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Search Results: 1 - 10 of 461790 matches for " A. Aprahamian "
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Status of eel fisheries, stocks and their management in England and Wales
M. Aprahamian,A. Walker
Knowledge and Management of Aquatic Ecosystems , 2008,
Abstract: All continental life stages of eel are exploited in England and Wales. The main fisheries for glass eel are by estuarine dip-nets in the southwest. The main fisheries for yellow and silver eel occur in southern and eastern England, with fyke nets being the preferred instrument. Fishing effort is not directly recorded but is inferred from licence sales. Around 1100 glass eel licenses were sold annually from 1980 to 1994, increasing to around 2500 in 1998, but declined to about 800 per annum since 2001. Declared catches of glass eel have been below 1–2 t since 2001, compared to 10–70 t in the 1970s and 1980s. Licence sales for yellow and silver eel fisheries (combined) varied from around 1100 to 2900 over the period 1983–2007, peaking in the mid-1980s, mid-1990s and again in 2005–2007. Declared catches peaked in the late 1980s and mid-1990s (peak 280 t), but have been low since 2001 (mean 29 t). Nett export data suggests catches may be 4 times higher. Eel landings are reported as bycatch from various marine fisheries around the UK coasts, with landings from 2001 to 2007 ranging from 0.2 to 13.7 t per annum. Assessment of stock status is based mainly on catch and catch per unit effort data as there has been little fishery independent survey of eels. There has been a general decreasing trend in both glass eel catches reported to the Agency and in nett export data. Comparing maximum catch levels in the late 1970s–early 1980s with minimum levels in the 2000s suggests that the catch has declined by around 75–95%. Trends in CPUE are similar, at least until 1998. It is concluded that current glass eel recruitment to the western coast of the UK is approximately 30% of the pre-1980 level of recruitment. Yellow and silver eel indices derived from HMRC nett exports or reported catches per licence sold, both suggest that the current estimate of stocks derived from these data are 20% those of the late 1980s and mid 1990s. The Reference Condition Model has been used to assess compliance with the EC target (EU regulation 1100/2007). Assessment is at the River Basin District (RBD) level and in most cases a single river has been used to represent the whole RBD. The assessment therefore has a low level of confidence attached to it, and work to improve this is outlined in the paper. A number of options to increase silver eel escapement are presented, specifically reducing fishing pressure, improving access and habitat quality, reducing the impacts of entrainment, controlling predators and stocking, with the interplay of options varying between rivers and basins depending on
The impact of global nuclear mass model uncertainties on $r$-process abundance predictions
M. Mumpower,R. Surman,A. Aprahamian
Physics , 2014,
Abstract: Rapid neutron capture or `$r$-process' nucleosynthesis may be responsible for half the production of heavy elements above iron on the periodic table. Masses are one of the most important nuclear physics ingredients that go into calculations of $r$-process nucleosynthesis as they enter into the calculations of reaction rates, decay rates, branching ratios and Q-values. We explore the impact of uncertainties in three nuclear mass models on $r$-process abundances by performing global monte carlo simulations. We show that root-mean-square (rms) errors of current mass models are large so that current $r$-process predictions are insufficient in predicting features found in solar residuals and in $r$-process enhanced metal poor stars. We conclude that the reduction of global rms errors below $100$ keV will allow for more robust $r$-process predictions.
Sensitivity of the r-process to nuclear masses
S. Brett,I. Bentley,N. Paul,R. Surman,A. Aprahamian
Physics , 2012, DOI: 10.1140/epja/i2012-12184-4
Abstract: The rapid neutron capture process (r-process) is thought to be responsible for the creation of more than half of all elements beyond iron. The scientific challenges to understanding the origin of the heavy elements beyond iron lie in both the uncertainties associated with astrophysical conditions that are needed to allow an r-process to occur and a vast lack of knowledge about the properties of nuclei far from stability. There is great global competition to access and measure the most exotic nuclei that existing facilities can reach, while simultaneously building new, more powerful accelerators to make even more exotic nuclei. This work is an attempt to determine the most crucial nuclear masses to measure using an r-process simulation code and several mass models (FRDM, Duflo-Zuker, and HFB-21). The most important nuclear masses to measure are determined by the changes in the resulting r-process abundances. Nuclei around the closed shells near N=50, 82, and 126 have the largest impact on r-process abundances irrespective of the mass models used.
The sensitivity of r-process nucleosynthesis to the properties of neutron-rich nuclei
R. Surman,M. Mumpower,J. Cass,A. Aprahamian
Physics , 2013,
Abstract: About half of the heavy elements in the Solar System were created by rapid neutron capture, or r-process, nucleosynthesis. In the r-process, heavy elements are built up via a sequence of neutron captures and beta decays in which an intense neutron flux pushes material out towards the neutron drip line. The nuclear network simulations used to test potential astrophysical scenarios for the r-process therefore require nuclear physics data (masses, beta decay lifetimes, neutron capture rates, fission probabilities) for thousands of nuclei far from stability. Only a small fraction of this data has been experimentally measured. Here we discuss recent sensitivity studies that aim to determine the nuclei whose properties are most crucial for r-process calculations.
The impact of individual nuclear properties on $r$-process nucleosynthesis
M. R. Mumpower,R. Surman,G. C. McLaughlin,A. Aprahamian
Physics , 2015,
Abstract: The astrophysical rapid neutron capture process or `$r$ process' of nucleosynthesis is believed to be responsible for the production of approximately half the heavy element abundances found in nature. This multifaceted problem remains one of the greatest open challenges in all of physics. Knowledge of nuclear physics properties such as masses, $\beta$-decay and neutron capture rates, as well as $\beta$-delayed neutron emission probabilities are critical inputs that go into calculations of $r$-process nucleosynthesis. While properties of nuclei near stability have been established, much still remains unknown regarding neutron-rich nuclei far from stability that may participate in the $r$ process. Sensitivity studies gauge the astrophysical response of a change in nuclear physics input(s) which allows for the isolation of the most important nuclear properties that shape the final abundances observed in nature. This review summarizes the extent of recent sensitivity studies and highlights how these studies play a key role in facilitating new insight into the $r$ process. The development of these tools promotes a focused effort for state-of-the-art measurements, motivates construction of new facilities and will ultimately move the community towards addressing the grand challenge of `How were the elements from iron to uranium made?'.
In-Band and Inter-Band B(E2) Values within the Triaxial Projected Shell Model
P. Boutachkov,A. Aprahamian,Y. Sun,J. A. Sheikh,S. Frauendorf
Physics , 2002,
Abstract: The Triaxial Projected Shell Model (TPSM) has been successful in providing a microscopic description of the energies of multi-phonon vibrational bands in deformed nuclei. We report here on an extension of the TPSM to allow, for the first time, calculations of B(E2) values connecting gamma- and gamma-gamma-vibrational bands and the ground state band. The method is applied to 166,168Er. It is shown that most of the existing B(E2) data can be reproduced rather well, thus strongly supporting the classification of these states as gamma-vibrational states. However, significant differences between the data and the calculation are seen in those B(E2) values which involve odd-spin states of the gamma-band. Understanding these discrepancies requires accurate experimental measurements and perhaps further improvements of the TPSM.
Low energy 0+ excitations in 158Gd
J.G. Hirsch,G. Popa,S.R. Lesher,A. Aprahamian
Revista mexicana de física , 2006,
Abstract: Estudios de alta precisión (p,t) del núcleo deformado 158Gd han permitido la observación de 13 estados excitados 0+ a una energía abajo de 3.1 MeV. Esta alta densidad de estados de baja energía y particularmente la medida de sus transiciones B (E2) al primer estado 2+ representa un desafío para cualquier modelo nuclear. Se utiliza el modelo pseudo SU (3), que describe adecuadamente un gran número de bandas excitadas en los isótopos de Dy, para estudiar el 158Gd. Encontramos que el modelo describe adecuadamente la mayoria de los estados, sin embargo, es posible que el hecho de excluir a los nucleones ocupando estados de paridad intrusa sea la principal limitante del modelo.
The impact of nuclear masses near $N=82$ on $r$-process abundances
M. Mumpower,D. -L. Fang,R. Surman,M. Beard,A. Aprahamian
Physics , 2014,
Abstract: We have performed for the first time a complete $r$-process mass sensitivity study in the $N=82$ region. We take into account how an uncertainty in a single nuclear mass propagates to influence important quantities of neighboring nuclei, including Q-values and reaction rates. We demonstrate that nuclear mass uncertainties of $\pm0.5$ MeV in the $N=82$ region result in up to an order of magnitude local change in $r$-process abundance predictions. We identify key nuclei in the study whose mass has a substantial impact on final $r$-process abundances and could be measured at future radioactive beam facilities.
Nuclear masses near $N=82$ that influence $r$-process abundances
M. Mumpower,R. Surman,M. Beard,D. -L. Fang,A. Aprahamian
Physics , 2014,
Abstract: Nuclear masses are one of the key ingredients of nuclear physics that go into astrophysical simulations of the $r$ process. Nuclear masses effect $r$-process abundances by entering into calculations of Q-values, neutron capture rates, photo-dissociation rates, beta-decay rates, branching ratios and the properties of fission. Most of the thousands of short-lived neutron-rich nuclei which are believed to participate in the $r$ process lack any experimental verification, thus the identification of the most influential nuclei is of paramount importance. We have conducted mass sensitivity studies near the $N=82$ closed shell in the context of a main $r$-process. Our studies take into account how an uncertainty in a single nuclear mass propagates to influence the relevant quantities of neighboring nuclei and finally to $r$-process abundances. We identify influential nuclei in various astrophysical conditions using the FRDM mass model. We show that our conclusions regarding these key nuclei are still retained when a superposition of astrophysical trajectories is considered.
The impact of individual nuclear masses on $r$-process abundances
M. R. Mumpower,R. Surman,D. -L. Fang,M. Beard,P. Moller,T. Kawano,A. Aprahamian
Physics , 2015, DOI: 10.1103/PhysRevC.92.035807
Abstract: We have performed for the first time a comprehensive study of the sensitivity of $r$-process nucleosynthesis to individual nuclear masses across the chart of nuclides. Using the latest version (2012) of the Finite-Range Droplet Model, we consider mass variations of $\pm0.5$ MeV and propagate each mass change to all affected quantities, including $Q$-values, reaction rates, and branching ratios. We find such mass variations can result in up to an order of magnitude local change in the final abundance pattern produced in an $r$-process simulation. We identify key nuclei whose masses have a substantial impact on abundance predictions for hot, cold, and neutron star merger $r$-process scenarios and could be measured at future radioactive beam facilities.
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