Fakultät für Physik - Digitale Hochschulschriften der LMU - Teil 04/05 Archiv

This thesis presents inclusive ep double and single differential cross section measurements for neutral current deep inelastic scattering of longitudinally polarized leptons on protons as a function of the negative four-momentum transfer squared Q2 and the Bjorken variable x. The data were collected in the years 2003-2007 in the H1 experiment at HERA with positively and negatively longitudinally polarized lepton beams of 27 GeV and a proton beam of 920 GeV corresponding to the centre-of-mass energy of sqrt{s} = 319 GeV. The integrated luminosity is about 330 pb^{-1}. An overview of the phenomenology of the deep inelastic scattering is given and the experimental apparatus is described. The NC cross section measurement procedure is presented and discussed in details. The measured cross sections are used to investigate electroweak effects at high Q2. The proton structure function xF_3, sensitive to the valence quarks in the proton, is measured. The polarization effects sensitive to the chiral structure of neutral currents are investigated. The Standard Model predictions are found to be in a good agreement with the measurement.

Laser-ion acceleration has been of particular interest over the last decade for fundamental as well as applied sciences. Remarkable progress has been made in realizing laser-driven accelerators that are cheap and very compact compared with conventional rf-accelerators. Proton and ion beams have been produced with particle energies of up to 50MeV and several MeV/u, respectively, with outstanding properties in terms of transverse emittance and current. These beams typically exhibit an exponentially decaying energy distribution, but almost all advanced applications, such as oncology, proton imaging or fast ignition, require quasimonoenergetic beams with a low energy spread. The majority of the experiments investigated ion acceleration in the target normal sheath acceleration (TNSA) regime with comparably thick targets in the μm range. In this thesis ion acceleration is investigated from nm-scaled targets, which are partially produced at the University of Munich with thickness as low as 3 nm. Experiments have been carried out at LANL’s Trident high-power and high-contrast laser (80,J, 500 fs, t=1054nm), where ion acceleration with these nano-targets occurs during the relativistic transparency of the target, in the so-called Breakout afterburner (BOA) regime. With a novel high resolution and high dispersion Thomson parabola and ion wide angle spectrometer, thickness dependencies of the ions angular distribution, particle number, average and maximum energy have been measured. Carbon C6+ energies reached 650MeV and 1GeV for unheated and heated targets, respectively, and proton energies peaked at 75MeV and 120MeV for diamond and CH2 targets. Experimental data is presented, where the conversion efficiency into carbon C6+ (protons) is investigated and found to have an up to 10fold (5fold) increase over the TNSA regime. With circularly polarized laser light, quasi-monoenergetic carbon ions have been generated from the same nm-scaled foil targets at Trident with an energy spread of as low as ±15% at a central energy of 35MeV. High resolution kinetic simulations show that the acceleration is based on the generation of ion solitons due to the circularly polarized laser. The conversion efficiency into monoenergetic ions is increased by an order of magnitude compared with previous results in the TNSA regime. The advances in ion energies and the control over the spectra mark an important basis for future research of laser-driven ion acceleration and might enable laser-based implementation of these applications in the future.

Chemotaxis, the ability of cells to detect and migrate directly towards a source of a chemically active agent, is the result of a sophisticated interplay of proteins within a complex regulatory network. However, partially redundant pathways that simultaneously mediate chemotaxis and dynamic protein distributions complicate the experimental identication of distinct signaling cascades and their inuence on chemotactic migration. Yet, increasingly precise generation and rapid modication of chemotactic stimuliin microuidic devices promise further insight into the basic principles of cellular feedback signaling. I developed a Chemotactic Gradient Generator (CGG) for the exposure of living cells to chemotactic gradient elds with alternating gradient direction based on a double T-junction microuidic chamber. A large extension of the concentration gradients enables the parallel exposure of several dozens of cells to identical chemotactic stimuli, allowing for a reliable quantitative analysis of the chemotactic migration behavior. Two pressure pumps and a syringe pump facilitate accurate control of the inow velocities at the individual ow chamber inlets, pivotal for precise manipulation of the chemotactic stimuli. The CGG combines homogeneous gradients over a width of up to 300 µm and rapid alterations of gradient direction with switching frequencies up to 0.7 Hz. Fast gradient switching in our experimental design facilitates cell stimulation at the intrinsic time scales of their chemotactic response as demonstrated by a gradual increase in the switching frequency of the gradient direction. We eventually observe a "chemotactically trapped" state of Dictyostelium discoideum (D. discoideum) cells at a switching rate of 0.01 Hz. Here, gradient switching proves too fast for the cells to respond to the altered gradient direction by migration. In contrast, we observe oscillatory runs at switching frequencies of less than 0.02 Hz. We distinguish between re-polymerizing cells that exhibit an internal re-organization of the actin cortex in response to chemotactic stimulation and stably polarized cells that gradually adjust their leading edge when the gradient is switched. To experimentally characterize both response types, we record cell shape and the intracellular distribution of actin polymerization activity. Cell shape is readily described by the eccentricity of the cell and to record F-actin polymerization dynamics we introduce a fluorescence distribution moment (FDM). Accurate description of the migratory response behavior facilitates a quantitative analysis of the inuence of both the experimental boundary conditions such as gradient shape, ongoing starvation of the cells, and in particular the inuence of distinct signaling cascades on chemotactic migration. Here, we demonstrate this ability of the GCC by inhibition of PI3-Kinase with LY 294002. PI3-Kinase initiates the formation of fresh pseudopods in the direction of the chemotactic gradient and therefore is one of the key signaling pathways mediating the chemotactic response. In shallow gradients and with ongoing starvation of the cells, we find a decreased ratio of re-polymerizing cells, pointing towards a diminished influence of PI3-Kinase. After inhibition of PI3-Kinase, cell re-polymerization in response to a switch in gradient direction is hindered at 5h of starvation, whereas at 7h of starvation evidence is found that chemotactic migration is more efficient. We observe the astonishing result that in dependency of the boundary conditions of the experiment inhibition of PI3-Kinase promotes an effective chemotactic response. Thus, the CGG for the rst time facilitates a quantitative analysis of the starvation time dependent effect of PI3-Kinase inhibition on D. discoideum chemotaxis.