198 research outputs found
Nonlinear Bethe-Heitler Pair Creation in an Intense Two-Mode Laser Field
We investigate electron-positron pair creation in the interaction of a
nuclear Coulomb field and a highly intense two-mode laser field. For
bichromatic laser fields, we examine the differences arising for commensurable
and incommensurable frequencies in a continuous variation of the laser
frequency ratio and the quantum interference effects, which may occur in the
commensurable case. We show that the interference manifests in the angular
distributions and the total pair-production rates of the created particles.
Additionally, by varying the amplitudes of the two modes we study pair creation
in a monochromatic laser wave of arbitrarily elliptical polarization.Comment: 12 pages, 4 figures, LPHYS'13 Conference Proceeding
Nonperturbative Bethe-Heitler pair creation in combined high- and low-frequency laser fields
The nonperturbative regime of electron-positron pair creation by a
relativistic proton beam colliding with a highly intense bichromatic laser
field is studied. The laser wave is composed of a strong low-frequency and a
weak high-frequency mode, with mutually orthogonal polarization vectors. We
show that the presence of the high-frequency field component can strongly
enhance the pair-creation rate. Besides, a characteristic influence of the
high-frequency mode on the angular and energy distributions of the created
particles is demonstrated, both in the nuclear rest frame and the laboratory
frame.Comment: 7 pages, 5 figure
Terahertz dynamic aperture imaging at stand-off distances using a Compressed Sensing protocol
In this text, results of a 0.35 terahertz (THz) dynamic aperture imaging
approach are presented. The experiments use an optical modulation approach and
a single pixel detector at a stand-off imaging distance of approx 1 meter. The
optical modulation creates dynamic apertures of 5cm diameter with approx 2000
individually controllable elements. An optical modulation approach is used here
for the first time at a large far-field distance, for the investigation of
various test targets in a field-of-view of 8 x 8 cm. The results highlight the
versatility of this modulation technique and show that this imaging paradigm is
applicable even at large far-field distances. It proves the feasibility of this
imaging approach for potential applications like stand-off security imaging or
far field THz microscopy.Comment: 9 pages, 13 figure
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Application of Terahertz radiation to soil measurements: Initial results
Developing soil sensors with the possibility of continuous online measurement is a major challenge in soil science. Terahertz (THz) electromagnetic radiation may provide the opportunity for the measurement of organic material density, water content and other soil parameters at different soil depths. Penetration depth and information content is important for a functional soil sensor. Therefore, we present initial research on the analysis of absorption coefficients of four different soil samples by means of THz transmission measurements. An optimized soil sample holder to determine absorption coefficients was used. This setup improves data acquisition because interface reflections can be neglected. Frequencies of 340 GHz to 360 GHz and 1.627 THz to 2.523 THz provided information about an existing frequency dependency. The results demonstrate the potential of this THz approach for both soil analysis and imaging of buried objects. Therefore, the THz approach allows different soil samples to be distinguished according to their different absorption properties so that relations among soil parameters may be established in future
Bethe-Heitler Pair Creation in a Bichromatic Laser Field
Within this thesis, the non-linear creation of electron-positron pairs in the superposition of a nuclear Coulomb field and a two-colour laser field of high intensity is studied. Primarily, two complementary scenarios are investigated:
On the one hand, if the two laser frequencies are commensurable, quantum interference may occur. This interference manifests in the total pair-creation rate and the angular distribution of the created particles, which are studied in the nuclear rest frame and the laboratory frame. Furthermore, the relative phase between the two laser modes allows to tune the strength of the terms arising from interference. Therefore, this parameter may be used to optimize the pair-creation yield.
On the other hand, for incommensurable frequencies, a set-up of largely differing frequencies is considered. This way, a strong laser field in the non-perturbative regime assisted by a single highly-energetic -photon is described. Due to the assistance of the latter, a strong enhancement of the total pair-creation rate can be found depending on the laser intensity. Additionally, the influence of the -photon on the angular and energetic distribution of the created particles is investigated, again in the nuclear rest frame and the laboratory frame.
Furthermore, the differences arising in the two former cases are directly compared by means of a continuous variation of the laser frequency ratio. This illustrates the strong modifications due to the interference in the commensurable case. Finally, for the special case of two modes with identical frequency, the total pair-creation rate is studied as a function of the ellipticity of the combined laser field. Here, the cases of a constant total field intensity and a constant maximum field intensity are compared
Drilling of shallow marine sulfide-sulfate mineralisation in south-eastern Tyrrhenian Sea, Italy; Seafloor sulfides, Tyrrhenian Sea, highsulfidation; hydrothermal systems, Palinuro
Semi-massive to massive sulfides with abundant late native sulfur were drilled in a shallowwater hydrothermal system in an island arc volcanic setting at the Palinuro volcanic complex in the Tyrrhenian Sea, Italy. Overall, 12.7 m of sulfide mineralisation were drilled in a sediment-filled
depression at a water depth of 630 - 650 m using the
lander-type Rockdrill I drill rig of the British Geological
Survey. Polymetallic (Zn, Pb, Sb, As, Ag) sulfides
overlie massive pyrite. The massive sulfide mineralisation contains a number of atypical minerals, including enargite-famatinite, tennantite-tetrahedrite, stibnite, bismuthinite, and Pb-,Sb-, and Ag-sulfosalts, that do not commonly occur in mid-ocean ridge massive sulfides. Analogous to subaerial epithermal deposits, the occurrence of these minerals and the presence of abundant native sulfur suggest an intermediate to high sulfidation and/or high oxididation state of the hydrothermal fluids in contrast to the near-neutral and reducing fluids from which base metal-rich massive sulfides along mid-ocean ridges typically form. Oxidised conditions during sulfide deposition are likely related to the presence of magmatic volatiles in the mineralising fluids that were derived from a degassing magma chamber below the Palinuro volcanic complex
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