3,984 research outputs found
Structure of Be probed via secondary beam reactions
The low-lying level structure of the unbound neutron-rich nucleus Be
has been investigated via breakup on a carbon target of secondary beams of
B at 35 MeV/nucleon. The coincident detection of the beam velocity
Be fragments and neutrons permitted the invariant mass of the
Be+ and Be++ systems to be reconstructed. In the case of
the breakup of B, a very narrow structure at threshold was observed in
the Be+ channel. Contrary to earlier stable beam fragmentation
studies which identified this as a strongly interacting -wave virtual state
in Be, analysis here of the Be++ events demonstrated that
this was an artifact resulting from the sequential-decay of the
Be(2) state. Single-proton removal from B was found to
populate a broad low-lying structure some 0.70 MeV above the neutron-decay
threshold in addition to a less prominent feature at around 2.4 MeV. Based on
the selectivity of the reaction and a comparison with (0-3)
shell-model calculations, the low-lying structure is concluded to most probably
arise from closely spaced J=1/2 and 5/2 resonances
(E=0.400.03 and 0.85 MeV), whilst the broad
higher-lying feature is a second 5/2 level (E=2.350.14 MeV). Taken
in conjunction with earlier studies, it would appear that the lowest 1/2
and 1/2 levels lie relatively close together below 1 MeV.Comment: 14 pages, 13 figures, 2 tables. Accepted for publication in Physical
Review
The principle of relative locality
We propose a deepening of the relativity principle according to which the
invariant arena for non-quantum physics is a phase space rather than spacetime.
Descriptions of particles propagating and interacting in spacetimes are
constructed by observers, but different observers, separated from each other by
translations, construct different spacetime projections from the invariant
phase space. Nonetheless, all observers agree that interactions are local in
the spacetime coordinates constructed by observers local to them.
This framework, in which absolute locality is replaced by relative locality,
results from deforming momentum space, just as the passage from absolute to
relative simultaneity results from deforming the linear addition of velocities.
Different aspects of momentum space geometry, such as its curvature, torsion
and non-metricity, are reflected in different kinds of deformations of the
energy-momentum conservation laws. These are in principle all measurable by
appropriate experiments. We also discuss a natural set of physical hypotheses
which singles out the cases of momentum space with a metric compatible
connection and constant curvature.Comment: 12 pages, 3 figures; in version 2 one reference added and some minor
modifications in sects. II and III mad
Isotopic distribution of fission fragments in collisions between 238U beam and 9Be and 12C targets at 24 MeV/u
Inverse kinematics coupled to a high-resolution spectrometer is used to
investigate the isotopic yields of fission fragments produced in reactions
between a 238U beam at 24 MeV/u and 9Be and 12C targets. Mass, atomic number
and isotopic distributions are reported for the two reactions. These
informations give access to the neutron excess and the isotopic distribution
widths, which together with the atomic-number and mass distributions are used
to investigate the fusion-fission dynamics.Comment: Submitted to PR
Probing Nuclear forces beyond the drip-line using the mirror nuclei N and F
Radioactive beams of O and O were used to populate the resonant
states 1/2, 5/2 and in the unbound F and F
nuclei respectively by means of proton elastic scattering reactions in inverse
kinematics. Based on their large proton spectroscopic factor values, the
resonant states in F can be viewed as a core of O plus a proton
in the 2s or 1d shell and a neutron in 1p. Experimental
energies were used to derive the strength of the 2s-1p and
1d-1p proton-neutron interactions. It is found that the former
changes by 40% compared with the mirror nucleus N, and the second by
10%. This apparent symmetry breaking of the nuclear force between mirror nuclei
finds explanation in the role of the large coupling to the continuum for the
states built on an proton configuration.Comment: 6 pages, 3 figures, 2 tables, accepted for publication as a regular
article in Physical Review
Global patterns of diapycnal mixing from measurements of the turbulent dissipation rate
The authors present inferences of diapycnal diffusivity from a compilation of over 5200 microstructure profiles. As microstructure observations are sparse, these are supplemented with indirect measurements of mixing obtained from (i) Thorpe-scale overturns from moored profilers, a finescale parameterization applied to (ii) shipboard observations of upper-ocean shear, (iii) strain as measured by profiling floats, and (iv) shear and strain from full-depth lowered acoustic Doppler current profilers (LADCP) and CTD profiles. Vertical profiles of the turbulent dissipation rate are bottom enhanced over rough topography and abrupt, isolated ridges. The geography of depth-integrated dissipation rate shows spatial variability related to internal wave generation, suggesting one direct energy pathway to turbulence. The global-averaged diapycnal diffusivity below 1000-m depth is O(10?4) m2 s?1 and above 1000-m depth is O(10?5) m2 s?1. The compiled microstructure observations sample a wide range of internal wave power inputs and topographic roughness, providing a dataset with which to estimate a representative global-averaged dissipation rate and diffusivity. However, there is strong regional variability in the ratio between local internal wave generation and local dissipation. In some regions, the depth-integrated dissipation rate is comparable to the estimated power input into the local internal wave field. In a few cases, more internal wave power is dissipated than locally generated, suggesting remote internal wave sources. However, at most locations the total power lost through turbulent dissipation is less than the input into the local internal wave field. This suggests dissipation elsewhere, such as continental margins
Spectroscopy of Na: shell evolution toward the drip line
Excited states in Na have been studied using the -decay of
implanted Ne ions at GANIL/LISE as well as the in-beam -ray
spectroscopy at the NSCL/S800 facility. New states of positive
(J=3,4) and negative (J=1-5) parity are proposed. The
former arise from the coupling between 0d protons and a 0d
neutron, while the latter are due to couplings with 1p or 0f
neutrons. While the relative energies between the J=1-4 states are
well reproduced with the USDA interaction in the N=17 isotones, a progressive
shift in the ground state binding energy (by about 500 keV) is observed between
F and Al. This points to a possible change in the proton-neutron
0d-0d effective interaction when moving from stability to the
drip line. The presence of J=1-4 negative parity states around 1.5
MeV as well as of a candidate for a J=5 state around 2.5 MeV give
further support to the collapse of the N=20 gap and to the inversion between
the 0f and 1p levels below Z=12. These features are discussed
in the framework of Shell Model and EDF calculations, leading to predicted
negative parity states in the low energy spectra of the F and O
nuclei.Comment: Exp\'erience GANIL/LISE et NSCL/S80
New pathway to bypass the 15O waiting point
We propose the sequential reaction process
O(,)O as a new pathway to bypass of the
O waiting point. This exotic reaction is found to have a surprisingly
high cross section, approximately 10 times higher than the
O(,)O. These cross sections were calculated after
precise measurements of energies and widths of the proton-unbound F low
lying states, obtained using the H(O,p)O reaction. The large
cross section can be understood to arise from the more
efficient feeding of the low energy wing of the ground state resonance by the
gamma decay. The implications of the new reaction in novae explosions and X-ray
bursts are discussed.Comment: submitte
Closed-loop separation control over a sharp edge ramp using Genetic Programming
We experimentally perform open and closed-loop control of a separating
turbulent boundary layer downstream from a sharp edge ramp. The turbulent
boundary layer just above the separation point has a Reynolds number
based on momentum thickness. The goal of the
control is to mitigate separation and early re-attachment. The forcing employs
a spanwise array of active vortex generators. The flow state is monitored with
skin-friction sensors downstream of the actuators. The feedback control law is
obtained using model-free genetic programming control (GPC) (Gautier et al.
2015). The resulting flow is assessed using the momentum coefficient, pressure
distribution and skin friction over the ramp and stereo PIV. The PIV yields
vector field statistics, e.g. shear layer growth, the backflow area and vortex
region. GPC is benchmarked against the best periodic forcing. While open-loop
control achieves separation reduction by locking-on the shedding mode, GPC
gives rise to similar benefits by accelerating the shear layer growth.
Moreover, GPC uses less actuation energy.Comment: 24 pages, 24 figures, submitted to Experiments in Fluid
Direct Observation of Propagating Gigahertz Coherent Guided Acoustic Phonons in Free Standing Single Copper Nanowires
We report on gigahertz acoustic phonon waveguiding in free-standing single
copper nanowires studied by femtosecond transient reflectivity measurements.
The results are discussed on the basis of the semianalytical resolution of the
Pochhammer and Chree equation. The spreading of the generated Gaussian wave
packet of two different modes is derived analytically and compared with the
observed oscillations of the sample reflectivity. These experiments provide a
unique way to independently obtain geometrical and material characterization.
This direct observation of coherent guided acoustic phonons in a single
nano-object is also the first step toward nanolateral size acoustic transducer
and comprehensive studies of the thermal properties of nanowires
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