77 research outputs found
Surface Modification of Titanium Dental Implants by Excimer Laser
The perfect osseointegration process of the dental implants depends among other factors on the surfact characteristics of the titanium. In this study enlarged mechanical roughness was produced by a laser-based technique, in order to decrease the healing period of the implant. There are different ways of forming laser induced surface
structures. In the case of mask projection techniques the surface can be modified in larger areas and surface patterns. An ArF nanosecond excimer laser was used in the experiments because of the advantageous properties of the excimer beams. Effective polishing by homogeneous laser illumination in the 3-5 J/cm2 fluence range was performed as confirmed by SEM and AFM studies. Holes of about
20 mm diameter and 10 mm in depth, with high aspect ratio and protrusions around the edges were ablated into the titanium surface with subsequent pulses of ns ArF excimer laser. To avoid easily breakable protrusions we applied excimer pulse durations of 0.5 picoseconds. In this case we obtained melting- and ridge-free ablation of titanium. The laser treatment influenced the chemical composition of the surface in two respects. On the one hand it
removed carbonaceous contamination as indicated by XPS and XRD measurements, demonstrating that cleaning of the surface does not alter the original crystalline structure. On the other hand, XPS measurements proved that pulsed laser oxidation in air increased the thickness of the surface oxide layer, promoting better osseointegration
Bimodality - a general feature of heavy ion reactions
Recently, is has been observed that events with the {\it same} total
transverse energy of light charged particles (LCP) in the quasi target region,
, show two quite distinct reaction scenarios in the
projectile domain: multifragmentation and residue production. This phenomenon
has been dubbed "bimodality". Using Quantum Molecular Dynamics calculations we
demonstrate that this observation is very general. It appears in collisions of
all symmetric systems larger than Ca and at beam energies between 50 A.MeV and
600 A.MeV and is due to large fluctuations of the impact parameter for a given
. Investigating in detail the bin in
which both scenarios are present, we find that neither the average fragment
momenta nor the average transverse and longitudinal energies of fragments show
the behavior expected from a system in statistical equilibrium, in experiment
as well as in QMD simulations. On the contrary, the experimental as well as the
theoretical results point towards a fast process. This observation questions
the conjecture that the observed bimodality is due to the coexistence of 2
phases at a given temperature in finite systems.Comment: accepted PR
Multiplicity correlations of intermediate-mass fragments with pions and fast protons in 12C + 197Au
Low-energy pi+ (E < 35 MeV) from 12C+197Au collisions at incident energies
from 300 to 1800 MeV per nucleon were detected with the Si-Si(Li)-CsI(Tl)
calibration telescopes of the INDRA multidetector. The inclusive angular
distributions are approximately isotropic, consistent with multiple
rescattering in the target spectator. The multiplicity correlations of the
low-energy pions and of energetic protons (E > 150 MeV) with intermediate-mass
fragments were determined from the measured coincidence data. The deduced
correlation functions 1 + R \approx 1.3 for inclusive event samples reflect the
strong correlations evident from the common impact-parameter dependence of the
considered multiplicities. For narrow impact-parameter bins (based on
charged-particle multiplicity), the correlation functions are close to unity
and do not indicate strong additional correlations. Only for pions at high
particle multiplicities (central collisions) a weak anticorrelation is
observed, probably due to a limited competition between these emissions.
Overall, the results are consistent with the equilibrium assumption made in
statistical multifragmentation scenarios. Predictions obtained with
intranuclear cascade models coupled to the Statistical Multifragmentation Model
are in good agreement with the experimental data.Comment: 9 pages, 11 figures, subm. to EPJ
Yield scaling, size hierarchy and fluctuations of observables in fragmentation of excited heavy nuclei
Multifragmentation properties measured with INDRA are studied for single
sources produced in Xe+Sn reactions in the incident energy range 32-50 A MeV
and quasiprojectiles from Au+Au collisions at 80 A MeV. A comparison for both
types of sources is presented concerning Fisher scaling, Zipf law, fragment
size and fluctuation observables. A Fisher scaling is observed for all the
data. The pseudo-critical energies extracted from the Fisher scaling are
consistent between Xe+Sn central collisions and Au quasi-projectiles. In the
latter case it also corresponds to the energy region at which fluctuations are
maximal. The critical energies deduced from the Zipf analysis are higher than
those from the Fisher analysis.Comment: 30 pages, accepted for publication in Nuclear Physics A, references
correcte
GATE : a simulation toolkit for PET and SPECT
Monte Carlo simulation is an essential tool in emission tomography that can
assist in the design of new medical imaging devices, the optimization of
acquisition protocols, and the development or assessment of image
reconstruction algorithms and correction techniques. GATE, the Geant4
Application for Tomographic Emission, encapsulates the Geant4 libraries to
achieve a modular, versatile, scripted simulation toolkit adapted to the field
of nuclear medicine. In particular, GATE allows the description of
time-dependent phenomena such as source or detector movement, and source decay
kinetics. This feature makes it possible to simulate time curves under
realistic acquisition conditions and to test dynamic reconstruction algorithms.
A public release of GATE licensed under the GNU Lesser General Public License
can be downloaded at the address http://www-lphe.epfl.ch/GATE/
The High-Acceptance Dielectron Spectrometer HADES
HADES is a versatile magnetic spectrometer aimed at studying dielectron
production in pion, proton and heavy-ion induced collisions. Its main features
include a ring imaging gas Cherenkov detector for electron-hadron
discrimination, a tracking system consisting of a set of 6 superconducting
coils producing a toroidal field and drift chambers and a multiplicity and
electron trigger array for additional electron-hadron discrimination and event
characterization. A two-stage trigger system enhances events containing
electrons. The physics program is focused on the investigation of hadron
properties in nuclei and in the hot and dense hadronic matter. The detector
system is characterized by an 85% azimuthal coverage over a polar angle
interval from 18 to 85 degree, a single electron efficiency of 50% and a vector
meson mass resolution of 2.5%. Identification of pions, kaons and protons is
achieved combining time-of-flight and energy loss measurements over a large
momentum range. This paper describes the main features and the performance of
the detector system
HADES experiment: di-lepton spectroscopy in p + p (2.2 GeV) and C+C (1 and 2 A GeV) collisions
The HADES (High Acceptance Di-Electron Spectrometer) is a tool designed for lepton pair (e+e−) spectroscopy in pion, proton and heavy ion induced reactions in the 1–2AGeV energy range. One of the goals of the HADES experiment is to study in-medium modifications of hadron properties like effective masses, decay widths, electromagnetic form factors etc. Such effects can be probed with vector mesons ( ρ,ω,ɸ ) decaying into e+e− channel. The identification of vector mesons by means of a HADES spectrometer is based on invariant mass reconstruction of e+e− pairs. The combined information from all spectrometer sub-detectors is used to reconstruct the di-lepton signal. The recent results from 2.2GeV p + p, 1AGeV and 2AGeV C+C experiments are presented.Diaz Medina, Jose, [email protected]
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