5,612 research outputs found
Slowing Down and Scattering of Ions in Solids
The interaction of particle beams with solids yields three parts, i.e. reflected particles, penetrating particles and trapped particles. At very low energies particle reflection is dominant, at very high energies penetration is the most important effect. Trapped particles are the result of energy loss processes, which on the other hand cause radiation damage in the solid. In the energy range discussed here, i.e. above energies where quantum effects, diffraction etc. are important and below energies where nuclear reactions, relativistic effects etc. may occur, the particle trajectories are classical. The energy loss process can be treated separately as nuclear and electronic stopping power. The collisions of the projectiles with target atoms are hence binary collisions involving a properly chosen screened Coulomb-potential. In single crystals the structural properties enable channeling, which is a very useful tool in sol id state analysis. The electronic stopping includes contributions from single collision processes and collective excitations. Both effects can be described by a dielectric response function. The range of applications covers analytical methods, means to modify solid state properties and also the production of thin films
Faint Gigahertz peaked spectrum sources and the evolution of young radio sources
GPS sources are the objects of choice to study the initial evolution of
extragalactic radio sources, since it is most likely that they are the young
counterparts of large scale radio sources. Correlations found between their
peak frequency, peak flux density and angular size provide strong evidence that
synchrotron self absorption is the cause of the spectral turnovers, and
indicate that young radio sources evolve in a self-similar way. The difference
in redshift distribution between young and old radio sources must be due to a
difference in slope of their luminosity functions, and we argue that this slope
is strongly affected by the luminosity evolution of the individual sources. A
luminosity evolution scenario is proposed in which GPS sources increase in
luminosity and large scale radio sources decrease in luminosity with time. It
is shown that such a scenario agrees with the local luminosity function of GPS
galaxies.Comment: Late, 6 pages, 2 figs. To appear in the proceedings of EVN/JIVE
Symposium No 4, New Astronomy Reviews (eds. Garrett et al.). For related
papers, see http://www.ast.cam.ac.uk/~snelle
SPUTTER DEPTH PROFILING OF OPTICAL WAVEGUIDES USING SECONDARY ION MASS SPECTROMETRY
The technique of sputter depth profiling by means of secondary ion mass spectrometry
of samples with high resistivity is reviewed. As examples we discuss optical waveguides made
in lithium niobate by titanium indiffusion and implantation and also yttrium iron garnet
waveguides grown by liquid phase epitaxy on gadolinium gallium garnet. Depth profiling of
these waveguide structures has been performed and the necessary precautions to prevent
charging by the primary ion beam are discussed. In some cases, coating with a metallic layer
is adequate, but a more universal method is charge neutralization by an additional electron
beam
No asymmetric outflows from Sagittarius A* during the pericenter passage of the gas cloud G2
The gas cloud G2 falling toward Sagittarius A* (Sgr A*), the supermassive
black hole at the center of the Milky Way, is supposed to provide valuable
information on the physics of accretion flows and the environment of the black
hole. We observed Sgr A* with four European stations of the Global Millimeter
Very Long Baseline Interferometry Array (GMVA) at 86 GHz on 1 October 2013 when
parts of G2 had already passed the pericenter. We searched for possible
transient asymmetric structure -- such as jets or winds from hot accretion
flows -- around Sgr A* caused by accretion of material from G2. The
interferometric closure phases remained zero within errors during the
observation time. We thus conclude that Sgr A* did not show significant
asymmetric (in the observer frame) outflows in late 2013. Using simulations, we
constrain the size of the outflows that we could have missed to ~2.5 mas along
the major axis, ~0.4 mas along the minor axis of the beam, corresponding to
approximately 232 and 35 Schwarzschild radii, respectively; we thus probe
spatial scales on which the jets of radio galaxies are suspected to convert
magnetic into kinetic energy. As probably less than 0.2 Jy of the flux from Sgr
A* can be attributed to accretion from G2, one finds an effective accretion
rate eta*Mdot < 1.5*10^9 kg/s ~ 7.7*10^-9 Mearth/yr for material from G2.
Exploiting the kinetic jet power--accretion power relation of radio galaxies,
one finds that the rate of accretion of matter that ends up in jets is limited
to Mdot < 10^17 kg/s ~ 0.5 Mearth/yr, less than about 20% of the mass of G2.
Accordingly, G2 appears to be largely stable against loss of angular momentum
and subsequent (partial) accretion at least on time scales < 1 year.Comment: 5 pages, 2 figures, 1 table; A&A Letter, in press (submitted 2015
February 26; accepted 2015 March 31
Dust in 3C324
The results of a deep submillimetre observation using SCUBA of the powerful
radio galaxy 3C324, at redshift z=1.206, are presented. At 850 microns,
emission from the location of the host radio galaxy is marginally detected at
the 4.2 sigma level, 3.01 +/- 0.72 mJy, but there is no detection of emission
at 450 microns to a 3 sigma limit of 21 mJy. A new 32 GHz radio observation
using the Effelsberg 100m telescope confirms that the sub-millimetre signal is
not associated with synchrotron emission. These observations indicate that both
the mass of warm dust within 3C324, and the star formation rate, lie up to an
order of magnitude below the values recently determined for radio galaxies at z
= 3 to 4. The results are compared with dust masses and star formation rates
derived in other ways for 3C324.Comment: 5 pages LaTeX, including 1 figure. Accepted for publication in MNRA
A new sample of faint Gigahertz Peaked Spectrum radio sources
The Westerbork Northern Sky Survey (WENSS) has been used to select a sample
of Gigahertz Peaked Spectrum (GPS) radio sources at flux densities one to two
orders of magnitude lower than bright GPS sources investigated in earlier
studies. Sources with inverted spectra at frequencies above 325 MHz have been
observed with the WSRT at 1.4 and 5 GHz and with the VLA at 8.6 and 15 GHz to
select genuine GPS sources. This has resulted in a sample of 47 GPS sources
with peak frequencies ranging from ~500 MHz to >15 GHz, and peak flux densities
ranging from ~40 to ~900 mJy. Counts of GPS sources in our sample as a function
of flux density have been compared with counts of large scale sources from
WENSS scaled to 2 GHz, the typical peak frequency of our GPS sources. The
counts can be made similar if the number of large scale sources at 2 GHz is
divided by 250, and their flux densities increase by a factor of 10. On the
scenario that all GPS sources evolve into large scale radio sources, these
results show that the lifetime of a typical GPS source is ~250 times shorter
than a typical large scale radio source, and that the source luminosity must
decrease by a factor of ~10 in evolving from GPS to large scale radio source.
However, we note that the redshift distributions of GPS and large scale radio
sources are different and that this hampers a direct and straightforward
interpretation of the source counts. Further modeling of radio source evolution
combined with cosmological evolution of the radio luminosity function for large
sources is required.Comment: LaTeX, 15 pages, 8 figs. To be published in A&AS. For more info see
http://www.ast.cam.ac.uk/~snelle
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