3,987 research outputs found
Microscopic Enhancement of Heavy-Element Production
Realistic fusion barriers are calculated in a macroscopic-microscopic model
for several soft-fusion heavy-ion reactions leading to heavy and superheavy
elements. The results obtained in such a realistic picture are very different
from those obtained in a purely macroscopic model. For reactions on 208:Pb
targets, shell effects in the entrance channel result in fusion-barrier
energies at the touching point that are only a few MeV higher than the ground
state for compound systems near Z = 110. The entrance-channel fragment-shell
effects remain far inside the touching point, almost to configurations only
slightly more elongated than the ground-state configuration, where the fusion
barrier has risen to about 10 MeV above the ground-state energy. Calculated
single-particle level diagrams show that few level crossings occur until the
peak in the fusion barrier very close to the ground-state shape is reached,
which indicates that dissipation is negligible until very late in the fusion
process. Whereas the fission valley in a macroscopic picture is several tens of
MeV lower in energy than is the fusion valley, we find in the
macroscopic-microscopic picture that the fission valley is only about 5 MeV
lower than the fusion valley for soft-fusion reactions leading to compound
systems near Z = 110. These results show that no significant
``extra-extra-push'' energy is needed to bring the system inside the fission
saddle point and that the typical reaction energies for maximum cross section
in heavy-element synthesis correspond to only a few MeV above the maximum in
the fusion barrier.Comment: 7 pages. LaTeX. Submitted to Zeitschrift fur Physik A. 5 figures not
included here. Complete preprint, including device-independent (dvi),
PostScript, and LaTeX versions of the text, plus PostScript files of the
figures, available at http://t2.lanl.gov/publications/publications.html or at
ftp://t2.lanl.gov/pub/publications/mehe
Composite Fermions in Negative Effective Magnetic Field: A Monte-Carlo Study
The method of Jain and Kamilla [PRB {\bf 55}, R4895 (1997)] allows numerical
generation of composite fermion trial wavefunctions for large numbers of
electrons in high magnetic fields at filling fractions of the form nu=p/(2mp+1)
with m and p positive integers. In the current paper we generalize this method
to the case where the composite fermions are in an effective (mean) field with
opposite sign from the actual physical field, i.e. when p is negative. We
examine both the ground state energies and the low energy neutral excitation
spectra of these states. Using particle-hole symmetry we can confirm the
correctness of our method by comparing results for the series m=1 with p>0
(previously calculated by others) to our results for the conjugate series m=1
with p <0. Finally, we present similar results for ground state energies and
low energy neutral excitations for the states with m=2 and p <0 which were not
previously addressable, comparing our results to the m=1 case and the p > 0,
m=2 cases.Comment: 11 page
Simultaneous Quantitative MRI Mapping of T1, T2* and Magnetic Susceptibility with Multi-Echo MP2RAGE.
The knowledge of relaxation times is essential for understanding the biophysical mechanisms underlying contrast in magnetic resonance imaging. Quantitative experiments, while offering major advantages in terms of reproducibility, may benefit from simultaneous acquisitions. In this work, we demonstrate the possibility of simultaneously recording relaxation-time and susceptibility maps with a prototype Multi-Echo (ME) Magnetization-Prepared 2 RApid Gradient Echoes (MP2RAGE) sequence. T1 maps can be obtained using the MP2RAGE sequence, which is relatively insensitive to inhomogeneities of the radio-frequency transmit field, [Formula: see text]. As an extension, multiple gradient echoes can be acquired in each of the MP2RAGE readout blocks, which permits the calculation of [Formula: see text] and susceptibility maps. We used computer simulations to explore the effects of the parameters on the precision and accuracy of the mapping. In vivo parameter maps up to 0.6 mm nominal resolution were acquired at 7 T in 19 healthy volunteers. Voxel-by-voxel correlations and the test-retest reproducibility were used to assess the reliability of the results. When using optimized paramenters, T1 maps obtained with ME-MP2RAGE and standard MP2RAGE showed excellent agreement for the whole range of values found in brain tissues. Simultaneously obtained [Formula: see text] and susceptibility maps were of comparable quality as Fast Low-Angle SHot (FLASH) results. The acquisition times were more favorable for the ME-MP2RAGE (â 19 min) sequence as opposed to the sum of MP2RAGE (â 12 min) and FLASH (â 10 min) acquisitions. Without relevant sacrifice in accuracy, precision or flexibility, the multi-echo version may yield advantages in terms of reduced acquisition time and intrinsic co-registration, provided that an appropriate optimization of the acquisition parameters is performed
Deconvolutionâbased distortion correction of EPI using analytic singleâvoxel pointâspread functions
Purpose To develop a postprocessing algorithm that corrects geometric distortions due to spatial variations of the static magnetic field amplitude, B0, and effects from relaxation during signal acquisition in EPI. Theory and Methods An analytic, complex pointâspread function is deduced for kâspace trajectories of EPI variants and applied to corresponding acquisitions in a resolution phantom and in human volunteers at 3 T. With the analytic pointâspread function and experimental maps of B0 (and, optionally, the effective transverse relaxation time, urn:x-wiley:07403194:media:mrm28591:mrm28591-math-0004) as input, a pointâspread function matrix operator is devised for distortion correction by a Thikonovâregularized deconvolution in image space. The pointâspread function operator provides additional information for an appropriate correction of the signal intensity distribution. A previous image combination algorithm for acquisitions with opposite phase blip polarities is adapted to the proposed method to recover destructively interfering signal contributions. Results Applications of the proposed deconvolutionâbased distortion correction (âDecoDisCoâ) algorithm demonstrate excellent distortion corrections and superior performance regarding the recovery of an undistorted intensity distribution in comparison to a multifrequency reconstruction. Examples include full and partial Fourier standard EPI scans as well as doubleâshot centerâout trajectories. Compared with other distortionâcorrection approaches, DecoDisCo permits additional deblurring to obtain sharper images in cases of significant urn:x-wiley:07403194:media:mrm28591:mrm28591-math-0005 effects. Conclusion Robust distortion corrections in EPI acquisitions are feasible with high quality by regularized deconvolution with an analytic pointâspread function. The general algorithm, which is publicly released on GitHub, can be straightforwardly adapted for specific EPI variants or other acquisition schemes
Changes in r-process abundances at late times
We explore changes in abundance patterns that occur late in the r process. As
the neutrons available for capture begin to disappear, a quasiequilibrium
funnel shifts material into the large peaks at A=130 and A=195, and into the
rare-earth "bump" at A=160. A bit later, after the free-neutron abundance has
dropped and beta-decay has begun to compete seriously with neutron capture, the
peaks can widen. The degree of widening depends largely on neutron-capture
rates near closed neutron shells and relatively close to stability. We identify
particular nuclei the capture rates of which should be examined experimentally,
perhaps at a radioactive beam facility.Comment: 8 pages, 14 figures included in tex
Entrada de nutrientes no solo pela ågua da chuva, e contribuição no processo da fitomassa aérea de capoeiras de diferente idades.
A entrada de nutrientes pela ĂĄgua da chuva em capoeiras com cinco e onze anos de idade. no nordeste paraense, foi avaliada em duas Ă©pocas do ano. A entrada dos nutrientes foi decomposta em: quantidades carreadas pela ĂĄgua da chuva (lNC), interceptação pela vegetação secundĂĄria. "throughfall"(THF), "stemflow''(Sf') e a lixiviação {(THF+SF)-( 1.25xINC)} As anĂĄlises quĂmicas foram obtidas pelos mĂ©todos usuais do Institut IĂŒr Bodenkunde und WaldcmĂŁrung. A chuva interna bruta mostrou um acrĂ©scimo nas quantidades de K, Na, Ca, Mg, e Si nas duas ĂĄreas estudadas, mesmo levando em consideração a concentração relativa causada pela interceptação da fitomassa. O incremento foi atribuĂdo Ă liberação de nutrientes pelas folhas. Ă s deposiçÔes de poeiras e excrementos da fauna entomolĂłgica. Da mesma forma, o incremento de S-So.4" e de nitrogĂȘnio orgĂąnico, responsĂĄvel pelo teor deste elemento no sistema, foi atribuĂdo Ă s perdas da vegetação. Com exceção do potĂĄssio em uma das Ă©pocas de coleta e do sĂłdio. os demais elementos analisados evidenciam maior incremento na chuva interna da capoeira com onze anos mostrando a importĂąncia da fitomassa no sistema. A maior quantidade de sĂłdio lixiviado na capoeira com cinco anos foi atribuĂda a algumas espĂ©cies encontradas na ĂĄrea.Editado por J. E. PĂ©faur
Evidence for magnetic clusters in NiV close to the quantum critical concentration
The d-metal alloy NiV undergoes a quantum phase transition from
a ferromagnetic ground state to a paramagnetic ground state as the vanadium
concentration is increased. We present magnetization, ac-susceptibility and
muon-spin relaxation data at several vanadium concentrations near the critical
concentration at which the onset of ferromagnetic order is
suppressed to zero temperature. Below , the muon data reveal a broad
magnetic field distribution indicative of long-range ordered ferromagnetic
state with spatial disorder. We show evidence of magnetic clusters in the
ferromagnetic phase and close to the phase boundary in this disordered
itinerant system as an important generic ingredient of a disordered quantum
phase transition. In contrast, the temperature dependence of the magnetic
susceptibility above is best described in terms of a magnetic quantum
Griffiths phase with a power-law distribution of fluctuation rates of dynamic
magnetic clusters. At the lowest temperatures, the onset of a short-range
ordered cluster-glass phase is recognized by an increase in the muon
depolarization in transverse fields and maxima in ac-susceptibility.Comment: 6 pages, 5 figures, submitted to Proceedings of SCES 201
Multi-echo investigations of positive and negative CBF and concomitant BOLD changes: Positive and negative CBF and BOLD changes
Unlike the positive blood oxygenation level-dependent (BOLD) response (PBR), commonly taken as an indication of an âactivatedâ brain region, the physiological origin of negative BOLD signal changes (i.e. a negative BOLD response, NBR), also referred to as âdeactivationâ is still being debated. In this work, an attempt was made to gain a better understanding of the underlying mechanism by obtaining a comprehensive measure of the contributing cerebral blood flow (CBF) and its relationship to the NBR in the human visual cortex, in comparison to a simultaneously induced PBR in surrounding visual regions. To overcome the low signal-to-noise ratio (SNR) of CBF measurements, a newly developed multi-echo version of a center-out echo planar-imaging (EPI) readout was employed with pseudo-continuous arterial spin labeling (pCASL). It achieved very short echo and inter-echo times and facilitated a simultaneous detection of functional CBF and BOLD changes at 3 T with improved sensitivity. Evaluations of the absolute and relative changes of CBF and the effective transverse relaxation rate, , the coupling ratios, and their dependence on CBF at rest, , indicated differences between activated and deactivated regions. Analysis of the shape of the respective functional responses also revealed faster negative responses with more pronounced post-stimulus transients. Resulting differences in the flow-metabolism coupling ratios were further examined for potential distinctions in the underlying neuronal contributions
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