Fakultät für Physik - Digitale Hochschulschriften der LMU - Teil 01/05

Das CRESST Experiment im Gran Sasso Untergrundlabor sucht nach Dunkler Materie in Form von schwach wechselwirkenden massiven Teilchen (WIMPs) ueber elastische Streuung an Kernen.

Die erste Phase von CRESST benutzte 262 g Saphir Kristalle als Absorber. Ein niedriger Untergrund, eine Langzeitstabilitaet des kryogenen Aufbaus, sowie eine niedrige Schwelle und eine hohe Sensitivitaet fuer leichte WIMPs wurden damit erreicht. In einer sehr stabilen Messung von 1.5 kg Tagen wurden ein Untergrund besser als 1 Ereignis/kg/Tag/keV ueber 20 keV und eine Schwelle von 580 eV erreicht. Saphir Detektoren sind besonders geeignet, um leichte WIMPs mit spin-abhaengiger Wechselwirkung zu messen. Die Ergebnisse verbessern existierende Limits in diesem Bereich.

Die zweite Phase von CRESST benutzt szintillierende Kristalle als Absorber. In einem Szintillator wird nach einer Energiedeposition neben Phononen auch Licht erzeugt. Kernrueckstoesse erzeugen weniger Licht als voll ionisierende Wechselwirkungen. Die gleichzeitige Messung des Phononen- und Lichtsignals ermoeglicht eine Identifizierung der Wechselwirkung. Erste Messungen mit 300 g CaWO4 Detektoren wurden im Aufbau am Gran Sasso durchgefuehrt. Trotz einiger technischer Probleme war eine Charakterisierung der Detektoren moeglich, die das grosse Potential der Ereignisdiskriminierung aufzeigt.

HI observations of high-velocity clouds (HVCs) indicate that they are

interacting with their ambient medium. In this thesis,

the question on the dynamical and thermal stabilization

of a cold dense neutral cloud in a hot, thin, and magnetized ambient

halo plasma is addressed. The results of two-dimensional

plasma-neutral gas simulations showing the dynamical evolution of neutral gas

clouds in a plasma flow are presented.

The simulations show the formation of a comet-like head-tail structure

combined with a magnetic barrier which exerts a stabilizing pressure on the

cloud and hinders hot plasma from diffusing into the cloud. Furthermore, the

magnetic barrier significantly reduces the heat conduction between the plasma

and the neutral gas. It is demonstrated that a sufficiently large (but small

compared to typical halo B-field strengths) component of the halo B-field

perpendicular to the cloud's motion effectively stabilizes HVCs against

disruption by Kelvin-Helmholtz- or Rayleigh-Taylor-instabilities and against

evaporation by heat transfer for periods comparable with typical observed

lifetimes of HVCs. The question on the stability of the clouds is addressed in

terms of the stripping of cloud material, the temperature profile of the cloud,

the dependence of the cloud's stability on the infall angle of the cloud,

two-component clouds, and the cloud's morphology. The results are compared with

observations of a compact HVC.

In addition, the observed strong H/alpha fluxes from cloud edges can be explained by applying the critical velocity effect to the magnetic barrier.

In a second part, the thesis addresses the phenomenon of cometary ion-tail disconnection events (DE) in terms of the theory of magnetic reconnection and by means of two-dimensional plasma-neutral gas simulations. The simulations show that either dayside magnetic diffusion and reconnection (when crossing the heliospheric current sheet) and nightside reconnection (without a switch in the magnetic polarity of the solar wind) lead to the disconnection of the plasma tail on the time scale of days, thus reproducing typical observed disconnection events.

In this thesis I report on experiments that enter a new regime in

the many body physics of ultracold atomic gases. A Bose-Einstein

condensate is loaded into a three-dimensional optical lattice

potential formed by a standing wave laser light field. In this

novel quantum system we have been able to both realize a quantum

phase transition from a superfluid to a Mott insulator, and to

observe the collapse and revival of a macroscopic matter wave

field.

Quantum phase transitions are driven by quantum fluctuations and

occur, even at zero temperature, as the relative strength of two

competing energy terms in the underlying Hamiltonian is varied

across a critical value. In the first part of this work I report

on the observation of such a quantum phase transition in a

Bose-Einstein condensate with repulsive interactions, held in a

three-dimensional optical lattice potential. In the superfluid

ground state, each atom is spread-out over the entire lattice,

whereas in the Mott insulating state, exact numbers of atoms are

localized at individual lattice sites. We observed the reversible

transition between those states and detected the gap in the

excitation spectrum of the Mott insulator.

A Bose-Einstein condensate is usually described by a macroscopic

matter wave field. However, a quantized field underlies such a

"classical" matter wave field of a Bose-Einstein condensate. The

striking behavior of ultracold matter due to the field

quantization and the nonlinear interactions between the atoms is

the focus of the second part of this work. The matter wave field

of a Bose-Einstein condensate is observed to undergo a series of

collapses and revivals as time evolves. Furthermore, we show that

the collisions between individual pairs of atoms lead to a fully

coherent collisional phase shift in the corresponding

many-particle state, which is a crucial cornerstone of proposed

novel quantum computation schemes with neutral atoms.

With these experiments we enter a new field of physics with

ultracold quantum gases. In this strongly correlated regime,

interactions between atoms dominate the behavior of the many-body

system such that it can no longer be described by the usual

theories for weakly interacting Bose gases. This novel quantum

system offers the unique possibility to experimentally address

fundamental questions of modern solid state physics, atomic

physics, quantum optics, and quantum information.

The energy and mass budget of the atmosphere is mainly determined

by the exchange at the earths surface. Here plant canopies play a

major role. The 'Localized Near Field Theorie' (LNF) of Raupach

(1989) describes the relation between concentration profiles and

source/sink profiles inside the plant canopy. This work describes

a general method based on the LNF to calculate source

distributions from measured concentration profiles inside a plant

canopy. The LNF is used for the first time to examine the effects

of the so called roughness sublayer. To apply the LNF during

arbitrary thermal stratification it is necessary to find

parametrisations for the input parameters standard deviation of

vertical velocity and Lagrangian integral time scale. These are

derived from values measured inside a pine forest. Using these

parametrisations the distribution of heat sources inside the

forest canopy is calculated from measured temperature profiles.

Am neuen Forschungsreaktor München II (FRM II) wird der Münchener Spaltfragmentbeschleuniger MAFF (Munich Accelerator for Fission Fragments) entwickelt.

Ziele dieses Projektes sind die Produktion von stabilen neuen schweren

Elementen (mit Z > 100) und Experimente zur Untersuchung der Kernstruktur

von neutronenreichen Spaltprodukten und des r-Prozesses in der Astrophysik.

Der LINAC von MAFF besteht aus einem RFQ, einer Boostersektion

mit drei Beschleunigertanks und einer energievariablen Sektion mit zwei

baugleichen 7-Spalt-Resonatoren.

Durch Ein- bzw. Abschalten des dritten Beschleunigertanks

können die Spaltfragmente mit zwei verschiedenen Energien

(4.15 MeV/u bzw. 5.40 MeV/u) in die 7-Spalt-Sektion eingeschossen werden.

Durch Nachbeschleunigen bzw. Abbremsen soll stufenlos ein Endenergiebereich

von 3.7 MeV/u bis 5.9 MeV/u abgedeckt werden können.

Alle Komponenten des LINAC sind als IH-Resonatoren ausgeführt.

Im Mittelpunkt dieser Arbeit steht die Entwicklung und der Bau der 7-Spalt-Resonatoren.

Diese wurden für eine mittlere Teilchengeschwindigkeit von β = 0.10 und einem Masse/Ladungs Verhältnis von A/q ≤ 6.3 ausgelegt.

Die Resonatoren sollen bei einer Frequenz von 202.56 MHz (CERN-Frequenz) und einer gepulsten Leistung bis max. 100 kW betrieben werden (Tastverhältnis = 1:10).

Nach Festlegung dieser Parameter wurden durch Simulationen mit dem Code MAFIA die Dimensionen eines 1:1 Modells bestimmt, an dem dann durch Variation der Geometrien die Resonatoreigenschaften experimentell untersucht wurden.

Mit den Programmen LINAC und LORASR wurde die Strahldynamik untersucht. Die Rechnungen zeigten transversal und longitudinal ein geringes Emittanzwachstum von unter 5 % für die extremsten Beschleunigungs- und Abbremsszenarien.

An einem Leistungsresonator wurden anschließend sowohl Niederleistungsmessungen zur Verifizierung der Rechnungen durchgeführt, als auch Hochleistungtests, um zu

demonstrieren, daß die geforderten Designspannungen und damit auch die entsprechenden Beschleunigungen erreicht werden können.

Dazu wurde am Maier-Leibnitz-Labor eine Test-Stahllinie aufgebaut, in der ein 16O5+ Ionenstrahl mit 4.15 MeV/u (DC und gepulst) in den Resonator eingeschossen und erfolgreich beschleunigt bzw. abgebremst wurde.

Die dabei ermittelte Shuntimpedanz von 120 MΩ/m für hohe Senderleistung läßt bei maximaler Senderleistung theoretisch eine Gesamtresonatorspannung von bis zu 2.4 MV erwarten, wodurch ein Endenergiebereich von 3.6 MeV/u bis 6.0 MeV/u abgedeckt werden kann.

Der IH-Resonator läßt eine einfache Modifikation der Driftr¨ohrenstruktur zu.

Es ist vorgesehen, bis zur Fertigstellung von MAFF, den schon gebauten Prototyp als 9-Spalt-Resonator in zwei einfach umzubauende Versionen beim REX-ISOLDEProjekt am CERN einzusetzen.

Die für eine Leistungssteigerung von 2.2 MeV/u auf 3.1 MeV/u nötigen Komponenten der Driftröhrenstruktur wurden bereits gefertigt und probeweise in den Resonatortank eingebaut.

Bei Niederleistungstest von Resonanzfrequenz und Spaltspannungsverteilung wurde eine sehr gute Übereinstimmung mit den Simulationen festgestellt.

Die gemessene Resonanzfrequenz lag dabei nur ca. 0.05% über der berechneten Frequenz.

Die Güte Q wurde bei den Niederleistungsmessungen mit 9833 bestimmt.

The inclusive rate of leptons produced from charmed particle decays (from Z -> charm anticharm) has been measured using a data sample of four million hgadronic Z decays recorded by the ALEPH experiment at LEP from 1991 to 1995. The analysis uses a double-tag and is self-calibrating, i.e. it makes minimum use of the Monte Carlo simulated information. Each event is divided into two hemispheres; hemispheres containing charm quarks are identified with a NEural Network anlaysis method. Lepton candidates are then searched for in the opposite hemisphere. Based on fully reconstructed D candidates, a value of BR(c -> l + X) = 9.09 +/- 0.61 % (l = e or mu) is obtained. The uncertainy includes both the statistical and systematic components.

Molecular machines are at the ultimate limit of miniaturization. Living organisms provide a variety of examples for such molecular machines, but in order to utilize and to control them, they need to be interfaced with the macroscopic world. On the other h and, there are synthetic molecular machines. Some have been interfaced already but usually in high vacuum at very low temperatures, which is clearly not desirable for technical applications. In this thesis, AFM-based single molecule force spectroscopy (S M FS) was utilized to investigate the mechanical change in single synthetic molecules upon environmental changes (external stimuli) in liquid environment at room temperature. The molecules are either from theory or from bulk experiments supposed to be ab le to convert such an external stimulus into mechanical work, which is a prerequisite for molecular motors. Three different types of molecules and various external energy inputs were investigated which led to the realization of a light driven synthetic mo le cular machine:

- Polyelectrolytes should, by OSF-theory, change their persistence length (and therefore the overall length at a constant force) with the Debye screening length of the solvent (which is manipulated by the salt concentration). Therefore, t he elasticity of the polyelectrolyte polyvinylamine, which could be covalently attached to the substrate and the AFM tip, was investigated in dependence on the salt concentration. It was found that the dependence of persistence length on salt concentr atio n is much smaller than expected from OSF-theory, which made this system less attractive for a molecular machine, but led to new theoretical insights.

- The adhesive properties of polyelectrolytes onto charged solid supports in aqueous solution are a subj ect of current research in industry and academia. A manipulation of polymer – substrate adhesion, e. g. at an AFM tip, could lead to a molecular 'grab and release' device. Therefore, the desorption force of single polyvinylamine-molecules from solid suppo rts was investigated. Polyvinylamine was physisorbed to a glass substrate and covalently attached to the cantilever. Then, the charge-charge interaction was manipulated by variation in salt concentration and polymer charge. While this has not led t o a sin gle molecule device yet, it gave new insights into the desorption of polyelectrolytes from charged substrates. The measurements performed here revealed that van der Waals forces and other non-covalent chemical interactions such as hydrogen and coo rdinative bonds can by far outweigh the electrostatic coulomb force (namely at short distances), and are therefore a more promising candidate for the tuning of adhesion forces.

- Elastin-based polypeptides have proven various kinds of energy conversion i n cross-linked bulk samples. The mechanism is based on a hydrophobic folding transition, which can be manipulated by temperature, salt, pH, electrochemistry, and/or by the composition (hydrophobicity) of the polymers. The difference between the folded and random state could be detected and investigated here at the level of individual polymer chains and characterized by the force-extension traces of the two polypeptides (GVGVP)nx251 and (GVGIP)nx260. Because of their different hydophobicity their folding t emperatu res lie above and below room temperature, respectively. With the polypeptide (GVGIP)nx260 the folded state was investigated extensively. All observations support the conjecture, that intermolecular aggregation dominates intramolecular aggregatio n. This i s further supported by the finding that neither a change in temperature nor the treatment with sodium dodecyl sulfate or guanidinium hydrochloride could force any of the two polypeptides from the folded to the random state or vice versa within a n experime nt, which in turn would be a prerequisite for a polypeptide based molecular motor.

- The most successful approach to building an AFM-interfaced molecula

The present work gives an overview on the experimental results of the spectroscopy in the second minimum of the double-humped fission barrier of ^240Pu and the derived conclusions on the structure of collective excitation modes in deformed nuclei. From the combined analysis of γ-decay and conversion electron measurements and an experiment on the transmission resonances in the prompt fission probability a detailed level scheme for the second minimum of an actinide nucleus could be established for the first time. Excitation energies, spins, parities and lifetimes of collective excitations above the groundstate band of the isomer up to β-vibrational multiphonon excitations could be established.

Contrary to the experiments in the high-spin region of superdeformed nuclei, which were promoted extraordinarily during the last years, the K-purity of the isomer states populated with low spins allows for a clear separation of vibrational and rotational excitations. Therefore low-lying collective vibrational bands could be identified unambiguously as K^π i=0^-, 1^- and 2^- -octupole bands, as well as the lowest-lying β-vibrational band. After determining the energy of the β-vibrational phonon it became possible to identify the resonance groups in the prompt fission probability as the third and fourth β-vibrational phonon. The complete spectroscopy of the K^ π =0^+ states in the transmission resonance experiment made it possible to determine the excitation energy of the fission isomeric ground state from measured level densities with a recently developed method. Very interesting was furthermore the observation of the predominant population of negative parity states (98 %) in the second minimum, most likely attributed to a filtering action of the inner and outer fission barrier. Finally the measured rotational bands give hints on the behaviour of the moments of inertia of superdeformed nuclei with increasing excitation energy.

The measurements could help to better understand the multiphonon states in the first minimum, where much stronger mixing with quasiparticle states occur.