Concept of Formation Length in Radiation Theory

The features of electromagnetic processes are considered which connected with finite size of space region in which final particles (photon, electron-positron pair) are formed. The longitudinal dimension of the region is known as the formation length. If some external agent is acting on an electron while traveling this distance the emission process can be disrupted. There are different agents: multiple scattering of projectile, polarization of a medium, action of external fields, etc. The theory of radiation under influence of the multiple scattering, the Landau-Pomeranchuk-Migdal (LPM) effect, is presented. The probability of radiation is calculated with an accuracy up to "next to leading logarithm" and with the Coulomb corrections taken into account. The integral characteristics of bremsstrahlung are given, it is shown that the effective radiation length increases due to the LPM effect at high energy. The LPM effect for pair creation is also presented. The multiple scattering influences also on radiative corrections in a medium (and an external field too) including the anomalous magnetic moment of an electron and the polarization tensor as well as coherent scattering of a photon in a Coulomb field. The polarization of a medium alters the radiation probability in soft part of spectrum. Specific features of radiation from a target of finite thickness include: the boundary photon emission, interference effects for thin target, multi-photon radiation. The experimental study of LPM effect is described. For electron-positron colliding beams following items are discussed: the separation of coherent and incoherent mechanisms of radiation, the beam-size effect in bremsstrahlung, coherent radiation and mechanisms of electron-positron creation.Comment: Revised review paper, 96 pages, 28 figures. Description of SLAC E-146 experiment removed, discussion of CERN SPS experiment adde

Bipolar distribution of deep-sea benthic foraminifera

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New and little-known Komokiacea (Foraminifera) from the bathyal and abyssal Weddell Sea and adjacent areas

Based on samples from the south-east Atlantic and Southern Ocean collected during the ANDEEP III campaign we describe three new species, Ipoa pennata sp. nov., Septuma stellata sp. nov. and Skeletonia variabilis gen. et sp. nov., of the enigmatic deep-sea foraminiferan superfamily Komokiacea. A further six species, Ipoa fragila Tendal & Hessler, 1977, Komokia multiramosa Tendal & Hessler, 1977, Normanina conferta (Norman, 1878), Septuma ocotillo Tendal & Hessler, 1977, S. brachyramosa Kamenskaya, 1993, and S. komokiformis Kamenskaya 1993, are redescribed. Together, these nine species occurred at 14 stations across the depth range 1549–4935 m. Normanina conferta was found at 11 stations (1579–4935 m); S. ocotillo (4526–4935 m), S. brachyramosa (1819–4730 m) and S. stellata (2603–4934 m) at six stations each; I. fragila (4649–4934 m) at five stations; K. multiramosa (4700–4935) and S. variabilis (4696–4932 m) at four stations each; I. pennata (4803–4934 m) at three stations and S. komokiformis (3103–4526 m) at two stations. Five species occur in both the North Atlantic Ocean and the Southern Ocean, suggesting that close faunal links exist between these areas. Three were first described from the North Pacific Ocean while others, including the three new species, are so far known only from the Southern Ocean

The biodiversity and biogeography of komokiaceans and other enigmatic foraminiferan-like protists in the deep Southern Ocean

We present a survey of komokiaceans and other relatively large, stercomata-bearing testate protists, presumed to be foraminifera, based on extensive ship-board sorting of samples collected at 13 sites (depth range 1820–4930 m) in the Weddell Sea and two sites in the SE Atlantic (Cape and Aguilas basins, north of the Antarctic Convergence) during the ANDEEP III expedition. Thirty-nine species occurred in the Weddell Sea and a further 11 in the SE Atlantic basins. Of these 50 species, 35 are undescribed. We assign, with a greater or lesser degree of certainty, 26 and 13 species to the komokiacean families Komokiidae and Baculellidae, respectively, and another 2 to the Komokiacea incertae sedis. We include in the Baculellidae an undescribed species in which very fine hair-like fibres, similar to those seen in some species currently included in this family, arise from the segments that make up the chain-like test. A further 11 chain-like species lack these fibres and we therefore exclude them from the Komokiacea. A final group of species includes a mixture of different forms, some of which exhibit komokiacean-like features. These assemblages were most diverse at abyssal sites in the central Weddell Sea (27–30 species per site). Above 4000 m, 1–8 species were present at individual sites and only two species, Normanina conferta and Septuma brachyramosa, occurred at depths <2000 m. One of these, N. conferta, was the most widely distributed species, occurring at 11 stations south of the Antarctic convergence as well as the Aguilas Basin. Many (31–61%) of the Southern Ocean ANDEEP species are recognised in the North Atlantic and 6 were previously described from the central North Pacific. Our results suggest that some komokiacean and chain-like species are widely distributed at abyssal depths in the oceans. They also support other evidence that many undescribed komokiacean species exist and highlight some of the difficulties involved in defining the morphological limits of this difficult taxon

Biodiversity and distribution of the genus Gromia (Protista, Rhizaria) in the deep Weddell Sea (Southern Ocean)

We present a survey of morphospecies of Gromia, a genus of testate protists, from bathyal and abyssal depths in the Weddell Sea and adjacent areas of the Southern Ocean. This material represents the most extensive and diverse available collection of deep-sea gromiids so far recorded. The twelve species, nine of which are undescribed, are recognized on the basis of morphological criteria, including the test size and shape, the appearance and structure of the oral capsule, and the characteristics of the test wall. Most species have a single oral capsule, which is circular in plan view with a conical nipple-like shape in lateral view. One morphospecies has three oral capsules. The appearance and structure of the wall displays great variability among Gromia species, ranging from very delicate and transparent with highly reflective highlights to relatively thick with distinct patterns of ridges covering the surface. More often, however, differences in wall structure are more subtle. Most morphospecies were distributed at bathyal depths along the continental margin, but one was sampled at »4,800 m, representing the first record of an abyssal gromiid. Concurrent with findings from other regions of the World’s oceans, the Weddell Sea gromiids were mostly found in surficial sediments in areas of elevated organic input, suggesting that deep-sea gromiids are likely to play an important role in carbon cycling in bathyal eutrophic regions through the ingestion and degradation of fresh organic matter

Monothalamous foraminiferans and gromiids (Protista) from western Svalbard: a preliminary survey

Monothalamous foraminifera were abundant in sediment samples from fjords and offshore areas around western Svalbard (water depth range 26–2472 m). The &gt;500 µm fractions of samples from the inner parts of fjords yielded numerous delicate “allogromiids” (organic-walled) “saccamminids” and “psammosphaerids” (agglutinated), including species assigned to Cylindrogullmia, Gloiogullmia, Nemogullmia and Toxisarcon. Larger, more robust, tubular agglutinated species were abundant in the outer reaches of Tempelfjord, Isfjord and Van Meijenfjord (Hyperammina subnodosa, Hippocrepinella crassa), on the current-influenced upper slope off Isfjord (Pelosina variabilis, Rhabdammina abyssorum), and the deeper part of the continental slope off Isfjord (e.g. Hyperammina crassatina). Oval and sausage-shaped organisms resembling gromiids (probably relatives of the foraminifera) were sometimes abundant in the fjords. Finer size fractions (63–500 µm) of fjord samples yielded a rich variety of monothalamous species. Among the allogromiids, Micrometula sp. was widely distributed, while Tinogullmia sp. and an undescribed species were restricted to single stations in Kongsfjord and Van Meijenfjord, respectively. Saccamminids common in the finer fractions included Conqueria spp., Psammophaga sp., and undescribed species with silvery, white and brownish tests. Many of the smaller allogromiids and saccamminids in our Svalbard samples resemble species found in the Gullmarfjord on the Swedish west coast.<br/

Bipolar gene flow in deep-sea benthic foraminifera

Despite its often featureless appearance, the deep-ocean floor includes some of the most diverse habitats on Earth. However, the accurate assessment of global deep-sea diversity is impeded by a paucity of data on the geographical ranges of bottom-dwelling species, particularly at the genetic level. Here, we present molecular evidence for exceptionally wide distribution of benthic foraminifera, which constitute the major part of deep-sea meiofauna. Our analyses of nuclear ribosomal RNA genes revealed high genetic similarity between Arctic and Antarctic populations of three common deep-sea foraminiferal species (Epistominella exigua, Cibicides wuellerstorfi and Oridorsalis umbonatus), separated by distances of up to 17 000 km. Our results contrast with the substantial level of cryptic diversity usually revealed by molecular studies, of shallow-water benthic and planktonic marine organisms. The very broad ranges of the deep-sea foraminifera that we examined support the hypothesis of global distribution of small eukaryotes and suggest that deep-sea biodiversity may be more modest at global scales than present estimates suggest

Widespread occurrence of nitrate storage and denitrification among Foraminifera and Gromiida

Benthic foraminifers inhabit a wide range of aquatic environments including open marine, brackish, and freshwater environments. Here we show that several different and diverse foraminiferal groups (miliolids, rotaliids, textulariids) and Gromia, another taxon also belonging to Rhizaria, accumulate and respire nitrates through denitrification. The widespread occurrence among distantly related organisms suggests an ancient origin of the trait. The diverse metabolic capacity of these organisms, which enables them to respire with oxygen and nitrate and to sustain respiratory activity even when electron acceptors are absent from the environment, may be one of the reasons for their successful colonization of diverse marine sediment environments. The contribution of eukaryotes to the removal of fixed nitrogen by respiration may equal the importance of bacterial denitrification in ocean sediments