204 research outputs found
Improved noise-adding radiometer for microwave receivers
Use of input switch and noise reference standard is avoided by using noise-adding technique. Excess noise from solid state noise-diode is coupled into receiver through directional coupler and square-wave modulated at low rate. High sensitivity receivers for radioastronomy applications are utilized with greater confidence in stability of radiometer
Quantum description and properties of electrons emitted from pulsed nanotip electron sources
We present a quantum calculation of the electron degeneracy for electron
sources. We explore quantum interference of electrons in the temporal and
spatial domain and demonstrate how it can be utilized to characterize a pulsed
electron source. We estimate effects of Coulomb repulsion on two-electron
interference and show that currently available nano tip pulsed electron sources
operate in the regime where the quantum nature of electrons can be made
dominant
Direct Imaging of Periodic Sub-wavelength Patterns of Total Atomic Density
Interference fringes of total atomic density with period and
for optical wavelength , have been produced in de Broglie
atom interferometer and directly imaged by means of an ``optical mask''
technique. The imaging technique allowed us to observe sub-wavelength periodic
patterns with a resolution of . The quantum dynamics near the
interference times as a function of the recoil phase and pulse areas has been
investigated.Comment: 4 pages, 4 figures, to be submitted to Phys. Rev. A; order
rearranged, references replaced and added, corrected typo
Absolute flux density calibrations of radio sources: 2.3 GHz
A detailed description of a NASA/JPL Deep Space Network program to improve S-band gain calibrations of large aperture antennas is reported. The program is considered unique in at least three ways; first, absolute gain calibrations of high quality suppressed-sidelobe dual mode horns first provide a high accuracy foundation to the foundation to the program. Second, a very careful transfer calibration technique using an artificial far-field coherent-wave source was used to accurately obtain the gain of one large (26 m) aperture. Third, using the calibrated large aperture directly, the absolute flux density of five selected galactic and extragalactic natural radio sources was determined with an absolute accuracy better than 2 percent, now quoted at the familiar 1 sigma confidence level. The follow-on considerations to apply these results to an operational network of ground antennas are discussed. It is concluded that absolute gain accuracies within + or - 0.30 to 0.40 db are possible, depending primarily on the repeatability (scatter) in the field data from Deep Space Network user stations
Spatio-temporal impact of climate change on the groundwater system
Given the importance of groundwater for food production
and drinking water supply, but also for the survival
of groundwater dependent terrestrial ecosystems (GWDTEs)
it is essential to assess the impact of climate change on this
freshwater resource. In this paper we study with high temporal
and spatial resolution the impact of 28 climate change
scenarios on the groundwater system of a lowland catchment
in Belgium. Our results show for the scenario period
2070–2101 compared with the reference period 1960–
1991, a change in annual groundwater recharge between
−20% and +7%. On average annual groundwater recharge
decreases 7%. In most scenarios the recharge increases during
winter but decreases during summer. The altered recharge
patterns cause the groundwater level to decrease significantly
from September to January. On average the groundwater
level decreases about 7 cm with a standard deviation between
the scenarios of 5 cm. Groundwater levels in interfluves and
upstream areas are more sensitive to climate change than
groundwater levels in the river valley. Groundwater discharge
to GWDTEs is expected to decrease during late summer and
autumn as much as 10%, though the discharge remains at
reference-period level during winter and early spring. As
GWDTEs are strongly influenced by temporal dynamics of
the groundwater system, close monitoring of groundwater
and implementation of adaptive management measures are
required to prevent ecological loss
On the relation between the Feynman paradox and Aharonov-Bohm effects
The magnetic Aharonov-Bohm (A-B) effect occurs when a point charge interacts
with a line of magnetic flux, while its dual, the Aharonov-Casher (A-C) effect,
occurs when a magnetic moment interacts with a line of charge. For the two
interacting parts of these physical systems, the equations of motion are
discussed in this paper. The generally accepted claim is that both parts of
these systems do not accelerate, while Boyer has claimed that both parts of
these systems do accelerate. Using the Euler-Lagrange equations we predict that
in the case of unconstrained motion only one part of each system accelerates,
while momentum remains conserved. This prediction requires a time dependent
electromagnetic momentum. For our analysis of unconstrained motion the A-B
effects are then examples of the Feynman paradox. In the case of constrained
motion, the Euler-Lagrange equations give no forces in agreement with the
generally accepted analysis. The quantum mechanical A-B and A-C phase shifts
are independent of the treatment of constraint. Nevertheless, experimental
testing of the above ideas and further understanding of A-B effects which is
central to both quantum mechanics and electromagnetism may be possible.Comment: 21 pages, 5 figures, recently submitted to New Journal of Physic
Centimeter-scale secondary information on hydraulic conductivity using a hand-held air permeameter on borehole cores
Saturated hydraulic conductivity (Ks) is one of the most important parameters determining groundwater flow and contaminant transport in both unsaturated and saturated porous media. Determining the small-scale variability of this parameter is key to evaluate implications on effective parameters at the larger scale. Moreover, for stochastic simulations of groundwater flow and contaminant transport, accurate models on the spatial variability of Ks are very much needed. While several well-established laboratory methods exist for determining Ks, investigating the small-scale variability remains a challenge. If several tens to hundreds of metres of borehole core has to be hydraulically characterised at the centimetre to decimetre scale, several hundreds to thousands of Ks measurements are required, which makes it very costly and time-consuming should traditional methods be used.
With reliable air permeameters becoming increasingly available from the late 80’s, a fast and effective indirect method exists to determine Ks. Therefore, the use of hand-held air permeameter measurements for determining very accurate small-scale heterogeneity about Ks is very appealing. Very little is known, however, on its applicability for borehole cores that typically carry a small sediment volume. Therefore, the method was tested on several borehole cores of different size, originating from the Campine basin, Northern Belgium. The studied sediments are of Miocene to Pleistocene age, with a marine to continental origin, and consist of sand to clayey sand with distinct clay lenses, resulting in a Ks range of 7 orders of magnitude. During previous studies, two samples were taken from borehole cores each two meters for performing constant head lab permeameter tests. This data is now used as a reference for the air permeameter measurements that are performed with a resolution of 5 centimetres.
Preliminary results indicate a very good correlation between the previously gathered constant head Ks data and the air permeability measurements, but a systematic bias seems to exist. A geostatistical analysis with cross-validation is performed to assess the predictive uncertainty on Ks, using both types of data.
We conclude that performing hand-held air permeameter measurements on undisturbed borehole cores provides a very cost-effective way to obtain very detailed information in the framework of stochastic simulation and conditioning of heterogeneous hydraulic conductivity fields
Entanglement Evolution in the Presence of Decoherence
The entanglement of two qubits, each defined as an effective two-level, spin
1/2 system, is investigated for the case that the qubits interact via a
Heisenberg XY interaction and are subject to decoherence due to population
relaxation and thermal effects. For zero temperature, the time dependent
concurrence is studied analytically and numerically for some typical initial
states, including a separable (unentangled) initial state. An analytical
formula for non-zero steady state concurrence is found for any initial state,
and optimal parameter values for maximizing steady state concurrence are given.
The steady state concurrence is found analytically to remain non-zero for low,
finite temperatures. We also identify the contributions of global and local
coherence to the steady state entanglement.Comment: 12 pages, 4 figures. The second version of this paper has been
significantly expanded in response to referee comments. The revised
manuscript has been accepted for publication in Journal of Physics
Constraining beyond the Standard Model nucleon isovector charges
At the TeV scale, low-energy precision observations of neutron
characteristics provide unique probes of novel physics. Precision studies of
neutron decay observables are susceptible to beyond the Standard Model (BSM)
tensor and scalar interactions, while the neutron electric dipole moment,
, also has high sensitivity to new BSM CP-violating interactions. To fully
utilise the potential of future experimental neutron physics programs, matrix
elements of appropriate low-energy effective operators within neutron states
must be precisely calculated. We present results from the QCDSF/UKQCD/CSSM
collaboration for the isovector charges and using lattice QCD
methods and the Feynman-Hellmann theorem. We use a flavour symmetry breaking
method to systematically approach the physical quark mass using ensembles that
span five lattice spacings and multiple volumes. We extend this existing
flavour breaking expansion to also account for lattice spacing and finite
volume effects in order to quantify all systematic uncertainties. Our final
estimates of the isovector charges are
and renormalised, where
appropriate, at in the scheme.Comment: 16 pages, 11 figures, 6 table
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