 1500 that is close to the threshold value for the appearance of quasifission process. It was found that mass-energy distributions of the reaction fragments differ significantly from those obtained in the 36Ar + 144Sm reaction leading to the formation of the same composite system of 180Hg at similar excitation energies of about 50 MeV. In the case of the reaction with 68Zn ions, the mass distribution of fissionlike fragments has a wide two-humped shape with maximum yields at 70 and 110 u for the light and heavy fragments, respectively, instead of 80 and 100 u observed in the fission of 180Hg formed in the 36Ar + 144Sm reaction. The difference is explained by an unexpectedly large contribution (more than 70%) of quasifission in the case of the 68Zn + 112Sn reaction.
Background: In heavy-ion-induced reactions the mechanism leading to the formation of the compound nucleus and the role of quasifission is still not clear. Purpose: Investigation of the quasifission process of superheavy composite systems with Z = 110-116 and comparison with properties of fusion-fission and quasifission of lighter composite systems. Method: Mass and energy distributions of fissionlike fragments formed in the reactions 48Ca +232 Th, 238U, 244Pu, and 248Cm at energies near the Coulomb barrier have been measured using the double-arm time-of-flight spectrometer CORSET at the U-400 cyclotron of the FLNR JINR. Results: The most probable fragment masses as well as total kinetic energies and their dispersions in dependence on the interaction energies and ion-target combinations have been studied for asymmetric and symmetric fragments formed in the reactions. The capture cross sections were obtained for the reactions 48Ca +244 Pu and 248Cm. The lower limits for fission barriers of 283−286Cn, 289−292Fl, and 293−296Lv compound nuclei were estimated. Conclusions: Analysis of the properties of symmetric fragments has shown that a significant part of these fragments may be attributed to fusion-fission process for the reactions 48Ca+238U, 244Pu, and 248Cm.
[Introduction] This work is a study of the influence of shell effects on the formation of binary fragments in damped collision. We have investigated binary reaction channels of the composite system with Z = 108 produced in the reaction 88 Sr + 176 Yb at an energy slightly above the Bass barrier ( E c.m. /E Bass = 1 . 03). Reaction products were detected by using the two-arm time-of-flight spectrometer CORSET at the K130 cyclotron of the Department of Physics, University of Jyv ̈ askyl ̈ a. The mass-energy distribution of primary binary fragments has been measured. For targetlike fragments heavier than 190 u, which correspond to a mass transfer as large as twenty nucleons or more, an enhancement of the yields is observed. This striking result can be ascribed to the proton shells at Z = 28 and 82 and implies the persistence of the shell effects in the formation of reaction fragments even for large mass transfers.
Multi-nucleon transfer reactions are nowadays the only known mean to produce neutron-rich nuclei in the Terra Incognita. The closed-shell region N=126 is crucial for both studying shell-quenching in exotic nuclei and the r-process, being its last "waiting-point". The choice of suitable reactions is challenging and a favorable case is 136Xe+208Pb, near the Coulomb barrier, because their neutron shell-closures play a stabilizing role, favoring the proton-transfer from lead to xenon. TOF-TOF data were analyzed to reconstruct the mass-energy distribution of the primary fragments. Preliminary results of an experiment held at Laboratori Nazionali di Legnaro with PRISMA, aimed at A and Z identification of the products, will be shown.
Purpose: The study of asymmetric and symmetric fission of 180,182,183Hg and 178Pt nuclei as a function of their excitation energy and isospin. Methods: Mass-energy distributions of fission fragments of 180Hg, 178Pt (two protons less than 180Hg), and 182Hg (two neutrons more than 180Hg) formed in the 36Ar+144Sm,142Nd, and 40Ca+142Nd reactions were measured at energies near and above the Coulomb barrier. Fission of 183Hg obtained in the reaction of 40Ca with 143Nd was also investigated to see if one extra neutron could lead to dramatic changes in the fission process due to the shape-staggering effect in radii, known in 183Hg. The measurements were performed with the double-arm time-of-flight spectrometer CORSET. Results: The observed peculiarities in the fission fragment mass-energy distributions for all studied nuclei may be explained by the presence of a symmetric fission mode and three asymmetric fission modes, manifested by the different total kinetic energies and fragment mass splits. The yield of symmetric mode grows with increasing excitation energy of compound nucleus. Conclusions: The investigated properties of asymmetric fission of 180,182,183Hg and 178Pt nuclei point out the existence of well-deformed proton shell at Z≈36 and a less deformed proton shell at Z ≈ 46.
Background: Observation of asymmetric fission of 180Hg has led to intensive theoretical and experimental studies of fission of neutron-deficient nuclei in the lead region. Purpose: The study of asymmetric and symmetric fission modes of 180,190Hg and 184,192,202Pb nuclei. Methods: Mass-energy distributions of fission fragments of 180,190Hg and 184Pb formed in the 36Ar+144,154Sm and 40Ca+144Sm reactions, respectively, at energies near the Coulomb barrier have been measured using the double-arm time-of-flight spectrometer CORSET and compared with previously measured 192,202Pb isotopes produced in the 48Ca+144,154Sm reactions. The mass distributions for 180,190Hg and 184,192,202Pb together with old data for 187Ir, 195Au, 198Hg, 201Tl, 205,207Bi, 210Po, and 213At [J. Nucl. Phys. 53, 1225 (1991)] have been decomposed into symmetric and asymmetric fission modes. The total kinetic-energy distributions for different fission fragment mass regions have been analyzed for 180,190Hg and 184Pb. Results: The stabilization role of proton numbers at Z≈36, 38, Z≈45, 46, and Z=28/50 in asymmetric fission of excited preactinide nuclei has been observed. The high (≈145−MeV) and the low (≈128−MeV) energy components have been found in the total kinetic-energy distributions of 180,190Hg fission fragments corresponding to the fragments with proton numbers near Z≈46 and Z≈36, respectively. In the case of fission of 184Pb only the low-energy component (≈135MeV) for the fragments with masses corresponding to the proton numbers Z≈36 and 46 has been found. Conclusions: The studied properties of asymmetric fission of 180,190Hg and 184,192,202Pb nuclei point out the existence of well deformed proton shell at Z≈36 and less deformed proton shell at Z≈46.
Background: The formation of superheavy nuclei in fusion reactions is suppressed by a competing quasifission process. The competition between the formation of the compound nucleus and the quasifission depends strongly on the reaction entrance channel. Purpose: The investigation of fission and quasifission processes in the formation of Z=120 superheavy composite systems in the 52,54Cr+248Cm and 68Zn+232Th reactions, and their comparison with the 64Ni+238U reaction at energies in the vicinity of the Coulomb barrier. Methods: Mass-energy distributions of fissionlike fragments formed in the reactions 52,54Cr+248Cm and 68Zn+232Th at energies near the Coulomb barrier were measured using the double-arm time-of-flight spectrometer CORSET. Results: Capture cross sections for the reactions under investigation were measured. The most probable fragment masses and total kinetic energies as well as their variances in dependence on the interaction energy were studied for asymmetric and symmetric fragments. The fusion probabilities were estimated from the analysis of mass-energy distributions. Conclusions: The estimated fusion probability drops down by a factor of 103 in the 54Cr+248Cm reaction compared to the reactions of 48Ca ions with actinides. Among the studied reactions, the 54Cr+248Cm is the most favorable one for the production of the superheavy element with Z=120.
Reaction products from the system 136Xe+208Pb at 136Xe ions laboratory energies of 700, 870, and 1020 MeV were studied by two-body kinematics and by a catcher-foil activity analysis to explore the theoretically proposed suitability of such reaction as a means to produce neutron-rich nuclei in the neutron shell closure N=126. Cross sections for products heavier than 208Pb were measured and were found sensibly larger than new theoretical predictions. Transfers of up to 16 nucleons from Xe to Pb were observed.
Compound nucleus fission and quasi-fission are both binary decay channels whose common properties make the experimental separation between them difficult. A way to achieve this separation could be to probe the angular momentum of the binary fragments. This can be done detecting gamma rays in coincidence with the two fragments. As a case study, the reaction 32S + 197Au near the Coulomb barrier has been performed at the Tandem ALTO facility at IPN ORSAY. ORGAM and PARIS, two different gamma detectors arrays, are coupled with the CORSET detector, a two-arm time-of-flight spectrometer. TOF-TOF data were analyzed to reconstruct the mass-energy distribution of the primary fragments coupled with gamma multiplicity and spectroscopic analysis. Preliminary results of will be shown.
The next years will see the completion of several new facilities at Istituto Nazionale di Fisica Nucleare – Laboratori Nazionali del Sud (LNS) opening up new possibilities in the fields of nuclear structure, nuclear dynamics, nuclear astrophysics and applications. These include a new line for high-intensity cyclotron beams, a new facility for in-flight production of radioactive ion beams, the PANDORA plasma trap for multidisciplinary studies and a high-power laser for basic science and applied physics. The nuclear physics community has organized a workshop to discuss the new physics opportunities that will be possible in the middle term (5–7 years) by employing state-of-the-art detection systems. A detailed discussion of the outcome from the workshop is presented in this report.