Variation of neutron yield from a titanium-tritide target during deuterium beam bombardment.

In the laboratory simulation of D-T fusion breeder blankets 14 MeV neutrons are produced by the bombardment of a titanium-tritide target with deuterium ions using accelerating voltages up to 500 keV and beam currents ranging from micro to milliamperes. For the accurate determination of tritium breeding ratios in the experimental assemblies an absolute determination of the total neutron yield over the irradiation period is required. The theoretical and experimental methods used to determine the ion composition of the deuterium beam the changing absolute yields and energy distributions of the neutrons emitted from the target during prolonged irradiation are described using the AAEC 14 MeV neutron generator as a typical example. Analysis of the measured data identified two ion species in the beam of the neutron generator. It was shown that after a 21-hour irradiation of the target with a 250 mu A beam (18.5C) at 200 kV the neutron output from the D-T reaction dropped from an initial value of 2 x 10 1 0 to 4 x 10 8 neutrons per second. The integrated neutron output over this period was estimated to be 2.05 x 10 14 of which about 24 per cent originated from the interaction of monatomic ions and 75 per cent from diatomic ions; less than one per cent arose from D-D reaction

Accelerated discovery of two crystal structure types in a complex inorganic phase field

The discovery of new materials is hampered by the lack of efficient approaches to the exploration of both the large number of possible elemental compositions for such materials, and of the candidate structures at each composition1. For example, the discovery of inorganic extended solid structures has relied on knowledge of crystal chemistry coupled with time-consuming materials synthesis with systematically varied elemental ratios2,3. Computational methods have been developed to guide synthesis by predicting structures at specific compositions4,5,6 and predicting compositions for known crystal structures7,8, with notable successes9,10. However, the challenge of finding qualitatively new, experimentally realizable compounds, with crystal structures where the unit cell and the atom positions within it differ from known structures, remains for compositionally complex systems. Many valuable properties arise from substitution into known crystal structures, but materials discovery using this approach alone risks both missing best-in-class performance and attempting design with incomplete knowledge8,11. Here we report the experimental discovery of two structure types by computational identification of the region of a complex inorganic phase field that contains them. This is achieved by computing probe structures that capture the chemical and structural diversity of the system and whose energies can be ranked against combinations of currently known materials. Subsequent experimental exploration of the lowest-energy regions of the computed phase diagram affords two materials with previously unreported crystal structures featuring unusual structural motifs. This approach will accelerate the systematic discovery of new materials in complex compositional spaces by efficiently guiding synthesis and enhancing the predictive power of the computational tools through expansion of the knowledge base underpinning them

Global Diversity of Brittle Stars (Echinodermata: Ophiuroidea)

This review presents a comprehensive overview of the current status regarding the global diversity of the echinoderm class Ophiuroidea, focussing on taxonomy and distribution patterns, with brief introduction to their anatomy, biology, phylogeny, and palaeontological history. A glossary of terms is provided. Species names and taxonomic decisions have been extracted from the literature and compiled in The World Ophiuroidea Database, part of the World Register of Marine Species (WoRMS). Ophiuroidea, with 2064 known species, are the largest class of Echinodermata. A table presents 16 families with numbers of genera and species. The largest are Amphiuridae (467), Ophiuridae (344 species) and Ophiacanthidae (319 species). A biogeographic analysis for all world oceans and all accepted species was performed, based on published distribution records. Approximately similar numbers of species were recorded from the shelf (n = 1313) and bathyal depth strata (1297). The Indo-Pacific region had the highest species richness overall (825 species) and at all depths. Adjacent regions were also relatively species rich, including the North Pacific (398), South Pacific (355) and Indian (316) due to the presence of many Indo-Pacific species that partially extended into these regions. A secondary region of enhanced species richness was found in the West Atlantic (335). Regions of relatively low species richness include the Arctic (73 species), East Atlantic (118), South America (124) and Antarctic (126)

Wounding induces dedifferentiation of epidermal Gata6+ cells and acquisition of stem cell properties

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Tritium breeding experiments in a fusion blanket assembly using a low-intensity neutron generator.

Experiments have been carried out to determine the accuracy with which tritium production rates (TPRs) can be measured in a fusion blanket assembly of non-spherical geometry by a non-central low intensity D-T neutron source. The tritium production was determined for samples of lithium carbonate containing high enrichments of 6 Li(96%) and 7 Li(99.9%). The measured data were used to check the accuracy with which the TPRs could be numerically predicted using current nuclear data and calculational methods. The numerical predictions from tritium production from the 7 Li samples agreed within the experimental errors of the measurements but 6 Li measurements which differ by more than 20 per cent from the predicted values which were observed in the lower half of the assembly