843 research outputs found
Novel insights into transfer processes in the reaction 16O+208Pb at sub-barrier energies
The collision of the doubly-magic nuclei O+Pb is a benchmark
in nuclear reaction studies. Our new measurements of back-scattered
projectile-like fragments at sub-barrier energies show show that transfer of 2
protons () is much more probable than -particle transfer.
transfer probabilities are strongly enhanced compared to expectations for the
sequential transfer of two uncorrelated protons; at energies around the fusion
barrier absolute probabilities for two proton transfer are similar to those for
one proton transfer. This strong enhancement indicates strong pairing
correlations in O, and suggests evidence for the occurrence of a nuclear
supercurrent of two-proton Cooper pairs in this reaction, already at energies
well below the fusion barrier.Comment: 5 pages, 3 figure
Effects of Nuclear Structure on Quasi-fission
The quasi-fission mechanism hinders fusion of heavy systems because of a mass
flow between the reactants, leading to a re-separation of more symmetric
fragments in the exit channel. A good understanding of the competition between
fusion and quasi-fission mechanisms is expected to be of great help to optimize
the formation and study of heavy and superheavy nuclei. Quantum microscopic
models, such as the time-dependent Hartree-Fock approach, allow for a treatment
of all degrees of freedom associated to the dynamics of each nucleon. This
provides a description of the complex reaction mechanisms, such as
quasi-fission, with no parameter adjusted on reaction mechanisms. In
particular, the role of the deformation and orientation of a heavy target, as
well as the entrance channel magicity and isospin are investigated with
theoretical and experimental approaches.Comment: Invited talk to NSRT12. To be published in Eur. Phys. J. Web of Con
The whole and its parts : why and how to disentangle plant communities and synusiae in vegetation classification
Most plant communities consist of different structural and ecological subsets, ranging from cryptogams to different tree layers. The completeness and approach with which these subsets are sampled have implications for vegetation classification. Nonâvascular plants are often omitted or sometimes treated separately, referring to their assemblages as âsynusiaeâ (e.g. epiphytes on bark, saxicolous species on rocks). The distinction of complete plant communities (phytocoenoses or holocoenoses) from their parts (synusiae or merocoenoses) is crucial to avoid logical problems and inconsistencies of the resulting classification systems. We here describe theoretical differences between the phytocoenosis as a whole and its parts, and outline consequences of this distinction for practise and terminology in vegetation classification. To implement a clearer separation, we call for modifications of the International Code of Phytosociological Nomenclature and the EuroVegChecklist. We believe that these steps will make vegetation classification systems better applicable and raise the recognition of the importance of nonâvascular plants in the vegetation as well as their interplay with vascular plants
Increased gravitational force reveals the mechanical, resonant nature of physiological tremor
Human physiological hand tremor has a resonant component. Proof of this is that its frequency can be modified by adding mass. However, adding mass also increases the load which must be supported. The necessary force requires muscular contraction which will change motor output and is likely to increase limb stiffness. The increased stiffness will partly offset the effect of the increased mass and this can lead to the erroneous conclusion that factors other than resonance are involved in determining tremor frequency. Using a human centrifuge to increase head-to-foot gravitational field strength, we were able to control for the increased effort by increasing force without changing mass. This revealed that the peak frequency of human hand tremor is 99% predictable on the basis of a resonant mechanism. We ask what, if anything, the peak frequency of physiological tremor can reveal about the operation of the nervous system.This work was funded by a BBSRC Industry Interchange Award
to J.P.R.S. and R.F.R. C.J.O. was funded by BBSRC grant
BB/I00579X/1. C.A.V. was funded by AâMidex (Aix-Marseille
Initiative of Excellence
Meta-Analyse programmatischer-prÀventiver Förderung der emotionalen und sozialen Entwicklung in der Primarstufe
Im Gegensatz zu klinischen Einrichtungen ist die Schule fĂŒr alle Kinder und Jugendliche zugĂ€nglich, sodass sie das wichtigste Setting fĂŒr prĂ€ventive MaĂnahmen im Bereich der emotionalen und sozialen Entwicklung ist. Internationale Studien bestĂ€tigen die Wirksamkeit schulischer prĂ€ventiver Förderung. Bei der Ăbertragung dieser Befunde auf Schulen in Deutschland besteht jedoch das Problem, dass die Wirkung von PrĂ€ventionsprogrammen nicht beliebig interkulturell ĂŒbertragbar ist. Eine ĂberprĂŒfung fĂŒr das deutsche Schulsystem ist vor diesem Hintergrund indiziert. Ziel dieser Arbeit ist es, die Wirksamkeit manualisierter, deutschsprachiger PrĂ€ventionsprogramme fĂŒr den Einsatz in der Schule meta-analytisch zu erfassen, ĂŒberblicksartig darzustellen und den Einfluss spezifischer Moderatoren auf die Programmeffekte zu ĂŒberprĂŒfen. Der Vergleich der Effekte der unterschiedlichen Studien erfolgt mittels Cohens d und der anschlieĂenden Berechnung der mittleren gewichteten EffektstĂ€rke nach Wilson. Entlang der inhaltlichen und methodischen Einschlusskriterien konnten insgesamt zwölf Programme fĂŒr die Primarstufe identifiziert werden. Zu diesen liegen zusammen 13 Studien mit insgesamt 169 Gruppenvergleichen vor. Im PrĂ€-Post-Vergleich liegt die durchschnittliche EffektstĂ€rke bei dw=0.15. FĂŒr die PrĂ€-Follow-up Analysen ergibt sich dw=0.23. Zudem konnten relevante EinflĂŒsse der Erhebungsmethode (r=.22) und der PrĂ€ventionsebene (r=.39) festgestellt werden. Auf Programmebene lassen sich die schrittweise Erarbeitung eines sozialen Problemlösezirkels (r=.31) und die Anzahl der Einheiten (r=.33) als relevante Einflussfaktoren fĂŒr die Wirksamkeit herausstellen. Implikationen fĂŒr das Fach emotionale und soziale Entwicklung werden diskutiert. (DIPF/Orig.
Influence of entrance-channel magicity and isospin on quasi-fission
The role of spherical quantum shells in the competition between fusion and
quasi-fission is studied for reactions forming heavy elements. Measurements of
fission fragment mass distributions for different reactions leading to similar
compound nuclei have been made near the fusion barrier. In general, more
quasi-fission is observed for reactions with non-magic nuclei. However, the
Ca+Pb reaction is an exception, showing strong evidence for
quasi-fission, though both nuclei are doubly magic. Time-dependent Hartree-Fock
calculations predict fast equilibration of in the two fragments early in
the collision. This transfer of nucleons breaks the shell effect, causing this
reaction to behave more like a non-magic one in the competition between fusion
and quasi-fission. Future measurements of fission in reactions with exotic
beams should be able to test this idea with larger asymmetries.Comment: accepted for publication in Physics Letters
Sub-barrier quasifission in heavy element formation reactions with deformed actinide target nuclei
Background: The formation of superheavy elements (SHEs) by fusion of two massive nuclei is severely
inhibited by the competing quasifission process. Low excitation energies favor SHE survival against fusion-fission
competition. In âcoldâ fusion with spherical target nuclei near 208Pb, SHE yields are largest at beam energies
significantly below the average capture barrier. In âhotâ fusion with statically deformed actinide nuclei, this is not
the case. Here the elongated deformation-aligned configurations in sub-barrier capture reactions inhibits fusion
(formation of a compact compound nucleus), instead favoring rapid reseparation through quasifission.
Purpose: To determine the probabilities of fast and slow quasifission in reactions with prolate statically deformed
actinide nuclei, through measurement and quantitative analysis of the dependence of quasifission characteristics
at beam energies spanning the average capture barrier energy.
Methods: The Australian National University Heavy Ion Accelerator Facility and CUBE fission spectrometer
have been used to measure fission and quasifission mass and angle distributions for reactions with projectiles
from C to S, bombarding Th and U target nuclei.
Results: Mass-asymmetric quasifission occurring on a fast time scale, associated with collisions with the tips of
the prolate actinide nuclei, shows a rapid increase in probability with increasing projectile charge, the transition
being centered around projectile atomic number ZP = 14. For mass-symmetric fission events, deviations of
angular anisotropies from expectations for fusion fission, indicating a component of slower quasifission, suggest
a similar transition, but centered around ZP ⌠8.
Conclusions: Collisions with the tips of statically deformed prolate actinide nuclei show evidence for two distinct
quasifission processes of different time scales. Their probabilities both increase rapidly with the projectile charge.
The probability of fusion can be severely suppressed by these two quasifission processes, since the sub-barrier
heavy element yield is likely to be determined by the product of the probabilities of surviving each quasifission
process.The authors acknowledge support from ARC Grants
No. FL110100098, No. DP130101569, No. FT120100760, No.
DE140100784, No. DP140101337, No. DP160101254, and
No. DP170102318, and support by the Federal Government
NCRIS program for operations of the ANU Heavy Ion Accelerator
Facility
- âŠ