Microscopic approach to heavy-ion reactions

Heavy-ion reactions are affected by the nuclear structure of the reactants and other dynamical processes during the collision. Theoretical studies of such reactions lead to a clearer understanding of prohibitive mechanisms to heavy-ion fusion. One application of this is superheavy element formation, where quasifission is the major competing process to the formation of a compound nucleus in this region of the nuclear chart. Microscopic approaches that use mean field approximations, such as the time-dependent Hartree-Fock (TDHF) theory, have been increasingly used to study low energy heavy-ion reactions. In addition to reactions, nuclear structure properties can also be studied with microscopic theory. Such approaches do not require empirical knowledge of the nucleus and can be used as a predictive tool for studying structure or reactions involving exotic nuclei. In this thesis, TDHF is used to study nuclear vibrations and heavy-ion reactions at near barrier energies. The TDHF approach is applied first to a systematic study of low-lying octupole and quadrupole vibrational modes of {40-54}Ca isotopes. Then, fusion reactions are calcuated for Ca+Sn systems, using the barrier energy as the main observable. Fusion reactions are first calculated with no couplings (static HF calculations) before including all dynamics that the mean field approximation offers (TDHF calculations). The addition of dynamics often results in a lower fusion barrier compared to the static barrier, with the exception of systems involving the neutron rich {52,54}Ca projectiles. The difference in fusion barriers between the two sets of calculations are explored by considering couplings to vibrational states and transfer channels. To separate the effects of individual couplings, the coupled-channels approach is used with the HF calculations of the vibrational states as inputs. Such couplings account for most of the lowering of the static barrier. For the most neutron rich systems, coupling to vibrational states does not account for the dynamical barrier increase. Transfer channels, in particular proton pickup, are explored within TDHF and may play a role in this increased barrier energy. Following from fusion reactions, the factors and dynamics that may hinder fusion in heavier systems are studied in the reactions 48Ca, 50Ti, 52Cr, 54Fe, 56,64Ni, 58Zn+208Pb at energies ranging from 0% to 30% above the static HF barrier. Although most of these systems fuse at energies within this range, some systems reseparate for which emission of nucleons was apparent at scission point. The contact times were in most cases less than 10 zs and the attempt at $N/Z$ equilibration of these systems happen on fast time scales (1-2 zs). Larger amounts of mass transfer occur in systems with longer neck times and fewer magic numbers such as 34S+232Th and Cr+Pt systems that were also calculated for comparison. Further studies are required to better understand the role of shell effects as well as nucleon emission in quasifission reactions, and subsequently reactions leading to superheavy element formation

Optimal Number, Location, and Size of Distributed Generators in Distribution Systems by Symbiotic Organism Search Based Method

This paper proposes an approach based on the Symbiotic Organism Search (SOS) for optimal determining sizing, siting, and number of Distributed Generations (DG) in distribution systems. The objective of the problem is to minimize the power loss of the system subject to the equality and inequality constraints such as power balance, bus voltage limits, DG capacity limits, and DG penetration limit. The SOS approach is defined as the symbiotic relationship observed between two organisms in an ecosystem, which does not need the control parameters like other meta-heuristic algorithms in the literature. For the implementation of the proposed method to the problem, an integrated approach of Loss Sensitivity Factor (LSF) is used to determine the optimal location for installation of DG units, and SOS is used to find the optimal size of DG units. The proposed method has been tested on IEEE 33-bus, 69-bus, and 118-bus radial distribution systems. The obtained results from the SOS algorithm have been compared to those of other methods in the literature. The simulated results have demonstrated that the proposed SOS method has a very good performance and effectiveness for the problem of optimal placement of DG units in distribution systems

Structural features of geostrophic circulation in open region of the South China Sea

On the basis of observation data of water temperature and salinity the mean seasonal geostrophic circulation in open region of the South China Sea (SCS) was computed by the dynamic method relative to the 800 decibar reference surface. The results of computation let go to following notices: In both main monsoons (winter and summer) there are two main geostrophic eddies: the anticlockwise eddy in the northern and northwestern part, and the clockwise eddy in the southern part of the SCS with corresponding divergent and convergent zones. The main frontal zones go along the middle latitudes of the sea from the southern continental shelf of Vietnam to the area west of Luzon Island. The strength and stability of the current in winter are higher than in summer. The Kuroshio has an enough strong branch intruding into the SCS through Bashi Strait in winter creating in the sea the water structure similar to that of the Northwest Pacific subtropical and tropical regions. In summer the Kuroshio water can intrude directly only into the area southwest of Taiwan

Dynamical effects in fusion with exotic nuclei

[Background] Reactions with stable beams have demonstrated a strong interplay between nuclear structure and fusion. Exotic beam facilities open new perspectives to understand the impact of neutron skin, large isospin, and weak binding energies on fusion. Microscopic theories of fusion are required to guide future experiments. [Purpose] To investigate new effects of exotic structures and dynamics in near-barrier fusion with exotic nuclei. [Method] Microscopic approaches based on the Hartree-Fock (HF) mean-field theory are used for studying fusion barriers in $^{40-54}$Ca+$^{116}$Sn reactions for even isotopes. Bare potential barriers are obtained assuming frozen HF ground-state densities. Dynamical effects on the barrier are accounted for in time-dependent Hartree-Fock (TDHF) calculations of the collisions. Vibrational couplings are studied in the coupled-channel framework and near-barrier nucleon transfer is investigated with TDHF calculations. [Results] The development of a neutron skin in exotic calcium isotopes strongly lowers the bare potential barrier. However, this static effect is not apparent when dynamical effects are included. On the contrary, a fusion hindrance is observed in TDHF calculations with the most neutron rich calcium isotopes which cannot be explained by vibrational couplings. Transfer reactions are also important in these systems due to charge equilibration processes. [Conclusions] Despite its impact on the bare potential, the neutron skin is not seen as playing an important role in the fusion dynamics. However, the charge transfer with exotic projectiles could lead to an increase of the Coulomb repulsion between the fragments, suppressing fusion. The effect of transfer and dissipative mechanisms on fusion with exotic nuclei deserve further studies.The authors are grateful to M. Dasgupta, D. J. Hinde, and A. S. Umar for stimulating discussions during this work. This research was undertaken with the assistance of resources from the National Computational Infrastructure (NCI), which is supported by the Australian Government. This research was supported under Australian Research Council’s Future Fellowship (Project No. FT120100760), Discovery Projects (Project No. DP140101337), and Laureate Fellowship (Project No. FL110100098) funding schemes

The Current Adoption of Dry-Direct Seeding Rice (DDSR) in Thailand and Lessons Learned for Mekong River Delta of Vietnam

The paper documents the joint study trip, organized by CCAFS Southeast Asia for Vietnamese rice researchers, extension workers, as well as local decision makers, to visit Thailand in April 2018. The goal of the study trip was to observe and learn the experience of Thai farmers on the large-scale adoption process of dry-direct seeding rice (DDSR), a viable alternative to address regional scarcity of fresh water in irrigation caused by the drought and salinity intrusion in the Mekong River Delta

IMPROVING PROFITABILITY OF INTEGRATED RICE-SHRIMP FARMING IN BRACKISH AREA: A CASE STUDY OF MEKONG DELTA, VIETNAM

Rotation between rice and shrimp farming by way of filter ponds and diversification of farm activities was studied with the aims of testing the efficiency of filter ponds and evaluating the combination of incorporating the growing of upland-crops on dikes of rice fields. Three groups of farmers participated in study trials which were the pilot group used a filter pond and applied new methods, the control group had a filter pond and applied the recommended new methods, and the normal practice (control) group not having a filter pond. Results showed that the rice-shrimp farming system using a filter pond improved water quality (pH, alkalinity and salinity) and reduced input costs. Growing upland crops on the dikes had a high financial return; though for both vegetables and the grass for the dairy cows will strongly depend on the market. On one hectare of land, farmers using a filter pond for rice-shrimp farming combined with upland-crops had a higher economic return than the traditional rice-shrimp farming system (2,812 compared with 854 USD/ha/year). However, implementing this model requires farmers to build filter ponds to aid freshwater storage, proper management skills and family labour resources. Diversification of farm activities such as integrated rice-shrimp culture may be a strategy for farmers for adapting to the impacts of climate change such as extreme weather events, less rain and saltwater intrusion