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

Fragmentation of spherical radioactive heavy nuclei as a novel probe of transient effects in fission

Peripheral collisions with radioactive heavy-ion beams at relativistic energies are discussed as an innovative approach for probing the transient regime experienced by fissile systems evolving towards quasi-equilibrium. A dedicated experiment using the advanced technical installations of GSI, Darmstadt, permitted to realize ideal conditions for the investigation of relaxation effects in the meta-stable well. Combined with a highly sensitive experimental signature, it provides a measure of the transient effects with respect to the flux over the fission barrier. Within a two-step reaction process, 45 proton-rich unstable spherical isotopes produced by projectile-fragmentation of a stable 238U beam have been used as secondary projectiles. The fragmentation of the radioactive projectiles on lead results in nearly spherical compound nuclei which span a wide range in excitation energy and fissility. The decay of these excited systems by fission is studied with a dedicated set-up which permits the detection of both fission products in coincidence and the determination of their atomic numbers with high resolution. The width of the fission-fragment nuclear charge distribution is shown to be specifically sensitive to pre-saddle transient effects and is used to establish a clock for the passage of the saddle point. The comparison of the experimental results with model calculations points to a fission delay of (3.3+/-0.7).10-21s for initially spherical compound nuclei, independent of excitation energy and fissility. This value suggests a nuclear dissipation strength at small deformation of (4.5+/-0.5).1021s-1. The very specific combination of the physics and technical equipment exploited in this work sheds light on previous controversial conclusions.Comment: 38 pages, 15 figure

On current ambiguity in the interpretation of fission at intermediate excitation energy

AbstractVarious approaches are currently used to interpret experimental data on fission. We critically examine a wide set of observables measured for fission of Po206,210 nuclei at medium excitation energy, and illustrate the ambiguity in current analysis. Dynamical calculations based on the four-dimensional Langevin equation using a macroscopic potential energy landscape are performed, and found to consistently describe available measurements. This observation calls into question the robustness of recent analysis based on statistical-model calculations and concluding, on the contrary, to substantial shell effects at the fission saddle point in Po206,210. The inconsistency in interpretation reached by the two approaches shows that, depending on the system, the conclusion can be strongly model-dependent. Although this may not be surprising, it emphasizes the today still limited reliability of firmly extracting fundamental nuclear properties from customary analysis

Sequential fissions of heavy nuclear systems

In Xe+Sn central collisions from 12 to 20 MeV/A measured with the INDRA 4$\pi$ multidetector, the three-fragment exit channel occurs with a significant cross section. In this contribution, we show that these fragments arise from two successive binary splittings of a heavy composite system. Strong Coulomb proximity effects are observed in the three-fragment final state. By comparison with Coulomb trajectory calculations, we show that the time scale between the consecutive break-ups decreases with increasing bombarding energy, becoming compatible with quasi-simultaneous multifragmentation above 18 MeV/A.Comment: 6 pages, 5 figures, contribution to conference proceedings of the Fifth International Workshop on Nuclear fission and Fission-Product Spectroscop

Composition, seasonal and interannual dynamics of zooplankton in Peter the Great Bay (Japan Sea)

Species structure, abundance and biomass of zooplankton, its seasonal and interannual dynamics are considered on the data of plankton surveys conducted in Peter the Great Bay in 2002-2013. All zooplankton species dwelling in the northwestern Japan Sea are presented in the fauna of Peter the Great Bay. Small-sized fraction of zooplankton prevais everywhere in the Bay (> 80 % of total number of animals). The zooplankton concentration in spring-summer (1280 mg/m3 on average) is in 1.4 times higher than in fall-winter. Both abundance and biomass of zooplankton are higher in the neritic zone in all seasons: the mean annual values are 53.17 thousand ind./m3 and 1370 mg/m3, respectively. The highest values are observed in the secondary bays, as the Amur Bay and Ussuri Bay (1455 mg/m3 on average). The Amur Bay is distinguished by high abundance of meroplankton, as well, in particular in its northern shallow area; its annual mean value is 3.65 thousand ind./m3 (twice higher than in the Ussuri Bay), the peaks are observed in summer and autumn. Abundance and biomass of zooplankton decrease gradually with the distance from the coast to the minimum values in the deep-water zone: 3.43 thousand ind./m3 and 485 mg/m3 on average, respectively. The total stock of zooplankton in Peter the Great Bay is estimated as 428. 103 t in spring and 636. 103 t in summer. Both seasonal and interannual dynamics of zooplankton abundance are more significant in the coastal areas, as the Amur and Ussuri Bays, where the changes are either similar (as in 2008, 2010-2013) or not (as in 2007, 2009). For example, low abundance of P. newmani but extremally high abundance of warm-water copepods (10 times higher than usually) and Bivalvia larvae were recorded in both bays in 2013. Fluctuations of large-sized oceanic species abundance have no essential impact on the total biomass of zooplankton in Peter the Great Bay. In general, the zooplankton abundance increased in 1.2-2.5 times during the decade, up to the maximum in 2010-2012, mainly because of Pseudocalanus newmani stock growth, but this species abundance decreased sharply in 2013 that caused 20 % reduction of the total zooplankton biomass

Mechanisms of subtropical plankton transport into the coastal waters of southern Primorye, a case of <i>Paracalanus parvus</i>

Results of long-term studies of zooplankton species in the Amur Bay (Japan Sea) are analyzed. Two seasonal «waves» of the allochtonous species are defined: the first in the early summer that is formed mainly by inter-zonal copepods spawning in the deep layers ( Metridia pacifica , Neocalanus plumchrus, Calanus glacialis ) and the second in the late summer that is represented by warm-water copepods of subtropical origin ( Calanus pacificus and Paracalanus parvus ) obviously transported from the southern Japan Sea by currents. The latter species is able to dominate in the zooplankton community of coastal waters in the early-autumn season. Variations of this mass species abundance in the Amur Bay under changes of wind-driven upwelling/downwelling circulation are investigated using an empirical advective model of its cross-shelf transport. There is found that strong summer southern on-shore winds (summer monsoon) are favourable for its high abundance but in these conditions it is transported toward the coast by the deep compensatory flow developed only after the summer monsoon change to the winter monsoon with opposite direction; that’s why the high abundance is observed in autumn, in September-October. In the years with weak summer monsoon the species is transported to the bay in mass by the surface on-shore wind-driven flow earlier, in August, but its number decreases after the monsoon change, so the maximum of abundance is not high. Patterns of the transport by cross-shelf currents depend supposedly on the depth of plankton concentration. Recent strengthening of summer monsoon causes heightening of P. parvus abundance in the coastal waters at southern Primorye, up to extreme high values in 2013, but climatic trend of the summer monsoon intensity is negative, so decreasing of this species transport toward Primorye coast and its abundance in the coastal waters, as the Amur Bay, can be expected in the nearest future