Erzeugung charakteristischer Röntgenstrahlung mittels relativistischer laserbeschleunigter Elektronen

Durch die Fokussierung ultrakurzer, hochintensiver Laserpulse auf dünne Metallfolien (Titan, Kupfer, Silber) wurde charakteristische Röntgen-K-Strahlung erzeugt und hinsichtlich der Abhängigkeit ihrer Ausbeute von der Laserintensität untersucht. Der experimentell gefundene Zusammenhang konnte durch Simulationsrechnungen qualitativ erklärt werden. In dem der Simulation zugrunde liegenden physikalischen Modell wird die Beschleunigung der Plasmaelektronen im Laserfeld durch eine relativistische Elektronenenergienverteilung und die im Festkörper durch Elektronenstöße verursachte K-Schalen-Ionisation durch einen entsprechenden relativistischen Wirkungsquerschnitt berücksichtigt. Neben der Intensitätsabhängigkeit der Strahlungsausbeute wurden auch der Einfluß der Laserpulsenergie sowie die Strahlungsquellgröße experimentell studiert

Dynamic viscoelasticity of actin networks cross-linked with wild-type and mutant [alpha]-actinin-4

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Physics, 2008.In title on title page, "[alpha]" appears as lower case Greek letter.Includes bibliographical references (p. 49-52).The actin cross-linker [alpha]-actinin-4 has been found indispensable for the structural and functional integrity of podocytes; deficiency or alteration of this protein due to mutations disturbs the cytoskeleton and results in kidney disease. This thesis presents rheological studies of in vitro actin networks cross-linked with wild-type and mutant a-actinin-4, which provide insight into the effect of the cross-linker on the mechanical properties of the networks. The frequency dependent viscoelasticity of the actin/[alpha]-actinin-4 networks is characterized by an elastic plateau at intermediate frequencies, and relaxation towards fluid properties at low frequencies. Since the elastic plateau is a consequence of cross-linking, its modulus increases with the [alpha]-actinin-4 concentration. Networks with wild-type and mutant a-actinin-4 differ significantly in their time scales: The relaxation frequencies of networks with the mutant cross-linker are an order of magnitude lower than that with the wild-type, suggesting a slower dissociation rate of mutant [alpha]-actinin-4 from actin, consistent with a smaller observed equilibrium dissociation constant. This difference can be attributed to an additional binding site, which is exposed as a result of the mutation. An increase in the temperature of networks with mutant [alpha]-actinin-4 appears to return the viscoelasticity to that of networks cross-linked by the wild-type. Moreover, the temperature dependence of the relaxation frequencies follows the Arrhenius equation for both cross-linkers. These results strongly support the proposition that the macroscopic relaxation of the networks directly reflects the microscopic dissociation rates of their constituents.by Sabine M. Volkmer Ward.S.M

Dynamic Viscoelasticity of Actin Cross-Linked with Wild-Type and Disease-Causing Mutant α-Actinin-4

The actin cross-linker α-actinin-4 has been found to be indispensable for the structural and functional integrity of podocytes; deficiency or alteration of this protein due to mutations results in kidney disease. To gain insight into the effect of the cross-linker on cytoskeletal mechanics, we studied the macroscopic rheological properties of actin networks cross-linked with wild-type and mutant α-actinin-4. The frequency-dependent viscoelasticity of the networks is characterized by an elastic plateau at intermediate frequencies, and relaxation toward fluid properties at low frequencies. The relaxation frequencies of networks with mutant α-actinin-4 are an order of magnitude lower than that with the wild-type, suggesting a slower reaction rate for the dissociation of actin and α-actinin for the mutant, consistent with a smaller observed equilibrium dissociation constant. This difference can be attributed to an additional binding site that is exposed as a result of the mutation, and can be interpreted as a difference in binding energy barriers. This is further supported by the Arrhenius-like temperature dependence of the relaxation frequencies