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E-Verhandlungen 1998
Programm und Abstacts der Sitzung HK 19

H. Abele, D. Dubbers, M. Klein, U. Schmidt, R. Simon und C. Stellmach
Physikalisches Institut der Universität Heidelberg, Philosophenweg 12, D-69120 Heidelberg, Germany

For Ultra Cold Neutrons (UCN) penetrating a thin ferromagnetic layer the coupling between spin and spatial wave function is not negligible. Here the kinetic energy of UCN is of the same order of magnitude than the energy of Zeeman splitting in a strong magnetic field. This leads to an additional phase of the precession angle of polarized UCN compared to the normal Lamor precession. Even if the potential barrier of the ferromagnetic layer is higher than the kinetic energy of the UCN the precession angle is still observable. Due to the fact that the precession angle is proportional to the flight time through a magnetic field, tunneling time for UCN penetrating a ferromagnetic layer can be measured. First experimental results will be presented.

H. Abele, A. Boucher, M. Klein, U. Schmidt und C. Stellmach
Physikalisches Institut der Universität Heidelberg, Philosophenweg 12, D-69120 Heidelberg, Germany

An unpolarized Ultra Cold Neutron (UCN) beam becomes polarized after passing through a suitable RF-field and static magnetic field. In contrast to commonly used spin filtering polarizer both spin components of the initial unpolarized UCN beam will be found in one final spin state. Therefore the intensity of the polarized neutrons is enhanced up to a factor of two compared to a spin filtering polarizer. Liouville's theorem is conserved due to the change in energy of one of the initial spin component. The efficiency of polarization is determined by the energy range and the divergence of the UCN and the homogeneity of the magnetic fields. Experimental results will be presented. Further more the rice of polarization in an initially unpolarized system is commonly regarded [1] as an indicator of parity nonconservation. In contrast to this general consideration the induced polarisation of UCN is an effect of purely electromagnetic origin and hence parity conserving.

[1] L. Stodolsky Phys.Lett. 50B, 352 (1974)

J. Kasper¹, H. Leeb¹ und R. Lipperheide^2
1Institut für Kernphysik, Technische Universität Wien , Wiedner Hauptstraß e 8-10, A-1040 Wien
²Hahn-Meitner-Institut Berlin, Glienickerstraß e 100, D-14109 Berlin

A novel neutron specular reflection method is presented which yields the full reflection coefficient (amplitude and phase) for non-magnetic and magnetic samples. It makes use of the interference effects between the reflections of a known reference layer and those of the investigated profile, which can be detected by polarisation measurements. For magnetic samples one can determine both non-spin-flip and spin-flip matrix elements and thus reconstruct internal magnetic fields. Simulations show good stability against measurement errors and the effects of surface roughness. The method is suited to provide valuable insight into the magnetic structure of multilayers and superlattices since it yields the necessary information for an unambiguous reconstruction of the scattering-length density and the magnetisation profile.

*) This work has been suported by Jubiläumsstiftung der Oesterreichischen Nationalbank, project number 6212.

•H. Leeb, M. Hochhold und F. Korinek
Institut für Kernphysik, TU Wien, Wiedner Hauptstraße 8-10, A-1040 Wien, Austria

Recently we proposed a tomographic extension of the three-dimensional neutron depolarization technique [1]. It is conjectured that the novel method allows the direct visualization of the magnetic domain structure within bulk ferromagnetic materials for the first time. The corresponding tomographic problem is much more involved than that of conventional x-ray tomography because of the tensorial character of the measured data. No analytical solution for the backprojection of the polarization data is presently known and it is even not clear whether a retrieval of the internal magnetic field distribution is possible. In this contribution we present numerical algorithms which take into account the tensorial character of the tomographic problem. Using simulated data we demonstrate the feasibility of the analysis of tomographic depolarization data. Our results indicate that the reconstruction of the magnetic domain structure within bulk materials is indeed possible. The experimental implementation of the method is discussed.

Work supported by FWF, project No. P10969-PHY

[1] M. Hochhold, H. Leeb and G. Badurek, J. Magn. Magn. Materials 157/158, 575 (1996).

•A.  Paul, S.  Röttger, A.  Zimbal und U.  Keyser
Physikalisch-Technische Bundesanstalt (PTB), Bundesallee 100, 38116 Braunschweig

Im Rahmen eines Forschungsvorhabens wird die rein kernphysikalische Methode der Isotopenanalyse durch die Messung der prompt emittierten g-Quanten nach thermischem Neutroneneinfang zu einem Meßplatz für die zerstörungsfreie qualitative und quantitative Bestimmung der Zusammensetzung beliebiger Proben ausgebaut.

Mittels der prompt (n,g)-Spektrometrie können Atommassendifferenzen mit Unsicherheiten bis zu 10^-10 und Vergleiche von Isotopenzusammensetzungen auf bis zu 10^-7 bestimmt werden. Die Grundlagen und deren Erweiterung bzgl. der möglichst genauen Bestimmung von Einfangquerschnitten thermischer Neutronen und daraus folgend die Bestimmung von absoluten Isotopenhäufigkeiten werden vorgestellt und diskutiert.

•Stefan Röttger, Annette Paul, Andreas Zimbal und Uwe Keyser

Physikalisch-Technische Bundesanstalt (PTB), Bundesallee 100,
38116 Braunschweig

Im Rahmen einer internationalen Kollaboration der metrologischen Staatsinstitute soll der Artefakt der Masseneinheit - das Urkilogramm - durch einen hochreinen, perfekten Einkristall aus natürlichem Silicium ersetzt werden. Dazu müssen neben den Atommassen der beteiligten Si-Isotope mit den Massenzahlen A=28,29,30 auch deren Isotopenhäufigkeiten so genau wie möglich bekannt sein.

Die einzige Methode, mit der all diese Fragestellungen zerstörungsfrei an massiven Proben gelöst werden können, ist die prompt (n,g)-Spektrometrie mit thermischen Neutronen.

Im Rahmen eines CRG-C Vertrages "`AVOGADRO"'-Projekt wird der neue Messaufbau am Hochfluß reaktor des ILL vorgestellt. Die bisher erzielten Ergebnisse für die Massendifferenzen und Isotopenverhältnisse werden vorgestellt und erste Experimente zur Verringerung der Unsicherheiten für die Einfangquerschnitte thermischer Neutronen diskutiert.

•G. Datzmann, G. Dollinger, C. Goeden, O. Schmelmer und H.-J. Körner

Physik-Department E12, TU-München, 85748 Garching

A new nanoprobe for energetic ions (e.g. 25 MeV protons) will be installed at the Munich Tandem accelerator. Therefore a superconducting multipole lens has been designed to achieve a beam diameter of 100 nm at the focal plane [1]. The facility aims to investigate special topics concerning solid state physics, biology and medicine. Compared to electron microscopy and microprobes using ions of lower energy, this method provides a reduced angular straggling in thick targets ( > 1 mm). Furthermore the high ion energies enable new experimental techniques. A 25 MeV protonbeam for example offer a good possibility for the detection of hydrogen in submicron dimensions, using coincident detection of elastic proton-proton-scattering events. Scanning Transmission Ion Microscopy (STIM)-experiments with heavy ions belong to another type of experiments planned for this facility. Due to the high stopping power of heavy ions in matter and a large 90^° magnetic analyser, it will be able to perform thickness or density measurements with a high accuracy.

[1] G. Dollinger et al., NIM B, 130 (1997) 51

•O. Beck¹, Y. Finkelstein², R. Moreh², D. Jäger¹, U. Kneissl¹, J. Margraf¹, H. Maser¹ und H.H. Pitz^1
1Institut für Strahlenphysik, Universität Stuttgart, D-70569 Stuttgart
²Physics Department, Ben-Gurion University of the Negev, Beer Sheva 84105, Israel

Nuclear resonance scattering of bremsstrahlung, employing nuclear self-absorption from the 3089 keV and the 3684 keV levels in ¹³C has been used to determine the effective temperatures T_|| and T_^ of the highly oriented pyrolytic graphite (HOPG) at 10 K, 295 K, 495 K, and 683 K in directions parallel and perpendicular to the hexagonal layers. The effective temperature of isotopic diamond (¹³C) at 295 K was also measured. The experiments were performed at the photon absorption and scattering setup [1] installed at the Stuttgart 4 MV Dynamitron accelerator. In HOPG the nuclear self-absorptions for parallel and perpendicular directions were quite different revealing a significant difference in the values T_|| and T_^. Comparison between the observed values and those deduced from the vibrational density of states of HPOG agree (within two standard deviations) in T_^, but the measured T_|| is about 25% higher. A larger deviation was observed in diamond. The results are discussed in view of the calculated phonon spectra of HOPG and diamond.
[1] R. Moreh et al., Phys. Rev. B 56 , (1997), 187.

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