2,944 research outputs found
Dielectric antenna effects in integrating line piezoelectric sensors for optoacoustic imaging
This work studies the adverse effects, as regards noise, of immersing in
water an integrating line piezoelectric detector devoted to optoacoustic
imaging. We found that the sensor, in conjunction with the acoustic coupling
medium (water), behaves as a resonant dielectric antenna. This phenomenon
limits the performance of the system because it efficiently captures unwanted
electromagnetic signals. The requirement of good acoustic coupling between the
water and the sensor precluded the use of a standard metallic shielding
enclosure. Therefore, we resorted to a silver-paint based electrical shield
deposited on the detector. This easy-to-implement and low-cost solution
significantly increases the signal to noise ratio and does not degrade the
acoustic performance. The noise reduction allows the use of a better
transimpedance amplifier with higher gain and bandwidth; thus achieving a very
sensitive, low-noise detection system.Comment: 7 pages, 7 figures, 1 table, submitted to Meas. Sci. Technol. on
March 24, 202
Stellar ArAr reactions and their effect on light neutron-rich nuclide synthesis
The ArAr ( = 35 d) and
ArAr (269 y) reactions were studied for the first time
with a quasi-Maxwellian ( keV) neutron flux for Maxwellian Average
Cross Section (MACS) measurements at stellar energies. Gas samples were
irradiated at the high-intensity Soreq applied research accelerator
facility-liquid-lithium target neutron source and the Ar/Ar and
Ar/Ar ratios in the activated samples were determined by
accelerator mass spectrometry at the ATLAS facility (Argonne National
Laboratory). The Ar activity was also measured by low-level counting at
the University of Bern. Experimental MACS of Ar and Ar, corrected
to the standard 30 keV thermal energy, are 1.9(3) mb and 1.3(2) mb,
respectively, differing from the theoretical and evaluated values published to
date by up to an order of magnitude. The neutron capture cross sections of
Ar are relevant to the stellar nucleosynthesis of light neutron-rich
nuclides; the two experimental values are shown to affect the calculated mass
fraction of nuclides in the region A=36-48 during the weak -process. The new
production cross sections have implications also for the use of Ar and
Ar as environmental tracers in the atmosphere and hydrosphere.Comment: 18 pages + Supp. Mat. (13 pages) Accepted for publication in Phys.
Rev. Let
Triple configuration coexistence in 44 S
The neutron-rich N=28 nucleus S44 was studied using the two-proton knockout reaction from Ar46 at intermediate beam energy. We report the observation of four new excited states, one of which is a strongly prolate deformed 4 + state, as indicated by a shell-model calculation. Its deformation originates in a neutron configuration which is fundamentally different from the "intruder" configuration responsible for the ground-state deformation. Consequently, we do not have three coexisting shapes in S44, but three coexisting configurations, corresponding to zero-, one-, and two-neutron particle-hole excitations. � 2011 American Physical Society
Is a minor-merger driving the nuclear activity in the Seyfert 2 galaxy NGC 2110?
We report on a detailed morphological and kinematic study of the isolated
non-barred nearby Seyfert 2 galaxy NGC 2110. We combine Integral Field optical
spectroscopy, with long-slit and WFPC2 imaging available in the HST archive to
investigate the fueling mechanism in this galaxy. Previous work (Wilson &
Baldwin 1985) concluded that the kinematic center of the galaxy is displaced
\~220 pc from the apparent mass center of the galaxy, and the ionized gas
follows a remarkably normal rotation curve. Our analysis based on the stellar
kinematics, 2D ionized gas velocity field and dispersion velocity, and high
spatial resolution morphology at V, I and Halpha reveals that: 1) The kinematic
center of NGC 2110 is at the nucleus of the galaxy. 2) The ionized gas is not
in pure rotational motion. 3) The morphology of the 2D distribution of the
emission line widths suggests the presence of a minor axis galactic outflow. 4)
The nucleus is blue-shifted with respect to the stellar systemic velocity,
suggesting the NLR gas is out-flowing due to the interaction with the radio
jet. 5) The ionized gas is red-shifted ~100 km/s over the corresponding
rotational motion south of the nucleus, and 240 km/s with respect to the
nuclear stellar systemic velocity. This velocity is coincident with the HI
red-shifted absorption velocity detected by Gallimore et al (1999). We discuss
the possibility that the kinematics of the south ionized gas could be perturbed
by the collision with a small satellite that impacted on NGC 2110 close to the
center with a highly inclined orbit. Additional support for this interpretation
are the radial dust lanes and tidal debris detected in the V un-sharp masked
image. We suggest that a minor-merger may have driven the nuclear activity in
NGC 2110.Comment: Full resolution images at
http://www.iaa.csic.es/~rosa/preprints/preprints.html or at
http://www.journals.uchicago.edu/ApJ/future.htm
-cluster ANCs for nuclear astrophysics
Background. Many important -particle induced reactions for nuclear
astrophysics may only be measured using indirect techniques due to small cross
sections at the energy of interest. One of such indirect technique, is to
determine the Asymptotic Normalization Coefficients (ANC) for near threshold
resonances extracted from sub-Coulomb -transfer reactions. This
approach provides a very valuable tool for studies of astrophysically important
reaction rates since the results are practically model independent. However,
the validity of the method has not been directly verified.
Purpose. The aim of this letter is to verify the technique using the
O(Li,)Ne reaction as a benchmark. The Ne nucleus
has a well known state at excitation energy of 5.79 MeV with a width of
28 eV. Reproducing the known value with this technique is an ideal opportunity
to verify the method.
Method. The 1 state at 5.79 MeV is studied using the -transfer
reaction O(Li,)Ne at sub-Coulomb energies.
Results. The partial width for the state at excitation energy
of 5.79 MeV is extracted and compared with the known value, allowing the
accuracy of the method to be evaluated.
Conclusions. This study demonstrates that extracting the Asymptotic
Normalization Coefficients using sub-Coulomb -transfer reactions is a
powerful tool that can be used to determine the partial width of near
threshold states that may dominate astrophysically important nuclear reaction
rates. \end{description
Probing the single-particle character of rotational states in F using a short-lived isomeric beam
A beam containing a substantial component of both the ,
ns isomeric state of F and its , 109.77-min ground
state has been utilized to study members of the ground-state rotational band in
F through the neutron transfer reaction , in inverse kinematics.
The resulting spectroscopic strengths confirm the single-particle nature of the
13/2 band-terminating state. The agreement between shell-model
calculations, using an interaction constructed within the shell, and our
experimental results reinforces the idea of a single-particle/collective
duality in the descriptions of the structure of atomic nuclei
Trace gases detection by photoacoustic technique based on a lineal chirp excitation scheme
We present a new photoacoustic gas-trace measurement setup, based on a chirped optical chopper. This method combines features of the resonant and pulsed techniques. To show the advantages of this setup, we carried out a comparison with the resonant method in samples of NO2 contained in a one-dimensional acoustic resonator. The results show the chirped technique allows carrying out short-term acquisitions with good signal-to-noise ratio
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