Jürgen Neuhaus

Diffusive motions in liquid medium-chain n-alkanes as seen by quasielastic time-of-flight neutron spectroscopy

Different diffusive motions in liquid C(32)H(66) on a picosecond time scale could be disentangled by resolution resolved quasielastic time-of-flight neutron spectroscopy (QENS). It is demonstrated that at all observation times, the dominating motion causes a Q(2) proportionality of the QENS signal, which indicates a Fickian diffusion mechanism. The observed motions can be characterized by an observation time dependent apparent diffusion coefficient D(a)(t(o)), which is up to one order of magnitude larger than the molecular self-diffusion coefficient D(s). By comparison with molecular dynamics simulations, the identified motions are attributed to displacements of hydrogen atoms reflecting not only global but also local molecular trajectories. Despite the rodlike shape of the molecules, the center of mass diffusion was found to be essentially isotropic. A coherent picture of the diffusional processes ranging from the fast tumbling of CH(2) groups to the slow long range molecular diffusion is presented.

Role of vibrational entropy in the stabilization of the high-temperature phases of iron

The phonon dispersions of the bcc and fcc phases of pure iron ({\alpha}-Fe, {\gamma}-Fe and {\delta}-Fe) at ambient pressure were investigated close to the respective phase transition temperatures. In the open bcc structure the transverse phonons along T1 [{\xi}{\xi}0] and T1 [{\xi}{\xi}2{\xi}] are of particularly low energy. The eigenvectors of these phonons correspond to displacements needed for the transformation to the fcc {\gamma}-phase. Especially these phonons, but also all other phonons soften considerably with increasing temperature. Comparing thermodynamic properties of the fcc and the two bcc phases it is shown that the high temperature bcc phase is stabilized predominantly by vibrational entropy, whereas for the stabilization of the fcc phase electronic entropy provides an equal contribution.

Scientific Review: The Time-of-Flight Spectrometer TOFTOF

The TOFTOF spectrometer is a disc chopper, time-of-flight spectrometer and is positioned in the neutron guide hall of the FRM-II, some 60 m away from the reactor core. All the instruments in the neutron guide hall, including the TOFTOF spectrometer, are powered with neutrons from the cold source operating with liquid D2 at 25 K. The remote position of the spectrometer, in combination with an elaborate shielding concept and the s-shaped curved primary neutron guide, which acts as a neutron velocity filter, results in an excellent signal-to-background ratio (see next). The primary spectrometer is equipped with a system of seven high-speed carbon fiber chopper discs and a focusing secondary neutron guide with supermirrors (m ≤ 3.6), which reduces the cross section of the primary beam from 44 mm (width) × 100 mm (height) to 23 mm × 47 mm. Including the divergence of 6 Å neutrons, the beam size at sample position is about 35 mm × 60 mm. It can be reduced in width and height with an automated double slit system 18 cm in front of the sample. The primary beam monitor, a calibrated fission chamber, is positioned directly behind this slit system.

Correspondent's Report: Forschungsneutronenquelle Heinz Maier-Leibnitz (FRM II)

Science with neutrons has a longstanding tradition in Germany, since its foundation in 1957, by Heinz Maier-Leibnitz, when the first neutron source Forschungsreaktor München (FRM) started operation. With this first nuclear reactor in Germany, a widely spread range of applications from neutron scattering to irradiations and medical cancer treatment was envisaged from the beginning. Following the motto of Maier-Leibnitz, “Do some thing new,” the development of neutron instrumentation played an important part in the activities at FRM. High precision measurements of the scattering length of thermal neutrons, neutron guides, small angle scattering, back scattering, turbine for ultra cold neutrons, and neutron irradiation at liquid He temperatures, all these techniques are strongly connected to the FRM, since most were applied first in Munich

E-learning neutron scattering

Volume 24 • Number 1 • 2013 Neutron News 18 Introduction to the VNT project Since experimental neutron scattering is mostly restricted to large-scale facilities, not all students have access to learning the technique at their home institution. Providing a freely accessible e-learning portal for neutron scattering is therefore an important outreach task in order to secure and educate the future users and scientists at neutron scattering facilities. This task and challenge has been taken up by the Integrated Infrastructure Initiative for Neutron Scattering and Muon Spectroscopy (NMI3). In 2010, we initiated investigations into the possibilities for e-learning neutron scattering. Subsequently, a pilot project started in 2011, through co-funding from the University of Copenhagen, that is now (2012) growing to a full-scale 4-year international e-learning project called Virtual Neutrons for Teaching (VNT) [1] with support from NMI3-II via the “E-learning neutron scattering” work package. The collaborators in the project are based at the University of Copenhagen and the research reactors FRM-II (Technical University of Munich) and ILL in Grenoble. The people involved in the collaboration span a broad range of competences from didactics and teaching in physics, through neutron scattering in theory, simulation and experiment to programming and web-design. We aim E-learning neutron scattering