Semitransparency in interaction-free measurements

We discuss the effect of semitransparency in a quantum-Zeno-like interaction-free measurement setup, a quantum-physics based approach that might significantly reduce sample damage in imaging and microscopy. With an emphasis on applications in electron microscopy, we simulate the behavior of probe particles in an interaction-free measurement setup with semitransparent samples, and we show that the transparency of a sample can be measured in such a setup. However, such a measurement is not possible without losing (i.e., absorbing or scattering) probe particles in general, which causes sample damage. We show how the amount of lost particles can be minimized by adjusting the number of round trips through the setup, and we explicitly calculate the amount of lost particles in measurements which either aim at distinguishing two transparencies or at measuring an unknown transparency precisely. We also discuss the effect of the sample causing phase shifts in interaction-free measurements. Comparing the resulting loss of probe particles with a classical measurement of transparency, we find that interaction-free measurements only provide a benefit in two cases: first, if two semitransparent samples with a high contrast are to be distinguished, interaction-free measurements lose less particles than classical measurements by a factor that increases with the contrast. This implies that interaction-free measurements with zero loss are possible if one of the samples is perfectly transparent. A second case where interaction-free measurements outperform classical measurements is if three conditions are met: the particle source exhibits Poissonian number statistics, the number of lost particles cannot be measured, and the transparency is larger than approximately 1/2. In all other cases, interaction-free measurements lose as many probe particles as classical measurements or more.Comment: 11 pages, 10 figure

Collective dipole excitations in the stable Cd isotopes

Die Grundlage der Arbeit bilden Kernresonanzfluoreszenz Experimente an den stabilen Cd Isotope. Mit Hilfe der Experimente konnten die Eigenschaften von Dipolanregungen bis zu einer Anregungsenergie von 4MeV in den Cd Isotopen bestimmt werden. Durch die Messung der Polarisation der Streustrahlung konnten insbesondere auch die Paritäten für eine Vielzahl von Zuständen ermittelt werden. Spezielle elektrische Dipolanregungen, die aus der Kopplung des ersten Quadrupol- und des ersten Oktupolphonons resultieren, konnten mit Hilfe eines einfachen Oszillationsansatzes quantitativ verstanden werden. Die Analyse dieser Anregung ergab, dass für die Cd Isotope mit Z=48 und den Te Isotopen mit Z=52 ein Core von N=Z=28 Protonen bzw. Neutronen gegen die übrigen Nukleonen der Isotope schwingt. Für die benachbarten Sn Isotope am Z=50 Schalenabschluss muss dagegen ein Core von N=Z=50 Protonen bzw. Neutronen angenommen werden um die Daten zu erklären. Es wurden auch Kandidaten für gemischt symmetrische Zustände, die mit magnetischen Dipolübergänge angeregt werden, gefunden.The PhD thesis is based on nuclear resonance fluorescence experiments for the stable Cd Isotopes. It was possible to extract information for dipole excitations with excitation energies up to 4MeV from the experiments. The use of Compton polarimeters allowed parity assignments for some excited states. Special electric dipole excitations resulting from the coupling of the first quadrupole- and the first octupole phonon could be explained within a simple oscillation ansatz. The analysis of these excitations shows that for the Cd isotopes with Z=48 and the Te isotopes with Z=52 a core of N=Z=28 protons respectivly neutrons oscillate against the other nucleons in the nuclei. For the Sn isotopes at the Z=50 shell closure a N=Z=50 core must be assumed to explain the data. Also candidates for mixed symmetry states are given. Mixed symmetry states are excited by magnetic dipol excitations