8,790 research outputs found
New Results on e+e- Line Emission in U+Ta Collisions
We present new results obtained from a series of follow-up e+e- coincidence
measurements in heavy-ion collisions, utilizing an improved experimental set-up
at the double-Orange beta-spectrometer of GSI. The collision system U+Ta was
reinvestigated in three independent runs at beam energies in the range
(6.0-6.4)xA MeV and different target thicknesses, with the objective to
reproduce a narrow sum-energy e+e- line at ~635 keV observed previously in this
collision system. At improved statistical accuracy, the line could not be found
in these new data. For the ''fission'' scenario, an upper limit (1 sigma) on
its production probability per collision of 1.3x10^{-8} can be set which has to
be compared to the previously reported value of [4.9 +- 0.8 (stat.) +- 1.0
(syst)]x10^{-7}. In the light of the new results, a reanalysis of the old data
shows that the continuous part of the spectrum at the line position is
significantly higher than previously assumed, thus reducing the production
probability of the line by a factor of two and its statistical significance to
< 3.4sigma.Comment: 15 pages, standard LaTeX with 3 included PS figures; Submitted to
Physics Letters
Measuring Identity in Individuals with Aphasia at Fontbonne University
This research proposal will outline how modifying a pre-existing identity scale can measure whether G.R.A.C.E. therapy at Fontbonne University works to increase a sense of identity in individuals with aphasia and potential next steps in furthering the study.https://griffinshare.fontbonne.edu/slp-posters/1011/thumbnail.jp
The dynamical hole in ultrafast photoassociation: analysis of the compression effect
Photoassociation of a pair of cooled atoms by excitation with a short chirped
laser pulse creates a dynamical hole in the initial continuum wavefunction.
This hole is manifested by a void in the pair wavefunction and a momentum kick.
Photoassociation into loosely bound levels of the external well in Cs_2
0(6S + 6P is considered as a case study. After the pulse, the
free evolution of the ground triplet state wavepacket is analyzed. Due to a
negative momentum kick, motion to small distances is manifested and a
compression effect is pointed out, markedly increasing the density of atom
pairs at short distance. A consequence of the hole is the redistribution of the
vibrational population in the ground triplet state, with population of the last
bound level and creation of pairs of hot atoms. The physical interpretation
makes use of the time dependence of the probability current and population on
each channel to understand the role of the parameters of the photoassociation
pulse. By varying such parameters, optimization of the compression effect in
the ground state wavepacket is demonstrated. Due to an increase of the short
range density probability by more than two orders of magnitude, we predict
important photoassociation rates into deeply bound levels of the excited state
by a second pulse, red-detuned relative to the first one and conveniently
delayed.Comment: 31 pages, 11 figure
First Energy and Angle differential Measurements of e^+e^- -pairs emitted by Internal Pair Conversion of excited Heavy Nuclei
We present the first energy and angle resolved measurements of e+e- pairs
emitted from heavy nuclei (Z>=40) at rest by internal pair conversion (IPC) of
transitions with energies of less than 2MeV as well as recent theoretical
results using the DWBA method, which takes full account of relativistic
effects, magnetic substates and finite size of the nucleus. The 1.76MeV E0
transition in Zr90 (Sr source) and the 1.77MeV M1 transition in Pb207 (Bi
source) have been investigated experimentally using the essentially improved
set-up at the double-ORANGE beta-spectrometer of GSI. The measurements prove
the capability of the setup to cleanly identify the IPC pairs in the presence
of five orders of magnitude higher beta- and gamma background from the same
source and to yield essentially background-free sum spectra despite the large
background. Using the ability of the ORANGE setup to directly determine the
opening angle of the e+e- pairs, the angular correlation of the emitted pairs
was measured. In the Zr90 case the correlation could be deduced for a wide
range of energy differences of the pairs. The Zr90 results are in good
agreement with recent theory. The angular correlation deduced for the M1
transition in Pb207 is in strong disagreement with theoretical predictions
derived within the Born approximation and shows almost isotropic character.
This is again in agreement with the new theoretical results.Comment: LaTeX, 28 pages incl. 10 PS figures; Accepted by Z.Phys.
Positron spectra from internal pair conversion observed in {238}U + {181}Ta collisions
We present new results from measurements and simulations of positron spectra,
originating from 238U + 181Ta collisions at beam energies close to the Coulomb
barrier. The measurements were performed using an improved experimental setup
at the double-Orange spectrometer of GSI. Particular emphasis is put on the
signature of positrons from Internal-Pair-Conversion (IPC) processes in the
measured e+ energy spectra, following the de-excitation of electromagnetic
transitions in the moving Ta-like nucleus. It is shown by Monte Carlo
simulations that, for the chosen current sweeping procedure used in the present
experiments, positron emission from discrete IPC transitions can lead to rather
narrow line structures in the measured energy spectra. The measured positron
spectra do not show evidence for line structures within the statistical
accuracy achieved, although expected from the intensities of the observed
transitions (E keV) and theoretical conversion
coefficients. This is due to the reduced detection efficiency for IPC
positrons, caused by the limited spatial and momentum acceptance of the
spectrometer. A comparison with previous results, in which lines have been
observed, is presented and the implications are discussed.Comment: LaTeX, 20 pages including 5 EPS figures; Accepted by Eur. Phys.Jour.
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Long-term balancing selection drives evolution of immunity genes in Capsella.
Genetic drift is expected to remove polymorphism from populations over long periods of time, with the rate of polymorphism loss being accelerated when species experience strong reductions in population size. Adaptive forces that maintain genetic variation in populations, or balancing selection, might counteract this process. To understand the extent to which natural selection can drive the retention of genetic diversity, we document genomic variability after two parallel species-wide bottlenecks in the genus Capsella. We find that ancestral variation preferentially persists at immunity related loci, and that the same collection of alleles has been maintained in different lineages that have been separated for several million years. By reconstructing the evolution of the disease-related locus MLO2b, we find that divergence between ancient haplotypes can be obscured by referenced based re-sequencing methods, and that trans-specific alleles can encode substantially diverged protein sequences. Our data point to long-term balancing selection as an important factor shaping the genetics of immune systems in plants and as the predominant driver of genomic variability after a population bottleneck
Femtosecond pulses and dynamics of molecular photoexcitation: RbCs example
We investigate the dynamics of molecular photoexcitation by unchirped
femtosecond laser pulses using RbCs as a model system. This study is motivated
by a goal of optimizing a two-color scheme of transferring
vibrationally-excited ultracold molecules to their absolute ground state. In
this scheme the molecules are initially produced by photoassociation or
magnetoassociation in bound vibrational levels close to the first dissociation
threshold. We analyze here the first step of the two-color path as a function
of pulse intensity from the low-field to the high-field regime. We use two
different approaches, a global one, the 'Wavepacket' method, and a restricted
one, the 'Level by Level' method where the number of vibrational levels is
limited to a small subset. The comparison between the results of the two
approaches allows one to gain qualitative insights into the complex dynamics of
the high-field regime. In particular, we emphasize the non-trivial and
important role of far-from-resonance levels which are adiabatically excited
through 'vertical' transitions with a large Franck-Condon factor. We also point
out spectacular excitation blockade due to the presence of a quasi-degenerate
level in the lower electronic state. We conclude that selective transfer with
femtosecond pulses is possible in the low-field regime only. Finally, we extend
our single-pulse analysis and examine population transfer induced by coherent
trains of low-intensity femtosecond pulses.Comment: 25 pages, 12 figure
Constraining the Nature of the Galactic Center X-ray Source Population
We searched for infrared counterparts to the cluster of X-ray point sources
discovered by Chandra in the Galactic Center Region (GCR). While the sources
could be white dwarfs, neutron stars, or black holes accreting from stellar
companions, their X-ray properties are consistent with magnetic Cataclysmic
Variables, or High Mass X-ray Binaries (HMXB) at low accretion-rates. A direct
way to decide between these possibilities and hence between alternative
formation scenarios is to measure or constrain the luminosity distribution of
the companions. Using infrared (J, H, K, Br-gamma) imaging, we searched for
counterparts corresponding to typical HMXB secondaries: spectral type B0V with
K<15 at the GCR. We found no significant excess of bright stars in Chandra
error circles, indicating that HMXBs are not the dominant X-ray source
population, and account for fewer than 10% of the hardest X-ray sources.Comment: 4 pages, 3 figures, 1 table, accepted in ApJ Letters for publicatio
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