<p>Познавање параметара нуклеарне структуре, попут шеме нивоа, функциjе густине стања и функциjе jачине прелаза су неопходне за разумевање<br />нуклеарних особина атомског jезгра. Ове величине су директно повезане са ефикасним пресеком, коjи jе битна физичка величина у нуклеарноj физици, као и теориjски са моделима jезгра. Прикупљање тачних и прецизних података о овим физичким величинама jе jедан од наjважниjих задатака у&nbsp; нискоенергетскоj нуклеарноj физици.У овом раду за одређивање ових величина коришћен jе метод двоструких гама каскада коjи се заснива на&nbsp; коинциденционоj детекциjи гама кваната коjи настаjу након захвата термалних неутрона на jезгрима мете. Циљ експеримента jе детекциjа два узастопна гама кванта коjи се емитуjу када се jезг-ро након захвата неутрона деексцитуjе са енергиjе захвата на основно или неко од нископобуђених стања.Jезгра коjа су испитивана у овом раду су⁹⁴Nb и⁵⁶Mn, а експерименти су извршени на BRR истра-живачком реактору у Будимпешти и FRM II истраживачком реактору на Техничком универзитету у Минхену, у Хаjнц Меjер-Лаjбниц центру, тим редом. Прикупљени експериментални подаци су анализирани и добиjене су информациjе о шеми нивоа и емитованих гама кваната приликом чега jе пронађен велики броj нових гама прелаза унутар ових jезгара (210 за ⁹⁴Nb, 50 за⁵⁶ Mn) , као и нових енергетских нивоа у овим jезгрима (29 за ⁹⁴ Nb, 24 за<br />⁵⁶ Mn). За детектоване каскаде израчунати су и интензитети. Након одређивања интензитета кас-када, за одређивање функциjе густине нивоа и jа-чине прелаза за<br />⁹⁴ Nb и ⁵⁶ Mn коришћен jе практични модел за гама распад неутронских резонанци, развиjен на Обjедињеном институту за нуклеарна истраживања у Дубни, Руска федерациjа. Такође, у овом раду jе одрађена и систематизациjа података за 43 jезгра коjа су раниjе испитивана овом методом и представљени су закључци добиjени овом систематизациjом.</p> / <p>Poznavanje parametara nuklearne strukture, poput šeme nivoa, funkcije gustine stanja i funkcije jačine prelaza su neophodne za razumevanje<br />nuklearnih osobina atomskog jezgra. Ove veličine su direktno povezane sa efikasnim presekom, koji je bitna fizička veličina u nuklearnoj fizici, kao i teorijski sa modelima jezgra. Prikupljanje tačnih i preciznih podataka o ovim fizičkim veličinama je jedan od najvažnijih zadataka u&nbsp; niskoenergetskoj nuklearnoj fizici.U ovom radu za određivanje ovih veličina korišćen je metod dvostrukih gama kaskada koji se zasniva na&nbsp; koincidencionoj detekciji gama kvanata koji nastaju nakon zahvata termalnih neutrona na jezgrima mete. Cilj eksperimenta je detekcija dva uzastopna gama kvanta koji se emituju kada se jezg-ro nakon zahvata neutrona deekscituje sa energije zahvata na osnovno ili neko od niskopobuđenih stanja.Jezgra koja su ispitivana u ovom radu su⁹⁴Nb i⁵⁶Mn, a eksperimenti su izvršeni na BRR istra-živačkom reaktoru u Budimpešti i FRM II istraživačkom reaktoru na Tehničkom univerzitetu u Minhenu, u Hajnc Mejer-Lajbnic centru, tim redom. Prikupljeni eksperimentalni podaci su analizirani i dobijene su informacije o šemi nivoa i emitovanih gama kvanata prilikom čega je pronađen veliki broj novih gama prelaza unutar ovih jezgara (210 za ⁹⁴Nb, 50 za⁵⁶ Mn) , kao i novih energetskih nivoa u ovim jezgrima (29 za ⁹⁴ Nb, 24 za<br />⁵⁶ Mn). Za detektovane kaskade izračunati su i intenziteti. Nakon određivanja intenziteta kas-kada, za određivanje funkcije gustine nivoa i ja-čine prelaza za<br />⁹⁴ Nb i ⁵⁶ Mn korišćen je praktični model za gama raspad neutronskih rezonanci, razvijen na Objedinjenom institutu za nuklearna istraživanja u Dubni, Ruska federacija. Takođe, u ovom radu je odrađena i sistematizacija podataka za 43 jezgra koja su ranije ispitivana ovom metodom i predstavljeni su zaključci dobijeni ovom sistematizacijom.</p> / <p>Knowledge of nuclear parameters, such as level scheme, level density function and radiative strength function is important in order to understand nuclear properties. These parameters are directly related to the value of the cross section, an important quantity in nuclear physics, and also, theoretically to nuclear models. Acquisition of precise and high quality data on these parameters is one of the most important<br />tasks in low-energy nuclear physics. To determine these parameters, in this thesis the method of the two step gamma cascade was used. This method is based on coincidence detection of gamma rays that are emitted after thermal neutron capture&nbsp; on the nucleus. The goal of the experiment is to record two consecutive gamma rays &nbsp; that are emitted as nucleus decays from the capture energy to the ground state<br />or some of the low-excited states.In this work, 94 Nb and 56 Mn nuclei were investigated and the experiments were carried out at the BRR research reactor at the&nbsp; PGAA facility of the Centre for Energy Research (MTA EK), Budapest, Hungary and FRM II research reactor at the Technische Universit&uml;at M&uml;unchen, Heinz Maier- Leibnitz&nbsp; Zentrum (MLZ), Garching, Germany, respectively. Collected data were analyzed and information about level scheme and emitted gamma rays were obtained. The analysis showed that a large number of new gamma transitions, as well as new energy levels (29 for 94 Nb, 24 for 56 Mn) was detected for these nuclei (210 for 94 Nb, 50 for 56 Mn). The intensities were also calculated for all recorded cascades. After calculating the intensities of cascades, to determine the level density function and radiative strength function the practical model of neutron resonances gamma decay developed at JINR, Dubna, Russia, was used. Also, in this thesis, the systematization of results obtained for 43 nuclei previously measured by this method,&nbsp; are presented.</p>

-

Cette thèse aborde la problématique de la durée de vie des électrons dans des puits quantiques unipolaires soumis à un fort champ magnétique selon l'axe de confinement. Plus particulièrement, le blocage de l'émission de phonons optiques longitudinaux en régime de confinement des électrons à 0-dimension est clairement démontré par la mise en évidence d'oscillations quantiques de la durée de vie. Cet effet est baptisé « résonance magnétophonon inter sous-bandes ». Le blocage de l'émission de phonons LO a été détecté sur la mesure de l'intensité de lasers à cascades quantiques GaAs/GaAlAs, émettant dans le moyen infra rouge autour de 10 µm, soumis à un champ balayé jusqu'à 60T. Le champ magnétique confine les électrons sur des orbites cyclotron, et transforme la dispersion quadratique de l'énergie des sous-bandes en niveaux de Landau discrets. Les oscillations de magnétophonons ont été également trouvées sur la magnétorésistance de la structure. La résonance magnétophonon inter sous-bande réalise une spectroscopie directe des modes de phonons pertinents pour la relaxation inter sous-bande. Entre autres, une contribution des modes de phonon LO de AlAs a été observée tandis que l'intervention attendue de modes confinés n'est pas apparue. La durée de vie inter sous-bandes en fonction du champ magnétique, obtenue par une méthode proposée, est comparée aux prévisions récentes d'un modèle de couplage fort électron-phonon (magnéto polarons). En conclusion, ces travaux ont dégagé des résultats et un nouvel outil performant de spectroscopie de phonons et de diagnostic des interactions électron-phonon dans un système à zéro dimension, analogue à un système de boites quantiques.

<p>Seagrasses constitute the basis for diverse and productive ecosystems worldwide. In East Africa, they provide important ecosystem services (e.g. fisheries) but are potentially threatened by increasing resource use and lack of enforced management regulations. The major aim of this PhD thesis was to investigate effects of anthropogenic distur-bances, primarily seaweed farming and coastal fishery, in East African seagrass beds. Seaweed farming, often depicted as a sustainable form of aquaculture, had short- and long-term effects on seagrass growth and abundance that cascaded up through the food web to the level of fishery catches. The coastal fishery, a major subsistence activity in the region, can by removing urchin predators indirectly increase densities of the sea urchin <i>Tripneustes gratilla</i>, which has overgrazed seagrasses in several areas. A study using simulated grazing showed that high magnitude leaf removal – typical of grazing urchins – affected seagrasses more than low magnitude removal, typical of fish grazing. Different responses in two co-occurring seagrass species furthermore indicate that high seagrass diversity in tropical seagrass beds could buffer overgrazing effects in the long run. Finally, a literature synthesis suggests that anthropogenic disturbances could drive shifts in seagrass ecosystems to an array of alternative regimes dominated by other or-ganisms (macroalgae, bivalves, burrowing shrimp, polychaetes, etc.). The formation of novel feedback mechanisms makes these regimes resilient to disturbances like seagrass recovery and transplantation projects. Overall, this suggests that resource use activities linked to seagrasses can have large-scale implications if the scale exceeds critical levels. This emphasizes the need for holistic and adaptive management at the seascape level, specifically involving improved techniques for seaweed farming and fisheries, protection of keystone species, and ecosystem-based management approaches.</p>

Dans les aciers des cuves des réacteurs nucléaires, l'irradiation neutronique engendre une densité numérique élevée (10^23 à 10^24 m-3) d'amas de solutés de quelques nanomètres de diamètre. Ces amas sont constitués d'un élément en sursaturation dans la matrice ferritique (Cu) ainsi que d'éléments solubles (Mn, Ni, Si, P) à la température d'irradiation (environ 300°C). Les travaux décrits dans ce mémoire ont pour objectifs de déterminer le phénomène responsable de la formation de ces amas (cascades de déplacements, précipitation homogène ou hétérogène) et d'obtenir des informations sur l'effet des différents solutés. La microstructure d'alliages modèles, après différentes expériences d'irradiation aux ions et aux électrons, est caractérisée par sonde atomique. La confrontation des résultats expérimentaux avec ceux obtenus à l'aide d'un modèle en dynamique d'amas calculant l'évolution de la population de défauts ponctuels lors des irradiations indique que la précipitation des amas de solutés est hétérogène, sur les amas de défauts ponctuels. La cinétique de précipitation est ralentie par les solutés autres que le cuivre.

<p>The abundance of predators and their impact on ecosystem dynamics is a vividly discussed topic in current ecology. In my studies, incorporating field observations, field experiments and theoretical modeling, I explored the importance of predators and predation in a low arctic tundra ecosystem in northern Norway. This involved observing the abundance and spatial activity of predators (raptors and small mustelids); manipulating the abundance of predators (spiders and birds) in an arthropod community; and exploring the theoretical consequences of intraguild predation on the coexistence among predators.</p><p>The results show that predation is important both in the arthropod assemblage and, depending on the productivity of the community, in the vertebrate assemblage. In arthropod communities predators are at least as abundant as their prey, whereas in the vertebrate part of ecosystem, predators are substantially less abundant than their prey. Still, in both cases predators had strong impact on their prey, influencing the abundance of prey and the species composition of prey assemblages. The impact of predation cascaded to the plant community both in the reticulate and complex arthropod food web and in the linear food chain-like vertebrate community. In the vertebrate-based community we could even observe the long time scale effect on plant community composition.</p><p>Within the predator community, exploitation competition and intraguild predation were the structuring forces. As the arthropod communities consist of predators with different sizes, intraguild predation is an energetically important interaction for top predators. As a consequence, they reduce the abundance of intermediate predators and the impact of intermediate predators on other prey groups. Moreover, being supported by intermediate predators, top predators can have stronger impact themselves on other prey groups.</p><p>In vertebrate communities, intraguild predation seems to be unimportant as energetic link, instead it manifests as an extreme version of interference competition. Therefore intraguild predation reduces the likelihood of coexistence, as it is due limited prey diversity and intense exploitative competition already precarious in the low arctic tundra.</p><p>In conclusion, predators have strong impact on their prey, especially in the more productive parts of the low arctic tundra. This applies even to the food webs with complex and reticulate structure, and these effects carry through the community both in the short time scale of population growth and on the long time scale of population generations.</p>

The abundance of predators and their impact on ecosystem dynamics is a vividly discussed topic in current ecology. In my studies, incorporating field observations, field experiments and theoretical modeling, I explored the importance of predators and predation in a low arctic tundra ecosystem in northern Norway. This involved observing the abundance and spatial activity of predators (raptors and small mustelids); manipulating the abundance of predators (spiders and birds) in an arthropod community; and exploring the theoretical consequences of intraguild predation on the coexistence among predators. The results show that predation is important both in the arthropod assemblage and, depending on the productivity of the community, in the vertebrate assemblage. In arthropod communities predators are at least as abundant as their prey, whereas in the vertebrate part of ecosystem, predators are substantially less abundant than their prey. Still, in both cases predators had strong impact on their prey, influencing the abundance of prey and the species composition of prey assemblages. The impact of predation cascaded to the plant community both in the reticulate and complex arthropod food web and in the linear food chain-like vertebrate community. In the vertebrate-based community we could even observe the long time scale effect on plant community composition. Within the predator community, exploitation competition and intraguild predation were the structuring forces. As the arthropod communities consist of predators with different sizes, intraguild predation is an energetically important interaction for top predators. As a consequence, they reduce the abundance of intermediate predators and the impact of intermediate predators on other prey groups. Moreover, being supported by intermediate predators, top predators can have stronger impact themselves on other prey groups. In vertebrate communities, intraguild predation seems to be unimportant as energetic link, instead it manifests as an extreme version of interference competition. Therefore intraguild predation reduces the likelihood of coexistence, as it is due limited prey diversity and intense exploitative competition already precarious in the low arctic tundra. In conclusion, predators have strong impact on their prey, especially in the more productive parts of the low arctic tundra. This applies even to the food webs with complex and reticulate structure, and these effects carry through the community both in the short time scale of population growth and on the long time scale of population generations.

Seagrasses constitute the basis for diverse and productive ecosystems worldwide. In East Africa, they provide important ecosystem services (e.g. fisheries) but are potentially threatened by increasing resource use and lack of enforced management regulations. The major aim of this PhD thesis was to investigate effects of anthropogenic distur-bances, primarily seaweed farming and coastal fishery, in East African seagrass beds. Seaweed farming, often depicted as a sustainable form of aquaculture, had short- and long-term effects on seagrass growth and abundance that cascaded up through the food web to the level of fishery catches. The coastal fishery, a major subsistence activity in the region, can by removing urchin predators indirectly increase densities of the sea urchin Tripneustes gratilla, which has overgrazed seagrasses in several areas. A study using simulated grazing showed that high magnitude leaf removal – typical of grazing urchins – affected seagrasses more than low magnitude removal, typical of fish grazing. Different responses in two co-occurring seagrass species furthermore indicate that high seagrass diversity in tropical seagrass beds could buffer overgrazing effects in the long run. Finally, a literature synthesis suggests that anthropogenic disturbances could drive shifts in seagrass ecosystems to an array of alternative regimes dominated by other or-ganisms (macroalgae, bivalves, burrowing shrimp, polychaetes, etc.). The formation of novel feedback mechanisms makes these regimes resilient to disturbances like seagrass recovery and transplantation projects. Overall, this suggests that resource use activities linked to seagrasses can have large-scale implications if the scale exceeds critical levels. This emphasizes the need for holistic and adaptive management at the seascape level, specifically involving improved techniques for seaweed farming and fisheries, protection of keystone species, and ecosystem-based management approaches.

The development flow region of stepped chutes was studied experimentally. Three configuration of chute bed slopes 3.5H:1V, 5H:1V, and 10H:1V were used to study the flow characteristics. Each model had five horizontal steps and with constant step height of 15 cm. Constant temperature anemometry was used to investigate the velocity field characteristics as well as local void fraction. Pressure transducers were used to examine the pressure distribution. The conditions of aerated and non-aerated cavity were studied. It was found that the temperature anemometry is a valuable tool in the study of water flow problems due to its good spatial and temporal resolution. It is recommended that the constant overheat ratio procedure should be used in dealing with non-isothermal water flows. Flow conditions along the development flow region were found to be quite complex with abrupt changes between steps depending whether or not the flow jet has disintegrated. The flow on this region does not resemble a drop structure and after the first step, the step cavity condition does not affect the flow parameters. Pressure distribution was also found to be complex. It was found that there are no conclusive pressure profiles either on the step treads nor on step risers. No correlation was observed with the values of pool depth. The instantaneous characteristics of the velocity field along the jet of a drop structure were also studied. It was concluded that the cavity condition does not affect the velocity field of the sliding jet. The shear stress layer at the jet/pool interface was quantified. / May 2006

Recent research has focused on the non-lethal effects of predator intimidation and fear, dubbed non-consumptive effects (NCEs), in which prey actively change their behavior and habitat use in response to predator chemical cues. Although NCEs can have large impacts on community structure, many studies have ignored differences in predator population structure and properties of the natural environment that may modify the magnitude and importance of NCEs. Here, I investigated the roles of predator size and density (i.e. biomass), as well as habitat characteristics, on predator risk assessment and the magnitude of consumptive and NCEs using blue crabs, mud crabs, and oyster prey as a model system. Predation experiments between blue crabs and mud crabs demonstrated that blue crabs consume mud crabs; however, the consumptive effects were dependent upon blue crab body size and habitat type. When mud crabs were exposed to chemical cues from differing biomasses of blue crabs in laboratory mesocosms, mud crab activity and predation on oysters was decreased in response to high biomass treatments (i.e. large and multiple small blue crabs), but not to low biomass predators (i.e single small blue crab), suggesting that risk associated with predator size is perceptible via chemical cues and is based on predator biomass. Further experiments showed that the perception of risk and the magnitude of the NCEs were affected by the sensory cues available and the diet of the blue crab predator. The NCE based on blue crab biomass was also demonstrated in the field where water flow can disperse cues necessary for propagating NCEs. Properties of water flow were measured within the experimental design and during the experiment and confirmed cage environments were representative of natural conditions and that patterns in NCEs were not associated with flow characteristics. These results affect species conservation and commercial fisheries management and demonstrate that we cannot successfully predict NCEs without considering predator size structure and the contexts under which we determine predator risk.