212 research outputs found
Impact of charge carrier injection on single-chain photophysics of conjugated polymers
Charges in conjugated polymer materials have a strong impact on the
photophysics and their interaction with the primary excited state species has
to be taken into account in understanding device properties. Here, we employ
single-molecule spectroscopy to unravel the influence of charges on several
photoluminescence (PL) observables. The charges are injected either
stochastically by a photochemical process, or deterministically in a
hole-injection sandwich device configuration. We find that upon charge
injection, besides a blue-shift of the PL emission and a shortening of the PL
lifetime due to quenching and blocking of the lowest-energy chromophores, the
non-classical photon arrival time distribution of the multichromophoric chain
is modified towards a more classical distribution. Surprisingly, the fidelity
of photon antibunching deteriorates upon charging, whereas one would actually
expect the number of chromophores to be reduced. A qualitative model is
presented to explain the observed PL changes. The results are of interest to
developing a microscopic understanding of the intrinsic charge-exciton
quenching interaction in devices
A future very-high-energy view of our Galaxy
The survey of the inner Galaxy with H.E.S.S. was remarkably successful in
detecting a wide range of new very-high-energy gamma-ray sources. New TeV
gamma-ray emitting source classes were established, although several of the
sources remain unidentified, and progress has been made in understanding
particle acceleration in astrophysical sources. In this work, we constructed a
model of a population of such very-high-energy gamma-ray emitters and
normalised the flux and size distribution of this population model to the
H.E.S.S.-discovered sources. Extrapolating that population of objects to lower
flux levels we investigate what a future array of imaging atmospheric
telescopes (IACTs) such as AGIS or CTA might detect in a survey of the Inner
Galaxy with an order of magnitude improvement in sensitivity. The sheer number
of sources detected together with the improved resolving power will likely
result in a huge improvement in our understanding of the populations of
galactic gamma-ray sources. A deep survey of the inner Milky Way would also
support studies of the interstellar diffuse gamma-ray emission in regions of
high cosmic-ray density. In the final section of this paper we investigate the
science potential for the Galactic Centre region for studying energy-dependent
diffusion with such a future array.Comment: Proceeding of "Heidelberg International Symposium on High Energy
Gamma-Ray Astronomy", held in Heidelberg, 7-11 July 2008, submitted to AIP
Conference Proceedings. 4 pages, 4 figure
The implications from CANGAROO-III observations of TeV blazar PKS 2155-304
We have observed the high-frequency-peaked BL Lacertae object PKS2155-304 in
2004, 2005 and 2006 with the CANGAROO-III imaging atmospheric Cherenkov
telescope, and have detected a signal above 660 GeV at the 4.8/sigma level
during the 2006 outburst period. Intranight flux variability on time scale of
half an hour is observed. From this variability time scale, the size of the TeV
gamma-ray emission region is restricted to 5x10^13\delta cm, and the super
massive black hole mass is estimated to be less than 1.9x10^8\delta M_{Solar},
where \delta is the beaming factor. The differential energy spectrum is
obtained, and an upper limit of the extragalactic infrared background light
(EBL) flux is derived under some assumption. We also fit a synchrotron self
Compton (SSC) model to the spectral energy distribution (SED) and derive the
beaming factor and magnetic field strength.Comment: 4 pages, 5 figures, proceedings of the "4th Heidelberg International
Symposium on High Energy Gamma-Ray Astronomy" July 7-11, 2008, Heidelberg,
German
Suzaku observation of TeV blazar the 1ES 1218+304: clues on particle acceleration in an extreme TeV blazar
We observed the TeV blazar 1ES 1218+304 with the X-ray astronomy satellite
Suzaku in May 2006. At the beginning of the two-day continuous observation, we
detected a large flare in which the 5-10 keV flux changed by a factor of ~2 on
a timescale of 5x10^4 s. During the flare, the increase in the hard X-ray flux
clearly lagged behind that observed in the soft X-rays, with the maximum lag of
2.3x10^4 s observed between the 0.3-1 keV and 5-10 keV bands. Furthermore we
discovered that the temporal profile of the flare clearly changes with energy,
being more symmetric at higher energies. From the spectral fitting of
multi-wavelength data assuming a one-zone, homogeneous synchrotron self-Compton
model, we obtain B~0.047 G, emission region size R = 3.0x10^16 cm for an
appropriate beaming with a Doppler factor of delta = 20. This value of B is in
good agreement with an independent estimate through the model fit to the
observed time lag ascribing the energy-dependent variability to differential
acceleration timescale of relativistic electrons provided that the gyro-factor
\xi is 10^5.Comment: 11 pages, 3 figures, Accepted for publication in ApJ
VHE Gamma-ray Afterglow Emission from Nearby GRBs
Gamma-ray Bursts (GRBs) are among the potential extragalactic sources of
very-high-energy (VHE) gamma-rays. We discuss the prospects of detecting VHE
gamma-rays with current ground-based Cherenkov instruments during the afterglow
phase. Using the fireball model, we calculate the synchrotron self-Compton
(SSC) emission from forward-shock electrons. The modeled results are compared
with the observational afterglow data taken with and/or the sensitivity level
of ground-based VHE instruments (e.g. STACEE, H.E.S.S., MAGIC, VERITAS, and
Whipple). We find that modeled SSC emission from bright and nearby bursts such
as GRB 030329 are detectable by these instruments even with a delayed
observation time of ~10 hours.Comment: Proceeding of "Heidelberg International Symposium on High Energy
Gamma-Ray Astronomy", held in Heidelberg, 7-11 July 2008, submitted to AIP
Conference Proceedings. 4 pages, 3 figures, 1 tabl
New Relativistic Particle-In-Cell Simulation Studies of Prompt and Early Afterglows from GRBs
Nonthermal radiation observed from astrophysical systems containing
relativistic jets and shocks, e.g., gamma-ray bursts (GRBs), active galactic
nuclei (AGNs), and microquasars commonly exhibit power-law emission spectra.
Recent PIC simulations of relativistic electron-ion (or electron-positron) jets
injected into a stationary medium show that particle acceleration occurs within
the downstream jet. In collisionless, relativistic shocks, particle (electron,
positron, and ion) acceleration is due to plasma waves and their associated
instabilities (e.g., the Weibel (filamentation) instability) created in the
shock region. The simulations show that the Weibel instability is responsible
for generating and amplifying highly non-uniform, small-scale magnetic fields.
These fields contribute to the electron's transverse deflection behind the jet
head. The resulting "jitter" radiation from deflected electrons has different
properties compared to synchrotron radiation, which assumes a uniform magnetic
field. Jitter radiation may be important for understanding the complex time
evolution and/or spectra in gamma-ray bursts, relativistic jets in general, and
supernova remnants.Comment: : 4 pages, 1 figure and 1 table, typos are corrected, submitted for
the Proceedings of The 4th Heidelberg International Symposium on High Energy
Gamma-Ray Astronomy, July 7-11, 2008, in Heidelberg, German
Deformation behaviour of ion-irradiated FeCr : A nanoindentation study
Understanding the mechanisms of plasticity in structural steels is essential for the operation of next-generation fusion reactors. This work on the deformation behaviour of FeCr, focusses on distinguishing the nucleation of dislocations to initiate plasticity, from their propagation through the material. Fe3Cr, Fe5Cr, and Fel OCr were irradiated with 20 MeV Fe3+ ions at room temperature to doses of 0.008 dpa and 0.08 dpa. Nanoindentation was then carried out with Berkovich and spherical indenter tips. Our results show that the nucleation of dislocations is mainly from pre-existing sources, which are not significantly affected by the presence of irradiation defects or Cr%. Yield strength, an indicator of dislocation mobility, increases with irradiation damage and Cr content, while work hardening capacity decreases mainly due to irradiation defects. The synergistic effects of Cr and irradiation damage in FeCr appear to be more important for the propagation of dislocations than for their nucleation.Peer reviewe
Characterising Ion-Irradiated FeCr : Hardness, Thermal Diffusivity and Lattice Strain
Ion-irradiated FeCr alloys are useful for understanding and predicting neutron damage in the structural steels of future nuclear reactors. Previous studies have largely focused on the structure of irradiation induced defects, probed by transmission electron microscopy (TEM), as well as changes in mechanical properties. Across these studies, a wide range of irradiation conditions has been employed on samples with different processing histories, which complicates the analysis of the relationship between defect structures and material properties. Furthermore, key properties, such as irradiation-induced changes in thermal transport and lattice strain, are little explored. Here we present a systematic study of Fe3Cr, Fe5Cr and Fe10Cr binary alloys implanted with 20 MeV Fe3+ ions to nominal doses of 0.01 dpa and 0.1 dpa at room temperature. Nanoindentation, transient grating spectroscopy (TGS) and X-ray micro-beam Laue diffraction were used to study the changes in hardness, thermal diffusivity and strain in the material as a function of damage and Cr content. Our results suggest that Cr leads to an increased retention of irradiation-induced defects, causing substantial changes in hardness and lattice strain. However, thermal diffusivity varies little with increasing damage and instead degrades significantly with increasing Cr content in the material. We find significant lattice strains even in samples exposed to a nominal displacement damage of 0.01 dpa. The defect density predicted from the lattice strain measurements is significantly higher than that observed in previous TEM studies, suggesting that TEM may not fully capture the irradiation-induced defect population. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.Peer reviewe
Microstructural and material property changes in severely deformed Eurofer-97
Severe plastic deformation changes the microstructure and properties of
steels, which may be favourable for their use in structural components of
nuclear reactors. In this study, high-pressure torsion (HPT) was used to refine
the grain structure of Eurofer-97, a ferritic/ martensitic steel. Electron
microscopy and X-ray diffraction were used to characterise the microstructural
changes. Following HPT, the average grain size reduced by a factor of
30, with a marked increase in high-angle grain boundaries. Dislocation density
also increased by more than one order of magnitude. The thermal stability of
the deformed material was investigated via in-situ annealing during synchrotron
X-ray diffraction. This revealed substantial recovery between 450 K - 800 K.
Irradiation with 20 MeV Fe-ions to 0.1 dpa caused a 20% reduction in
dislocation density compared to the as-deformed material. However, HPT
deformation prior to irradiation did not have a significant effect in
mitigating the irradiation-induced reductions in thermal diffusivity and
surface acoustic wave velocity of the material. These results provide a
multi-faceted understanding of the changes in ferritic/martensitic steels due
to severe plastic deformation, and how these changes can be used to alter
material properties.Comment: 59 pages, 19 figure
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