4. X-RAY LABORATORY ________________________________________________________________________________________________

4.1. PREFACE

The compulsory financial cut in the budget of the Faculty of Science also affected the X-ray laboratory in 1997. More than half of the laboratory personnel were paid using external funding. Altogether MFIM 1.67, about MFIM 0.5 more than in 1996, was obtained from various sources, mainly from the Academy of Finland, TEKES and the European Synchrotron Radiation Facility (ESRF). Two members of the laboratory, Veijo Honkimäki and Pekka Suortti spent the whole of 1997 at ESRF. This trend is likely to be continued which means ever increasing efforts to cover the annual budget with external financial support.

The main development in the laboratory equipment was the purchase of a surface diffractometer from Huber at the end of 1997. This was made possible by common financing from a group within the Faculty of Science; Department of Inorganic Chemistry and Department of Physics. After the installation of the necessary electronics and controlling devices the diffractometer can be used to study surfaces and thin films, area in common for many research groups in the departments of Physics and Chemistry.

Synchrotron radiation has been used for many experiments. Altogether 14 scientific visits were accomplished in 1997; to ESRF, to the National Synchrotron Light Source (NSLS) and to the Hamburg Synchrotron Radiation Laboratory (HASYLAB). The main topics of the experiments were (i) high resolution inelastic scattering and fluorescence spectroscopy (ESRF, NSLS), (ii) surface physics (HASYLAB) and (iii) small angle scattering (HASYLAB). A co-operative project with the Finnish Space Research Centre was also accomplished at NSLS; the properties of a semiconductor detector were studied for use in an instrument in an X-ray telescope to be used by a satellite to be launched in 1998.

Experimental work in our home laboratory largely concentrated on the applications of wide angle (WAXS) and small angle (SAXS) X-ray scattering. There were research partners from the Helsinki University of Technology (tailored polymers), Department of Chemistry (PVF and PVDF films and membranes, ion-implanted SrS:Ce thin films, potato tubers) and the Finnish Forest Research Institute (structure of wood cells) included in several research projects.

The international exchange schemes, including those with the Russian Academy of Sciences and Charles University (The Czech Republic), were continued. The four-circle diffractometer was involved in these projects to study, for example, structure studies of some non-linear optical materials, high-temperature superconductors and uranium intermetallic compounds.

Industrial collaboration with the Finnish Science Center Heureka and Quantronics Oy aimed at production of an Expo-Flu Fluorescence Analyser. The prototype was constructed at Quantronics Oy and has been installed at Heureka. It was also introduced to the European Science Centre community in November in Brussels.

Seppo Manninen
Laboratory Chairman

4.2. SOLID STATE SPECTROSCOPY

THE CuKalpha1,2 h HYPERSATELLITES AND CORRELATED HYPERSATELLITES

Keijo Hämäläinen, Simo Huotari, C.-C. Kao* and M. Deutsch**

We have continued the project to measure emission line satellites related to the multiple ionization. The present interests have been the hypersatellites (HS) and correlated hypersatellites (CHS) which originate in transitions with two K holes as the initial state. Their theoretical cross sections in Cu are 3 and 6 orders of magnitude, respectively, lower than that of the corresponding diagram Kalpha1,2 lines. Although important for elucidating the intra-K-shell electronic correlations, only a single measurement exists for the HS, in which the Ka1h line is masked by the overlapping WLalpha2 line. For the CHS, the only extent measurement claims cross sections 3 orders of magnitude larger than the theoretical value. The experiments have been carried out using synchrotron radiation at the beamline X25 at the Brookhaven National Laboratory.

* NSLS, Brookhaven National Laboratory, USA ** Bar-Ilan University, Israel

INELASTIC SCATTERING STUDY OF ADVANCED SEMICONDUCTOR AND HIGH TEMPERATURE SUPERCONDUCTOR MATERIALS

Keijo Hämäläinen, Jarkko Laukkanen, Seppo Manninen, L. Dobrzynski* T. Buslaps**, A. Shukla** and A. Bansil***

This project is related to the studies of the basic electronic properties of selected semiconductor and high-Tc materials. Zn1-xMgxSe is a wide gap semiconductor, one of the possible blue laser candidates. The role of the Mg doping is to control the band gap size. Compared with GaN, measured previously, they have similar band gaps but quite different cohesion energies and long term stabilities. The reason might arise from different type of bonding which can be studied using inelastic scattering from single crystal samples. La2-xSrxCuO4 is a high temperature superconductor which has been often used as a target of various theoretical approaches. Our goal has been to trace the Fermi surface features. All experiments have been made at ESRF, in 1998 new experiments with varying the Sr content will be performed.

* Department of Physics, Univ. Warsaw, Poland
** ESRF, Grenoble, France
*** Dept. of Physics, Northeastern University, Boston, USA

FLUORESCENCE ANALYSIS OF NOBLE METALS IN JEWELLERY AND MINE ROCKS

Jarkko Laukkanen, Keijo Hämäläinen, Seppo Manninen, P. Jalas*, J. Vierumäki* and Veijo Honkimäki**

Our research on applications of fluorescence analysis of noble metals in samples with varying shape, composition and origin have been continued and expanded. Together with our industrial partner, we have designed and developed a self-contained, automatic quantitative fluorescence analyzer based on the fundamental parameter approach, for the science center and museum visitor use. The prototype unit is currently installed for the permanent main exhibition at the Finnish science center Heureka. Since it's introduction the unit has been delivering several hundred analyses every week reliably.

Research on the quantitative fluorescence analyzer for mine rock samples has advanced to the development model stage for a portable and/or on-line fluorescence analyzer. The prototype will be built during the year 1998.

* Quantronics Oy, Helsinki, Finland
** ESRF, Grenoble, France

MAGNETIC COMPTON SCATTERING OF HIGH ENERGY X-RAYS

J.E. McCarthy*, M.J. Cooper**, Veijo Honkimäki*, T. Tschentscher*, Pekka Suortti*, S. Gardelis**, Keijo Hämäläinen, Seppo Manninen and D.N. Timms***

The final analysis of the high energy magnetic Compton scattering data, measured in 1995-6 at ESRF has been made [1]. X-ray photon energies up to 1 MeV were used to study the relativistic scattering cross section for circularly polarised photons. The magnetic scattering contribution was obtained by taking the difference between the scattered data measured using the opposite magnetisation directions. The other aim was to determine the optimal photon energy in this type of studies. The experimental cross section as a function of the incident energy could be explained by spin-dependent relativistic calculations.

* ESRF, Grenoble, France
** Physics Dept., Univ. Warwick, Coventry, U.K.
*** Physics Div., Univ. Portsmouth, Portsmouth, U.K.

[1] J.E. McCarthy, M.J. Cooper, V. Honkimäki, T. Tschentscher, P. Suortti, S. Gardelis, K. Hämäläinen, S. Manninen and D.N. Timms, Nucl. Instr. Meth. in Phys. Res. A 401 (1997) 463

ELECTRON CORRELATION EFFECTS IN SIMPLE METALS

Keijo Hämäläinen, Seppo Manninen, J.A. Soininen, W. Caliebe*, C.-C. Kao**, J.B. Hastings** and M. Krisch***

Ultra high resolution Compton scattering spectroscopy has been used to study the electron correlation effect to the momentum density of conduction electrons. Data on Li, Be and Na has been obtained using the high resolution spectrometers at NSLS and ESRF. Combined with a theoretical analysis we try to understand the correlation effect and separate its contribution when Fermi surface studies are considered. Both the magnitude and the energy dependence of this effect have been studied and the results are compared with those obtained using different techniques like positron annihilation, for example.

* Inst. für Festkörperphysik FZ Jülich, Germany
** NSLS, Brookhaven National Laboratory, USA
*** ESRF, Grenoble, France

CHARACTERIZATION OF THE SIXA Si(Li) DETECTOR

Simo Huotari, Keijo Hämäläinen and T. Tikkanen*

The Silicon X-ray Array (SIXA) detector is a focal plane instrument of the SODART x-ray telescope to be launched with the Spectrum-X-Gamma satellite in 1998. It is an array of 19 separate Si(Li) detector elements designed to study galactic and extragalactic x-ray sources in the energy range between 500 eV and 20 keV. The transfer standard model of one element of the SIXA was studied for characterization and calibration of the spectrometer. The thickness of the cathode electrode was measured at the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory (BNL) [1]. The detector response (i.e. line shape and efficiency) in different operating temperatures and in different parts of the crystal was studied using high photon energies (60 keV) generated by a tungsten anode x-ray tube. The techniques developed will be used later to calibrate the actual 19 on-board satellite elements of the spectrometer.

* Dept. of Astronomy, Univ. Helsinki, Finland

[1] T. Tikkanen, K. Hämäläinen and S. Huotari, Nucl. Instr. and Meth. in Phys. Res. A {bf 403} (1998) 425

MANY-BODY EFFECTS IN INELASTIC X-RAY SCATTERING

J.A. Soininen, Keijo Hämäläinen, Seppo Manninen and B. Barbiellini*

Inelastic x-ray scattering is a widely used tool in the study of the electronic structure of solids. Theories which are currently employed in interpreting the experimental results rely mainly on the independent particle formalism. Recent experimental and theoretical studies have shown that there is a need to find a way to incorporate many-body effects to the description of the scattering process. Our project is to study theoretically and numerically the effects of many-body interactions to the inelastic x-ray scattering cross section. At the moment we are studying numerically the electron-electron interactions in the free electron gas. This should give us qualitative, and to certain degree quantitative, understanding how these interactions change the scattering cross section.

* Physics Department, University of California, Los Angeles, USA

CORRELATION IN MOMENTUM SPACE: A QUANTUM MONTE CARLO STUDY

J. A. Soininen, A. Harju*, Seppo Manninen, Keijo Hämäläinen and R. M. Nieminen*

Variational quantum Monte Carlo (VMC) method is a powerful tool for studying many-electron systems such as atoms, molecules and solids. VMC calculations provide energies of small atoms and molecules that are comparable to configuration-interaction (CI) calculations. Although VMC is able to provide good ground state energies, it is not always clear how well the VMC wave function describes other properties, such as the momentum density, of the system studied. We study the radial momentum distributions and isotropic Compton profiles of various VMC wave functions. By comparing these results with results of other methods, such as CI and density functional theory, we are attempting to determine what types of VMC wave functions are most capable of describing the momentum densities of many-electron systems. This will enable us to produce VMC wave functions that will provide not only accurate energies but also other correlation properties of atomic and molecular systems in a accurate way.

* Laboratory of Physics, Helsinki University of Technology, Finland

POLARIZATION OF NONRESONANT INELASTIC X-RAY SCATTERING

T. Åberg* and Sami Heinäsmäki

Polarization properties of x-ray scattering have been analyzed using density matrix approach. The general cross section is expressed in terms of Stokes parameters of incoming and detected radiation. The dependence on the experimental arrangement is made explicit. Circular dichroism is considered for both randomly oriented and polarized target systems. A simple example illustrates how incident left- and right-handed circularly polarized x-rays can be used to obtain orientational information. We also examine the validity of the impulse (Compton) and optical (Raman) approximation for scattering by 1s electrons of hydrogen and hydrogen-like ions using an exact scaling law.

* Laboratory of Physics, Helsinki University of Technology, Finland

1. T. Åberg and S. Heinäsmäki, J. Phys. B 30 (1997) 4827

RESONANCE FLUORESCENCE INDUCED AUTOIONIZATION

Sami Heinäsmäki and T. Åberg*

We use multichannel resonant scattering theory for an extension of the conventional theory of autoionization resonances in photoabsorption. It incorporates autoionization induced by resonance fluorescence. As a result we obtain a modified Fano profile formula. The result shows that the new mechanism has in certain cases a strong influence on the shape of the resonances.

* Laboratory of Physics, Helsinki University of Technology, Finland

1. S. Heinäsmäki and T. Åberg, J. Phys. B, in press

NEW INTERPRETATION OF RARE-EARTH ION YIELDS

Sami Heinäsmäki, G. Kutluk* and T. Åberg**

We use a simple potential-well model to study the 4d -> 4f giant resonances in Ce, Sm, Eu, and Gd. We show that by photoionization of a d electron in a spherical potential well into a short-lived f scattering state a typical asymmetric ion yield curve is produced. We further show that the resonance profiles can be parametrized according to the Fano profile of photoionization cross section and explain why.

* Hiroshima Synchrotron Radiation Center, Higashi-Hiroshimashi, Japan
** Laboratory of Physics, Helsinki University of Technology, Finland

DYNAMIC STRUCTURE FACTOR FOR GaN

Heikki Sutinen, Simo Huotari, Keijo Hämäläinen, Seppo Manninen and C.-C. Kao*

GaN is an interesting material as it has an energy gap in the blue light region and thus is a potential candidate for a blue laser construction. The dynamic structure factor has been studied both experimentally and theoretically. Inelastic scattering experiments were accomplished at the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory using a high resolution spectrometer and a good quality GaN single crystal. The electronic structure was solved using the the Full Potential Linearized Augmented Plane Wave-package WIEN97 [1]. The calculation of the dynamic structure factor is a combination of free electron theory and real band structure. Calculations are in reasonable agreement with the experiment.

* NSLS, Brookhaven National Laboratory, USA

1. P. Blaha, K. Schwartz and J. Luitz, WIEN97, Vienna University of Technology 1997. Improved and updated Unix version of the original copyrighted WIEN-code by P. Blaha, K. Schwartz, P. Sorantin and S. B. Trickey, Comput. Phys. Commun. 59 (1990) 399

4.3. STUDIES ON CRYSTALLINE MATERIALS

STRUCTURE OF HIGH-Tc SUPERCONDUCTORS

M.A. Gosálvez, Merja Blomberg, V.N. Molchanov* and V.I. Simonov*

As a result of the activity during the previous year (very accurate determination of the crystallographic fine structure of YBa2Cu4O8 both in normal and superconducting states) it was inferred that the continuous phase transition seemed to be correlated to measurable changes in the behavior of the lattice parameters, in addition to changes in interatomic distances and thermal motion of the atoms [1]. Similar behavior has been found using uniaxial pressure experiments [2]. This year a very careful set of systematic measurements, pushing the accuracy of the equipment to its limit, were carried out in order to determine if significant changes in lattice parameters can be sensed accurately by x-ray diffraction technique in continuous phase transitions. Analysis of data is currently in progress.

* Institute of Crystallography, Russian Academy of Sciences, Moscow, Russia

1. V.N. Molchanov, V.I. Simonov, M.K. Blomberg, M.J. Merisalo, JETP Letters 66 (1997) 534
2. C. Meingast, J. Karpinski, E. Jilek and E. Kaldis, Physica C 209 (1993) 591

STRUCTURE OF NONLINEAR OPTICAL MATERIALS

M.A. Gosálvez, Merja Blomberg, N.I. Sorokina*, I.A. Verin* and V.I. Simonov*

The studies on KTiOPO4 have been continued. The evolution of the lattice parameters of this crystal was determined in the range of temperatures from 20 to 300 K. This revealed a very dif-ferent behavior from the other compound in the same family, TlTiOPO4, studied earlier in this laboratory. Furthermore, the unit-cell volume shows a change in slope at ~100 K not shown by the other compound. This might be related to the effects of a phase transition that does not occur for the rest of the family in this range of temperatures. The data is still being interpreted.

A study of the low-temperature (20 K) structure of the crystal (Cs,Rb)TiOAsO4 has also been started. The aim is the analysis of the crystallographic fine structure, avoiding as much as possible the uncertainty induced by thermal vibrations of the atoms. The installation of the new control software required parallel development of routines in order to attain the 3-dimensional recording of more than 8300 reflections. Currently half of them have already been recorded.

* Institute of Crystallography, Russian Academy of Sciences, Moscow, Russia

DISPLACEMENT INDUCED SATELLITE REFLECTIONS IN Cs3Sb2I9

M.A. Gosálvez, Merja Blomberg, M.S. Novikova* and V.I. Simonov*

The hexagonal structure lattice parameters of the crystal Cs3Sb2I9 were determined at different temperatures between 50 and 294 K. Special interest was delivered to the measurements below 100 K where the new software proved to be an excellent tool. Very meaningful reciprocal lattice maps were easily collected and a complete structure of satellite peaks was disclosed. The intensities of the main peaks were observed to decrease as the temperature was reduced. These results can be explained by assuming the existence of sinusoidal displacement disorder with polarization and propagation vectors along [0 1 0] and [1 2 1] directions, respectively. If a peak at position (0 k l) is allowed by the group symmetry, satellites will appear at positions ±0.66 (0 2 1). An additional set of satellites at positions ± 0.5 (0 0 1) can be explained with a propagation vector along [0 0 1] and similar polarization direction. There might exist a phase transition in the type of disorder since below 65K some of the latter satellites fade off to the benefit of satellites at ±0.5 (0 1 0). Results are still being derived.

* Institute of Crystallography, Russian Academy of Sciences, Moscow, Russia

TWIN BOUNDARY MOTION STUDIES ON Ni2MnGa

M.A. Gosálvez, Merja Blomberg and K. Ullakko*

Contacts in order to establish collaboration with the Helsinki University of Technology in the area of magnetic shape memory materials have been made. As an example, preliminary preparation of a suitable sample and experimental set-up for the study of the mechanisms that enable large magnetically induced strains in the martensitic phase of Ni2MnGa were undertaken. Magnetostriction as the responsible mechanism has been denied. The proposed explanation [1] involves twin boundary motion, which would favor reversible size increase for those domains correctly oriented with respect to the magnetic field. We believe that following the effect of the magnetic field on the intensity and shape of a number of reflections will provide a final proof for this explanation.

* Helsinki University of Technology, Finland

1. K. Ullakko, J.K. Huang, C. Kantner, R.C. O'Handley and V.V. Kokorin, Appl. Phys. Lett. 69 (1996) 1966

STUDIES ON URANIUM INTERMETALLICS

M.A. Gosálvez, Merja Blomberg, R. Kuzel*, L. Dobiásová*, L. Havela** and V. Sechovsk´y**

The study of the temperature dependence of the structure of uranium intermetallics UNiGa, URhAl, URuAl and URuGa has been continued. The non-statistical occupation of the lattice sites around the uranium atom observed in previous studies cannot be explained by purely geometrical arguments. Instead hybridization of the uranium valence electrons and those of its neighboring atoms must be assumed. Accurate determination of the occupation factors of those lattice sites is required to know exactly how the non-statistic assignment occurs. To support the theory of hybridization as an explanation, the aspherical atom model should be used to refine the structure, since this model allows for symmetry adapted deformation of the electron clouds around the atom. By performing this task at different temperatures above and below the magnetic ordering point, it will be possible to see how the delocalization of electrons develops the magnetism in these systems.

* Department of Semiconductor Physics, Charles University, Prague, Czech Republic
** Department of Metal Physics, Charles University, Prague, Czech Republic

4.4. STUDIES ON WEAKLY ORDERED MATERIALS

X-RAY SCATTERING STUDIES OF GRAFTED AND SULFONATED POLY(VINYLIDENE FLUORIDE) MEMBRANES

Kaija Jokela, Milja Karjalainen, Mika Torkkeli, M. Paronen*, Ritva Serimaa, Sakari Vahvaselkä, Veli Eteläniemi and F. Sundholm*

X-ray scattering studies of grafted and sulfonated poly(vinylidene fluoride) (PVDF) membranes with high proton-conductivity were continued. Earlier studies at room temperature have shown how grafting, sulfonation and crosslinkers affect the structure and proton conductivity of membranes.[1] More information about sulfonic-acid-water clusters, preferred orientation and thermal behaviour of membranes was received from SAXS, ASAXS (Hasylab/DESY) and WAXS measurements in 1997. These results made it possible to undestand more about the structure and thermal behaviour of membranes, which is important from the applications' point of view.

* Department of Chemistry, University of Helsinki, Finland

1. S. Hietala, S. Holmberg, M. Karjalainen, J. Näsman, M. Paronen, R. Serimaa, F. Sundholm, and S. Vahvaselkä, J. Mater. Chem. 7 (1997) 721-726

DIRECT SULFONATION OF POLY(VINYL FLUORIDE)

Kaija Jokela, Milja Karjalainen, Mika Torkkeli, M. Paronen*, Ritva Serimaa, Sakari Vahvaselkä, Veli Eteläniemi, Eero Rauhala and F. Sundholm*

Structural investigations of irradiated and sul-fonated poly(vinyl fluoride) (PVF) films have been continued by WAXS and SAXS methods, with the purpose of producing new types of proton conductors. The irradiation was performed with 2.5 MeV protons at the Accelerator Laboratory in Helsinki and with 175 keV electrons at Åbo Academi in Turku. The best conductivities were obtained for the films, which were proton beam irradiated, have large water uptake, large ion exchange capacity, and low crystallinity. SAXS intensity curves of proton irradiated samples showed features that could be attributed to ionic groups.

* Department of Chemistry, University of Hel-sinki, Finland

STUDIES ON ION IMPLANTED SrS:Ce THIN FILMS

Milja Karjalainen, Reijo Lappalainen, Sanna Sevanto, Ritva Serimaa, W.-M. Li*, M. Ritala* and M. Leskelä*

Strontium sulfide is a promising blue phosphor material for full-colour thin film electroluminescent displays. It has been observed that implantation of impurity elements, e.g. F, increases the photoluminescence intensity. The recrystallization of the SrS-based films was observed using the X-ray diffraction method and the diffusion of the implanted atoms was studied using nuclear resonance broadening and Rutherford backscattering techniques as a function of temperature.

* Department of Chemistry, University of Helsinki, Finland

X-RAY SCATTERING STUDY OF NATIVE POTATO TUBERS

T. Väänänen*, Mika Torkkeli, Kaija Jokela, M. Toivola* and Ritva Serimaa

In potato starch granules crystalline amylopectin forms lamellae that are separated by amorphous amylose regions. The purpose of the study is to follow structural changes in commercial cultivars and hybrids being developed by small and wide angle x-ray scattering experiments during winter storage and as a function of temperature. The results will be correlated with the processing behaviour of the tubers. The research involves also studies of potato glycoalkaloids and is carried out in collaboration with the departments of Plant Production, Pharmacy and Physics, University of Helsinki.

* Department of Chemistry, University of Helsinki, Finland

AMORPHOUS BIOLOGICALLY ACTIVE PLATINUM PYRIMIDINE COMPLEXES

Ritva Serimaa, Veli Eteläniemi, Mika Torkkeli, T. Laitalainen*, J. Pitkänen* and Timo Paakkari

The purpose of the project is to develop synthesis for a biologically active amorphous Pt complex. Results of structural studies by anomalous wide angle x-ray scattering, small-angle x-ray scattering, and x-ray absorption methods, Pt-nuclearity in the solid state, and the size of aggregates in aqueous solution, are correlated with the biological activity.[1]

* Department of Chemistry, University of Helsinki, Finland

1. R. Serimaa, V. Eteläniemi, T. Laitalainen, A. Bienenstock, S. Vahvaselkä and T. Paakkari, Inorg. Chem. 36 (1997) 5574

XSW STUDIES OF OXIDATION OF SILICON SURFACES

Veli Eteläniemi, E.G. Michel*, C. Sánchez-Hanke** and G. Materlik**

Surfaces and thin layers have received increasing attention under last 10-20 years. Several new techniques have been introduced and nowadays order of mono layer (1 ML) thick or even thinner layers can be easily studied using several techniques. X-ray Standing Wave (XSW) is one of those. XSW measurements of a few atomic layers thick samples can be carried out only using synchrotron radiation. We have applied XSW method several years and under the last studies we have improved our possibilities so that low species as oxygen can be studied. Oxidation is a key problem in many studies and this mechanism is not allways well understood. The oxidation rate of the clean Si surface is low and the saturation coverage is less than 1 ML. This hinders the formation of a thick SiO2 layer which is needed in semiconductor technology. There are also other applications for oxidation layers. The oxidation rates can be enhanced using catalytic materials as metals. The disadvantage is that most of the metals cannot be removed from the surface at moderate temperature. However, alkali metals (AM) are an exception. Therefore we have studied the oxidation process on the clean surface, and on the Na and K covered surfaces. In the clean surface studies one O 1s XPS peak was observed whereas the AM covered surface showed two O 1s peaks. The reason for that is the chemical shift and it gives for us a chance to distinguish whether the peak can be assigned to Si-O or AM-O interference. Thus we were able to figure out the adsorption sites of O on clean and AM coverage surfaces and deduce whether the adsorption process is similar in all studied cases. The oxygen dose were varied from 0.1 up to 3.5 L (1 L = 10­6 Torr sec).

* Dto. de Física de la Materia Condensada and Instituto Universitario de Ciencia de Materiales "Nicolás Cabrera" Universidad Autónoma de Madrid, Spain
** HASYLAB, Hamburg, Germany

EXAFS, AN ELEMENT SENSITIVE METHOD

Veli Eteläniemi

In an EXAFS study absorption of x-rays is measured as a function of photon energy near an absorption edge of the chosen species. Therefore EXAFS is an element sensitive method that gives information about the local arrangement of the atoms around the chosen species. Usually EXAFS shows only the short range order information as the atomic distances to the neighbouring atoms, the number of atoms at each distance and sometimes it is possible to distinguish each neighbouring element. EXAFS method can be applied for all kinds of materials: solid, liquid, amorphous and crystalline samples. We have studied using our laboratory facilities the local environment of Cu, Pt and Bi atoms in solid samples where their shares have been 1% or more.

STUDYING STRUCTURES WITHIN STRUCTURES USING X-RAY SCATTERING

Mika Torkkeli, Ritva Serimaa, M. Saariaho*, J. Ruokolainen*, O. Ikkala*, G. ten Brinke**

During the past two years we have studied the self organization of side chain grafted polymer chains. These transform quite readily from a homogenous to lamellar morphology at a so called order-disorder transition temperature. This temperature may be adjusted almost at will by varying the repulsion between the base polymer and the side groups. In principle different morphologies might emerge based on structural analogy to block-comb copolymers, but so far only lamellar morphologies have been observed. Block co-polymers are known to exhibit a rich variety of morphologies. As a novel approach, if one of the polymer blocks is side chain grafted then structures within structures are formed. At present, TEM images reveal only the larger scale block structure; to see the structure within the blocks and transitions in either, temperature dependent scattering measurement is still the best tool available. Our experimental setup requires two separate scans at the small angle (SAXS) range (60 Å to 1000 Å) and medium angle (MAXS) range (10 Å to 150 Å) to cover effectively the relevant region in k-space. Development of sample preparation techniques and measurement automatics to facilitate imaging of oriented samples is being planned.

* Department of Technical Physics, Helsinki University of Technology, Finland
** Department of Polymer Science and Materials Science Center, University of Groningen, The Netherlands

A STUDY OF THE STRUCTURE OF WOOD CELLS

Seppo V. Andersson, Ritva Serimaa, Tanja Suni, Timo Paakkari, E. Pesonen*, P. Saranpää* and M. Treacy**

The purpose of the study is to develop fast methods for determination of the mean microfibril angles (MFA) of wood using x-ray diffraction and small angle x-ray scattering. The results are compared with those obtained by using polarization microscope. Variation of MFA of Norway spruce (Picea abies (L.) Karst.), Scots pine (Pinus sylvestris L.) and Sitka spruce (Picea sitchensis) is studied. Furthermore we investigate the relationship between MFA and shrinkage properties, especially in the juvenile wood zone.

* Finnish Forest Research Institute METLA, Vantaa, Finland
** Forest Products Department, Dublin, Ireland