1,195 research outputs found
A Goldstone Theorem in Thermal Relativistic Quantum Field Theory
We prove a Goldstone Theorem in thermal relativistic quantum field theory,
which relates spontaneous symmetry breaking to the rate of space-like decay of
the two-point function. The critical rate of fall-off coincides with that of
the massless free scalar field theory. Related results and open problems are
briefly discussed
Novel imaging and quality assurance techniques for ion beam therapy: a Monte Carlo study
Ion beams exhibit a finite and well defined range in matter together with an “inverted” depth-dose profile, the so-called Bragg peak. These favourable physical properties may enable superior tumour-dose conformality for high precision radiation therapy. On the other hand, they introduce the issue of sensitivity to range uncertainties in ion beam therapy. Although these uncertainties are typically taken into account when planning the treatment, correct delivery of the intended ion beam range has to be assured to prevent undesired underdosage of the tumour or overdosage of critical structures outside the target volume. Therefore, it is necessary to define dedicated Quality Assurance procedures to enable in-vivo range verification before or during therapeutic irradiation. For these purposes, Monte Carlo transport codes are very useful tools to support the development of novel imaging modalities for ion beam therapy. In the present work, we present calculations performed with the FLUKA Monte Carlo code and preliminary experimental studies
On the mixing property for a class of states of relativistic quantum fields
Let be a factor state on the quasi-local algebra of
observables generated by a relativistic quantum field, which in addition
satisfies certain regularity conditions (satisfied by ground states and the
recently constructed thermal states of the theory). We prove that
there exist space and time translation invariant states, some of which are
arbitrarily close to in the weak* topology, for which the time
evolution is weakly asymptotically abelian
Influence of local surface albedo variability and ice crystal shape on passive remote sensing of thin cirrus
Airborne measurements of solar spectral radiance reflected by cirrus are
performed with the HALO-Solar Radiation (HALO-SR) instrument onboard the High
Altitude and Long Range Research Aircraft (HALO) in November 2010. The data
are used to quantify the influence of surface albedo variability on the
retrieval of cirrus optical thickness and crystal effective radius. The
applied retrieval of cirrus optical properties is based on a standard two-wavelength approach utilizing measured and simulated reflected radiance in
the visible and near-infrared spectral region. Frequency distributions of the
surface albedos from Moderate resolution Imaging Spectroradiometer (MODIS)
satellite observations are used to compile surface-albedo-dependent lookup
tables of reflected radiance. For each assumed surface albedo the cirrus
optical thickness and effective crystal radius are retrieved as a function of
the assumed surface albedo. The results for the cirrus optical thickness are
compared to measurements from the High Spectral Resolution Lidar (HSRL). The
uncertainty in cirrus optical thickness due to local variability of surface
albedo in the specific case study investigated here is below 0.1 and thus
less than that caused by the measurement uncertainty of both instruments. It
is concluded that for the retrieval of cirrus optical thickness the surface
albedo variability is negligible. However, for the retrieval of crystal effective
radius, the surface albedo variability is of major importance,
introducing uncertainties up to 50%. Furthermore, the influence of the
bidirectional reflectance distribution function (BRDF) on the retrieval of
crystal effective radius was investigated and quantified with uncertainties
below 10%, which ranges below the uncertainty caused by the surface albedo
variability. The comparison with the independent lidar data allowed for
investigation of the role of the crystal shape in the retrieval. It is found that
if assuming aggregate ice crystals, the HSRL observations fit best with the
retrieved optical thickness from HALO-SR
Airborne observations of clouds in the Central Arctic during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition
Parameterizing anisotropic reflectance of snow surfaces from airborne digital camera observations in Antarctica
The surface reflection of solar radiation comprises an important boundary condition for solar radiative transfer simulations. In polar regions above snow surfaces, the surface reflection is particularly anisotropic due to low Sun elevations and the highly anisotropic scattering phase function of the snow crystals. The characterization of this surface reflection anisotropy is essential for satellite remote sensing over both the Arctic and Antarctica. To quantify the angular snow reflection properties, the hemispherical-directional reflectance factor (HDRF) of snow surfaces was derived from airborne measurements in Antarctica during austral summer in 2013/14. For this purpose, a digital 180∘ fish-eye camera (green channel, 490–585 nm wavelength band) was used. The HDRF was measured for different surface roughness conditions, optical-equivalent snow grain sizes, and solar zenith angles. The airborne observations covered an area of around 1000 km × 1000 km in the vicinity of Kohnen Station (75∘0′ S, 0∘4′ E) at the outer part of the East Antarctic Plateau. The observations include regions with higher (coastal areas) and lower (inner Antarctica) precipitation amounts and frequencies. The digital camera provided upward, angular-dependent radiance measurements from the lower hemisphere. The comparison of the measured HDRF derived for smooth and rough snow surfaces (sastrugi) showed significant differences, which are superimposed on the diurnal cycle. By inverting a semi-empirical kernel-driven bidirectional reflectance distribution function (BRDF) model, the measured HDRF of snow surfaces was parameterized as a function of solar zenith angle, surface roughness, and optical-equivalent snow grain size. This allows a direct comparison of the HDRF measurements with the BRDF derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) satellite product MCD43. For the analyzed cases, MODIS observations (545–565 nm wavelength band) generally underestimated the anisotropy of the surface reflection. The largest deviations were found for the volumetric model weight fvol (average underestimation by a factor of 10). These deviations are likely linked to short-term changes in snow properties
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