Neutron macromolecular crystallography at the FRM IIThe neutron single crystal diffractometer BIODIFF

The research reactor Heinz Maier-Leibnitz (FRM II) is a modern high flux neutron source which feeds at the present 27 state of the art instruments. The newly build neutron single crystal diffractometer BIODIFF is especially designed to collect data from crystals with large unit cells. The main field of application is the structure analysis of proteins, especially the determination of hydrogen atom positions. BIODIFF is a joint project of the Forschungszentrum Jülich (FZJ/JCNS) and the Forschungs-Neutronenquelle Heinz Maier-Leibnitz (FRM II). Typical scientific questions addressed are the determination of protonation states of amino acid side chains and the characterization of the hydrogen bonding network between the protein and an inhibitor or substrate. BIODIFF is designed as a monochromatic instrument. By using a highly orientated pyrolytic graphite monochromator (PG002) the diffractometer is able to operate in the wavelength range of 2.4 Å to about 5.6 Å. Contaminations of higher order wavelengths are removed by a neutron velocity selector. To cover a large solid angle and thus to minimize the data collection time the main detector of BIODIFF consists of a neutron imaging plate system in a cylindrical geometry. A Li/ZnS scintillator CCD camera is available for additional detection abilities. The main advantage of BIODIFF is the possibility to adapt the wavelength to the size of the unit cell of the sample crystal while operating with a clean monochromatic beam that keeps the background level low. BIODFF is equipped with a standard Oxford Cryosystem “Cryostream 700+” which allows measurements in the temperature regime from 90K up to 500K

Fractal diffusion in high temperature polymer electrolyte fuel cell membranes

© 2018 Author(s). The performance of fuel cells depends largely on the proton diffusion in the proton conducting membrane, the core of a fuel cell. High temperature polymer electrolyte fuel cells are based on a polymer membrane swollen with phosphoric acid as the electrolyte, where proton conduction takes place. We studied the proton diffusion in such membranes with neutron scattering techniques which are especially sensitive to the proton contribution. Time of flight spectroscopy and backscattering spectroscopy have been combined to cover a broad dynamic range. In order to selectively observe the diffusion of protons potentially contributing to the ion conductivity, two samples were prepared, where in one of the samples the phosphoric acid was used with hydrogen replaced by deuterium. The scattering data from the two samples were subtracted in a suitable way after measurement. Thereby subdiffusive behavior of the proton diffusion has been observed and interpreted in terms of a model of fractal diffusion. For this purpose, a scattering function for fractal diffusion has been developed. The fractal diffusion dimension dw and the Hausdorff dimension df have been determined on the length scales covered in the neutron scattering experiments

Polymers: When S(Q,t) is not what you think! The role of dynamic RPA

Investigating a (polymeric) soft-matter system with several components often is aimed at the observation of the motions/mobilities of one of the components. The normal approach to this request in the context of quasielastic neutron scattering (e.g. NSE) is to put contrast on the targeted component, typically by leaving it hydrogenated and have the rest (surrounding, matrix) deuterated. In simple systems with only one molecular species as a homo-polymer melt with a few of the molecules contrasted (e.g. 10% h in 90% d) -provided the h-and d-molecular varieties behave sufficiently equalthis strategy yields a valid single chain (molecule) structure factor S(Q,t)/S(Q), even for a 50/50% mixture. However, if the component of interest and the matrix are of different kind, unexpected distortions of the observed S(Q,t) may occur. Interpretation of such results in terms of the single chain structure factor would then lead to erroneous conclusions. In this contribution the conditions, under which these distortions will occur, are discussed and how dynamic RPA may help to cope with them is explained. A practical method to apply this correction to polymer and similar problems is presented and an experimental verification is discussed [1, 12]

Das SV4-Programmsystem : Ein Betriebsprogramm für die effiziente Nutzung von (Dreiachsen-) Neutronenspektrometern

In Zusammenarbeit mit der Gruppe Zwoll des Zentrallabors für Elektronik (ZEL) wurde dieSteuerungselektronik des Dreiachsen-Neutronenspektrometers SV4a am DIDO Reaktor vollständigerneuert. Im Rahmen dieser Erneuerung wurde ein neues Konzept für das Steuerprogramm desExperimentrechners verwirklicht.Die PDPll der alten Konfiguration wurde durch emen IBM AT03 Rechner und dasCAMAC-Bussystem durch das SMP-Bussystem (Siemens) ersetzt. Eine eigenständige, festprogrammierteSteuereinheit (SS, Siemens) gibt in Abhängigkeit von Endschalter- undSignalisierungskontakten die Motorleistungstreiber frei und schützt damit das Instrument vor Zerstörungendurch Fehlbedienung oder Rechnerfehlfunktionen. Weitere periphere Geräte sind überden IEC-Bus ansteuerbar. Neben einer Erhöhung der Zuverlässigkeit und Senkung der Wartungskostenwurde durch die Umstellung eine weitergehende Automatisierung derSpektrometerfunktionen erreicht. Insbesondere sind ab jetzt die Probentemperatur, ein (odermehrere) durch Spannungen repräsentierte externe Feldparameter, die beiden Kippwinkel am Probenortsowie die Strahlbegrenzerblenden vor und hinter der Probe rechnersteuerbar. Die Blendenund Kristallkippungen für die Justage des Monochromators und der Filter amMonochromatorausgang, die bisher zwar elektrisch angetrieben, aber nur über Handbedienung gefahrenwerden konnten, sind jetzt automatischen, rechnergesteuerten Prozeduren zugänglich.Der Experimentrechner (DIDO-MASTER) kann mit einem weiteren Fernsteuerrechnerkommunizieren, dabei kann der Anschluß über Develnet oder über Direktleitungen erfolgen. DasSpektrometer kann in identischer Weise von beiden kommunizierenden Rechnern bedient werden,diese Funktion wird auch zur Bereitstellung einer vor Ort benötigten Handsteuerung benutzt.Im Rahmen der Erneuerung wurde auch ein vollständig neues Steuerprogramm mit bisher indiesem Zusammenhang unüblichen Eigenschaften erstellt. Es wurde der Tatsache Rechnung[...

Untersuchung von Benzol und Toluolschichten auf den 0001 Flächen von Graphit mittels Neutronenstreuung

The structures of benzene ($C_{6}H_{6}, C_{6}D_{6}$ ) and toluene ($C_{6}H_{5}-CH_{3}, C_{6}D_{5}-CD_{3}$) monolayers on the basal planes of graphite have been investigated by neutron diffraction. The dynamics of the benzene layer has been studied by observing the incoherently, inelastically scattered neutrons using the time-of-flight method. The main results are: Above a phase transition temperature T$_{c} \approx$ 145 K benzene on the basal planes of graphite forms a quasi 2D-fluid with high compressibility. Below this temperature it forms a registered $\sqrt{7}$ x $\sqrt{7}$ structure with the benzene rings lying parallel to the surface. For toluene a fluid phase exists above 140 K, between 70 K and 140 K it forms an incommensurate layer and below 70 K a 3x3 structure has been observed.The fluid phase of adsorbed benzene shows a broad quasielastic scattering indicating an effective surface diffusion coefficient of 10$^{-4}$ cm$^{2}$/s at 200 K. The inelasic spectrum of the solid $\sqrt{7}$ x $\sqrt{7}$ phase has been compared with an appropriate lattice dynamical model. The comparison with the data reveals, that the $\sqrt{7}$ x $\sqrt{7}$ (T = 100 K) can be considered as a fairly anharmonic 2D-solid with a static external poten ial due to the substrate