Fission dynamics of intermediate-fissility systems: a study within a stochastic three-dimensional approach

The system of intermediate fissility 132Ce has been studied experimentally and theoretically to investigate the dissipation properties of nuclear matter. Cross sections of fusion-fission and evaporation-residue channels together with light charged particle multiplicities in both channels, their spectra, light charged particle-evaporation residue angular correlations, and mass-energy distribution of fission fragments have been measured. Theoretical analysis has been performed using a multidimensional stochastic approach coupled with a Hauser-Feshbach treatment of particle evaporation. The main conclusions are that the full one-body shape-dependent dissipation mechanism allows the reproduction of the full set of experimental data and that after a time τd=5×10−21 s from the equilibrium configuration of the compound nucleus, fission decay can occur in a time that can span several orders of magnitude

Evaporation Channel as a Tool to Study Fission Dynamics

The dynamics of the fission process is expected to affect the evaporation residue cross section because of the fission hindrance due to the nuclear viscosity. Systems of intermediate fissility constitute a suitable environment for testing such hypothesis, since they are characterized by evaporation residue cross sections comparable or larger than the fission ones. Observables related to emitted charged particle, due to their relatively high emission probability, can be used to put stringent constraints on models describing the excited nucleus decay and to recognize the effects of fission dynamics. In this work model simulations are compared with the experimental data collected via the ^{32}S + ^{100}Mo reaction at E_{lab}= 200 MeV. By comparing an extended set of evaporation channel observables the limits of the statistical model and the large improvement coming by using a dynamical model are evidenced. The importance of using a large angular covering apparatus to extract the observable is stressed. The opportunity to measure more sensitive observables by a new detection device in operation at LNL are also discussed.Comment: v1: 7 pages, 6 figure

Clustering effects in 48Cr composite nuclei produced via 24Mg + 24Mg reaction

In the framework of studying clustering effects in N=Z light nuclei, an experiment was carried out to get information on the properties of the 48 Cr composite nuclei produced via the 24 Mg +24 Mg reaction. In particular, the study regards the 48 Cr at 60 MeV of excitation energy where a resonance with a narrow width (170 KeV) has been found by measuring the elastic and anelastic channels. To determine the deformation of this state, evaporative Light Charged Particles (LCP) are measured and compared to the Statistical Model (SM) predictions, which are very sensitive to nuclear deformation. The experiment was performed at LNL using the 8ĒLP apparatus to select LCPs and a Parallel Plate Avalanche Counter (PPAC) system to detect the Evaporation Residues (ER). Preliminary results on the measurements of ER – LCP and LCP – LCP angular correlations are presented which indicate the presence of a very large deformation.The analysis will proceed with the extraction of the LCP energy spectra and with the angular correlations for high multiplicity channels to determine the deformation

Fission Dynamics: The Quest of a Temperature Dependent Nuclear Viscosity

oai:ojs2.jnp.chitkara.edu.in:article/2This paper presents a journey within some open questions about the current use of a temperature dependent nuclear viscosity in models of nuclear fission and proposes an alternative experimental approach by using systems of intermediate fissility. This study is particularly relevant because: i) systems of intermediate fissility offer a suitable frame-work since the intervals between the compound nucleus and scission point temperatures with increasing excitation energy are much smaller than in the case of heavier systems, ii) the dependence of viscosity on the temperature may change with the fissility of the composite system; iii) the opportunity to measure also observables in the evaporation residues channel translates into a larger set of effective constraints for the models

clustering effects in 48cr composite nucleus produced via the reaction 24mg 24mg at the excitation energy of 60 mev

The reaction 24Mg + 24Mg was used to produce the composite nucleus 48Cr at 60 MeV of excitation energy where a narrow resonance (170 KeV) has been found by measuring the elastic and inelastic channels. To determine the occurrence of deformation of this compound nucleus and its possible connection with the resonances and the hypothetical cluster structure, evaporative Light Charged Particles (LCP) were measured and compared to Statistical Model (SM) predictions. The experiment was performed at LNL using the 8πLP apparatus. The comparison of the evaporation residue-LCP coincidence angular distributions and LCP energy spectra with SM calculations supports the presence of a very large deformation of the composite nucleus 48Cr that scales with the angular momentum

Evaporation and fission decay of (132)Ce compound nuclei at E(x)=122 MeV: some limitations of the statistical model

Light charged particle (LCP) emission in the evaporation residue (ER) and fusion fission (FF) channels have been studied for the 200 MeV 32S + 100Mo reaction, leading to 132Ce composite nuclei at E x =122 MeV. The main goal was to study the decay of 132 Ce on the basis of an extended set of observables, to get insights on the fission dynamics. The proton and alpha particle energy spectra, their multiplicities, ER-LCP angular correlations, ER and FF angular distributions, and ER and FF cross-sections were measured. The measured observables were compared with the Statistical Model (SM). Using standard parameters, the model was able to reproduce only the pre-scission multiplicities and the FF and ER cross-sections. The calculation was observed to strongly overestimate the proton and alpha particle multiplicities in the ER channel. Disagreements were also observed for the ER-LCP correlations, the LCP energy spectra and the ER angular distribution. By varying the SM input parameters over a wide range of values, it is shown that it is not possible to reproduce all the observables simultaneously with a unique set of parameters. The inadequacy of the model in reproducing the ER particle multiplicities is also observed analysing data from the literature for other systems in the A ≈ 150 and E x ≈ 100−200 MeV region. These results indicate serious limitations about the use of the SM in extracting information on fission dynamics

Study of the threshold anomaly effect in the reaction 7 Li+ 208 Pb at energies around the Coulomb barrier

The elastic scattering in the reaction7Li+208Pb was investigated in the bombarding energy range from 25 to39 MeV. The real and imaginary parts of the optical potential were analyzed by using a phenomenological potential. A dispersion relation analysis is presented in order to investigate the threshold anomaly effect. It is concluded that7Li has an intermediate behavior between the tightly bound nuclei suchas16O and the loosely bound nuclei such as6Li where the lack of the threshold anomaly is unambiguously observed.Reaction cross sections are also extracted from the elastic scattering data and its comparison with the ones of other systems has been performed to draw hints on the effect of the breakup channel

High intensity neutrino oscillation facilities in Europe

The EUROnu project has studied three possible options for future, high intensity neutrino oscillation facilities in Europe. The first is a Super Beam, in which the neutrinos come from the decay of pions created by bombarding targets with a 4 MW proton beam from the CERN High Power Superconducting Proton Linac. The far detector for this facility is the 500 kt MEMPHYS water Cherenkov, located in the Fréjus tunnel. The second facility is the Neutrino Factory, in which the neutrinos come from the decay of μ+ and μ− beams in a storage ring. The far detector in this case is a 100 kt magnetized iron neutrino detector at a baseline of 2000 km. The third option is a Beta Beam, in which the neutrinos come from the decay of beta emitting isotopes, in particular He6 and Ne18, also stored in a ring. The far detector is also the MEMPHYS detector in the Fréjus tunnel. EUROnu has undertaken conceptual designs of these facilities and studied the performance of the detectors. Based on this, it has determined the physics reach of each facility, in particular for the measurement of CP violation in the lepton sector, and estimated the cost of construction. These have demonstrated that the best facility to build is the Neutrino Factory. However, if a powerful proton driver is constructed for another purpose or if the MEMPHYS detector is built for astroparticle physics, the Super Beam also becomes very attractive

Binary fragmentations of excited nuclear systems in the 372 MeV Fe-56+Th-232 reaction

The binary fragmentation of the excited superheavy system of Z=116 formed in the reaction 372 MeV Fe-56+Th-232 has been investigated. The fragment masses and kinetic energies were determined through measurement of the fragment velocities by the time-of-flight method. Neutron spectra were measured in coincidence with the fragments at several angles with respect to the fragment direction, which were analyzed to deduce the total as well as the pre-scission neutron multiplicities. We analyzed the correlations between fragment mass and kinetic energy, gated and nongated by the coincidence with neutrons, to learn about the dynamics of the reaction with respect to the two-body exit channels. The events in the near-mass-symmetric valley region appear to receive a significant contribution from the asymmetric mode of fission of the superheavy compound nucleus Z=116, which can be due to the influence of the closed proton and neutron shells of Z=50 and N=82 in the light fragments. From the observed number of pre-scission neutrons, it is inferred that the time scales of the fissionlike reactions leading to near-mass-symmetric splits are rather large, of the order of several times 10(-20) s. The average number of prompt neutrons emitted in the spontaneous fission of such a superheavy nucleus is nu=(12 +/- 1), as deduced from the neutron measurements