Narrow Results

The fusion-fission excitation functions for the decay of compound nucleus ²¹⁵Fr*, formed in ¹¹B+²⁰⁴Pb and ¹⁸O+¹⁹⁷Au reaction channels, are studied on the Dynamical Cluster-decay Model (DCM), showing entrance channel independence, in agreement with experiments, not invoking any quasi-fission (qf) process in either of the two channels.

In this paper, in order to produce high-energy neutrons, the design of a compact, portable and high efficiency reactor based on a plasma focus device is investigated. The COMSOL Multiphysics software was used to investigate and optimize 16 plasma focus devices with different stored energies for producing neutrons. To keep the fission reactor at the subcritical condition, fuel assembly containing a specific combination of fission fuel of ${}^{238}$U – ${}^{239}$Pu was used. As a result of energetic neutrons produced in the plasma focus device, no radioactive waste remained in the reactor. In addition, an efficient gas cooling system that absorbed and transferred heat was designed using the COMSOL Multiphysics software. The absorbed heat was transferred to the thermo-photo-voltaic converter, and thus electricity was generated with an efficiency of about 40%. Electrical and thermal energies are calculated in each shot for every plasma focus device. Finally, we showed that high frequency plasma focus devices had a high potential to produce energy.

The decay of compound nucleus ²⁰²Pb^*, formed in entrance channel reaction ⁴⁸Ca+¹⁵⁴Sm at different incident energies, is studied by using the dynamical cluster-decay model (DCM) where all decay products are calculated as emissions of preformed clusters through the interaction barriers. The calculated results show an excellent agreement with experimental data for the fusion-evaporation residue cross-section σ_ER together with the fusion-fission cross-section σ_FF (taken as a sum of the energetically favored symmetric and near symmetric A=65–75 plus complementary fragments), and the competing, non-compound-nucleus quasi-fission cross-section σ_QF where the entrance channel is considered not to loose its identity (and hence with preformation factor P₀=1). The interesting feature of this study is that the three decay processes (ER, FF and QF) are quite comparable at low energies, ER being the most dominant, whereas at higher energies FF becomes most probable followed by ER and QF. The prediction of two fission windows, the symmetric fission (SF) and the near symmetric fission (nSF) whose contribution is more at lower incident energies, suggests the presence of a fine structure effect in the fusion-fission of ²⁰²Pb^*. This result is attributed to the shell effects (magic shells) playing effective role in the fragment preformation yields for ⁴⁸Ca+¹⁵⁴Sm reaction at lower excitation energies, giving rise to "shoulders", to an otherwise Gaussian FF mass distribution, responsible for the QF process. As a further verification of this result, absence of "shoulders" (hence, the QF component) in the decay of ¹⁹²Pb^* due to ⁴⁸Ca+¹⁴⁴Sm reaction is also shown to be given by the calculations, in agreement with experiments. The only parameter of the model is the neck-length ΔR which shows that the ER occurs first, having the largest values of ΔR, and the FF and QF processes occur almost simultaneously at lower incident energies but the FF takes over QF at higher incident energies. In other words, the three processes occur in different time scales, QF competing with FF at lower incident energies.

The three-dimensional Langevin equation (LE) and Transition State Model (TSM) are applied to calculate prescission neutron multiplicity and anisotropy of fission fragment angular distribution for ¹⁶O+¹⁸¹Ta and ¹⁶O+²⁰⁸Pb systems. Dynamical calculations were carried out considering one-body dissipation. Theoretical results of pre-scission neutron multiplicity and anisotropy have been compared with experimental data for given systems. Calculations show that the results of anisotropy based on dynamical approach are in better agreement with the experimental data as compared with the TSM (in saddle point).

The dynamics involved in the decay of light mass nuclei formed in asymmetric channels ¹²C + ²⁸Si, ¹¹B + ²⁸Si and ¹²C + ²⁷Al have been investigated using the dynamical cluster-decay model (DCM). In reference to the experimentally measured charge particle cross-sections, the fragment masses contributing towards the decay of ⁴⁰Ca* and ³⁹K* nuclei have been identified using spherical choice of fragmentation. Also, the role of entrance channel has been investigated by studying the decay of ³⁹K* nuclear system formed in two different reactions at same excitation energy. The behavior of fragmentation potential, preformation probability, penetrability and emission time, is analyzed to figure out the favorable mass fragments, their relative emergence and the entrance channel effects observed in the decay of light mass nuclei. In addition to this, the cross-sections for the light particles (LPs) and heavier charge fragments have been estimated for the compound nucleus (CN) decay. Besides this, one of the noncompound nucleus (nCN) process, deep inelastic collision (DIC) has been addressed in context of DCM approach for the first time. The cross-sections obtained in framework of DCM for both CN and nCN processes are found to have nice agreement with the available experimental data.

The pathway toward the production of superheavy elements crosses with the one of an in depth study of the fission and quasifission processes. Quasifission affects the probability of forming a compound nucleus after capture and fission plays a decisive role during the cooling of the compound nucleus. To search for optimal reactions to produce superheavy elements in fusion reactions and to provide an estimate of their production cross section measurements of the capture and fusion cross sections along with the survival probabilities are a mandatory task. In this presentation an overview of the impact of fission and quasifission over the synthesis of superheavy elements is discussed along with the need to perform measurements over a wide range of masses and energies.

The overlap in the mass symmetric region of the reaction products from fusion-fission and quasi-fission complicates the assignment of symmetric events to complete fusion on the basis of the mass distribution alone. Additional observables, besides mass distribution, should be used. The method proposed here relies on the fact that fusion-fission and quasifission are characterized by a different timescale. Within this framework we performed a detailed study to find out if timescales can be probed via angular momentum as measured via γ rays multiplicity. The proof of principle was carried out by measuring the γ rays in coincidence with two fragments in the reaction ³²S + ¹⁹⁷Au at beam energy near the Coulomb barrier. The experiment has been performed at the Tandem ALTO facility at IPN Orsay using a detection setup consisting of ORGAM and PARIS γ detector arrays coupled with the CORSET time-of-flight spectrometer. Preliminary results on the sensitivity of this method to distinguish reaction channels with different dynamics are discusses.

Systems of intermediate fissility are characterized by an evaporation residues (ER) cross section comparable or larger than the fission cross section, and by a relatively higher probability for charged particle emission in the pre-scission channel. In a theoretical framework in which time scale estimates of the fission process rely on statistical model calculations, the analysis of particle emission in the ER channel is the source of additional constraints on the statistical and dynamical models. This contribution will offer an overall view of the systems studied so far and will provide new clues for interesting and contradictory physical cases. In particular, the analysis of the systems ³²S + ¹⁰⁰Mo at E_Lab = 200 MeV and ¹⁸O + ¹⁵⁰Sm at E_Lab = 122 MeV will be presented. Recent advancement will be briefly discussed on the interplay between the fission and the evaporation residues channels as implied by a dynamical (Langevin) approach to the description of the fission process.

Mass-energy distributions and capture cross sections have been measured in the reactions ⁴⁸Ca + ²⁰⁸Pb, ²³²Th, ²³⁸U, ²⁴⁴Pu, and ²⁴⁸Cm at energies around the Coulomb barrier. The properties of the fusion-fission and quasifission processes were studied in dependence on the interaction energies and ion-target combinations. In the reaction ⁴⁸Ca+²⁰⁸Pb the fusion-fission process is dominant. In the reactions of 48Ca with actinide targets the asymmetric quasifission becomes the dominant process. Nevertheless, the analysis of mass and energy distributions has shown that a significant part of symmetric fragments in the range ACN/2 ± 20 u may be attributed to fusion-fission process for the reactions ⁴⁸Ca+²³⁸U, ²⁴⁴Pu, and ²⁴⁸Cm. Using this analysis we estimated the fusion-fission crosssections σ_FF, the survival probabilities W_sur and the lower limits for fission barriers of ^254-256No, ^283-286Cn, ^289-292Fl, and ^293-296Lv compound nuclei.

The properties of the mass and energy distributions of fissionlike fragments formed in the reactions ³⁶S, ⁴⁸Ca, ⁴⁸Ti, ⁶⁴Ni + ²³⁸U at energies around the Coulomb barrier have been analyzed to define the systematic trend of compound nucleus fission and quasifission in hot fusion reactions with actinide targets. The measurements have been carried out at the U400 cyclotron of the FLNR, JINR using the double-arm time-of-flight spectrometer CORSET. The most probable fragment masses as well as total kinetic energies and their dispersions in dependence on the interaction energies have been investigated for asymmetric and symmetric fragments for the studied reactions. The fusion probabilities have been deduced from the analysis of mass and energy distributions. It was found that for the studied reactions fusion probability depends exponentially on mean fissility parameter of the system. For the reactions with actinide nuclei leading to the formation of superheavy elements the fusion probabilities are of several orders of magnitude higher than in the case of cold fusion reactions.

The systematic study of fission fragment yields under different initial conditions provides a valuable experimental benchmark for fission models that aim to understand this complex decay channel and to predict reaction product yields. Inverse kinematics coupled to the use of a high-resolution spectrometer is shown to be a powerful tool to identify and measure the inclusive isotopic yields of fission fragments. In-flight fusion-fission was used to produce secondary beams of neutron-rich isotopes in the collision of a ²³⁸U beam at 24 MeV/u with ⁹Be and ¹²C targets at GANIL using the LISE3 fragment-separator. Unique A,Z,q identification of fission products was attained with the dE-TKE-Bρ-ToF measurement technique. Mass, and atomic number distributions are reported for the two reactions that show the importance of different reaction mechanisms for these two targets.

The properties of the mass and energy distributions of fissionlike fragments formed in the reactions ⁴⁸Ca,⁵⁸Fe + ²⁰⁸Pb, ³⁶S,⁴⁸Ca,⁴⁸Ti,⁶⁴Ni + ²³⁸U, ⁴⁸Ca + ²³²Th,²⁴⁴Pu,²⁴⁸Cm at energies around the Coulomb barrier have been analyzed to define the systematic trend of compound nucleus fission and quasifission in cold and hot fusion reactions. The measurements have been carried out at the U400 cyclotron of the FLNR, JINR using the double-arm time-of-flight spectrometer CORSET. The fusion probabilities have been deduced from the analysis of mass and energy distributions. It was found that for the studied reactions fusion probability depends exponentially on mean fissility parameter of the system. For the reactions with actinide nuclei leading to the formation of superheavy elements the fusion probabilities are of several orders of magnitude higher than in the case of cold fusion reactions.

The reaction ³⁴S + ¹⁸⁶W at E_lab = 160 MeV was investigated with the aim of diving into the features of the fusion-fission process. Gamma rays coincident with binary reaction fragments were measured using the high efficiency gamma-ray spectrometer ORGAM at the TANDEM Accelerator facility of I.P.N., Orsay, and the time-of-flight spectrometer for fission fragments registration CORSET of the Flerov Laboratory of Nuclear Reactions (FLNR), Dubna. Evidence of symmetric and asymmetric fission modes were observed in the mass and TKE distributions, occurring due to shell effects in the fragments. The coupling of the ORGAM and CORSET setups enables the FF-γ coincident measurement which offers the opportunity to extract the isotopic distribution of the fragments of different masses formed in the aforementioned reaction and to find the exact neutron multiplicity, the average spin and average angular momenta. Details regarding the experimental setup, methods of processing the acquisitioned data and preliminary results are presented.