Showing papers by "University of Ljubljana published in 1976"

Collision of two black holes: Theoretical framework

Abstract: Highly nonspherical time-dependent collisions between black holes may be powerful sources of gravitational radiation. We consider various attempts at estimating the efficiency of the generation of radiation by such collisions. To determine the actual efficiency as well as to understand the details of the dynamical coalescence of black-hole event horizons, we have developed a numerical method for solving the Einstein gravitational field equations in these high-velocity strong-field regions. The head-on collision of two nonrotating vacuum black holes is chosen as an example of our technique. We use the geometrodynamical model of a black hole as an Einstein-Rosen bridge. The initial data to be evolved are the time-symmetric conformally flat data discovered by Misner. A new set of spatial coordinates for these data is derived. Then the general space plus time decomposition of Einstein's equations is presented and specialized to the axisymmetric nonrotating case. Details of the evolution will be given in later papers. (AIP)

Proton- 14 N double resonance study of the structural phase transitions in the perovskite type layer compound (CH 3 NH 3 ) 2 CdCl 4 .

Abstract: Using a proton-nitrogen double resonance technique we have determined the quadrupole coupling of14N in the room temperature orthorhombic (Cmca), the low temperature tetragonal (P42/ncm), and the monoclinic low temperature (P21/c) phases of (CH3NH3)2CdCl4. In all these phases all nitrogens are chemically equivalent demonstrating that the disorder in the orientations and H-bonding arrangements of the CH3-NH3 groups in theC m c a andP42/ncm phases is indeed dynamic and not static. In the monoclinic phase the14N quadrupole coupling constant equalse2qQ/h=880 kHz and the asymmetry parameter isη=0.20, wherease2qQ/h=790 kHz,η=0.1 in the tetragonal low temperature phase ande2qQ/h=726 kHz,η=0.21 in the room temperature orthorhombic phase. The observed increase in the14N quadrupole coupling constant on going from the orthorhombic phase to the tetragonal low temperature phase which is coupled with a simultaneous decrease in the asymmetry parameter can be understood in terms of a partial freezing in of the dynamic disorder in the C-N bond directions whereas the14N quadrupole coupling tensor in the monoclinic phase is characteristic of a frozen in C-N bond in a deformed lattice, where the N-H — Cl bonds are of different length.

Vibrational Spectra, Hydrogen Bonding, and Ferroelectricity in the PbHPO4 Family

Abstract: The infrared and Raman spectra of polycrystalline PbHPO4, PbHAsO4, CaHPO4, and BaHPO4 were measured at room temperature in the range between 4000 and 30 cm−1, and the temperature dependence of the dielectric constant of these materials was studied. The OHO hydrogen bonds in PbHPO4 and PbHAsO4 are found to be very similar to those in KH2PO4 and KH2AsO4 and the XO4 groups (X = P, As) are – in contrast to CaHPO4 and BaHPO4 – as well close to being tetrahedral. The symmetry of the soft mode in PbHPO4 is deduced and it is found that the ferroelectric transition could be of second order as far as group theory is concerned. A microscopic model of the ferroelectric orderdisorder phase transition based on the interaction between the chains of OHO bonds linking the PO4 groups is proposed. Es wurden die Infrarot- und Ramanspektren von polykristallinem PbHPO4, PbHAsO4, CaHPO4 und BaHPO4 bei Raumtemperatur im Bereich zwischen 4000 und 30 cm−1 gemessen und die Temperaturabhangigkeit der Dielektrizitatskonstante dieser Materialien untersucht. Die OHO-Wasserstoffbindungen in PbHPO4 und PbHAsO4 sind denen in KH2PO4 und KH2AsO4 sehr ahnlich, und die XO4-Gruppen (X = P, As) sind – im Gegensatz zu CaHPO4 und BaHPO4 – ebenfalls nahezu tetraedrisch. Es wird die Symmetrie der weichen Mode in PbHPO4 abgeleitet und gefunden, das die ferroelektrische Umwandlung vom Standpunkt der Gruppentheorie von zweiter Art sein konnte. Fur die Phasenumwandlung ferroelektrische Ordnung-Unordnung wird auf der Grundlage der Wechselwirkung zwischen den die PO4-Gruppen verbindenden Ketten der OHO-Bindungen ein mikroskopisches Modell vorgeschlagen.

Some international differences in treatment and survival in breast cancer.

Abstract: In a recent study, 5-year survival rates for breast cancer patients in Boston (Massachusetts), Glamorgan (Wales), Slovenia (Yugoslavia) and Tokyo (Japan) were 57.3%, 49.5%, 41.9% and 74.9%, respectively. In this report, data are presented on the types of treatment used in the four areas and on the relationship of differences in treatment practices to the differences in survival rates. Generally, surgically treated patients who also had radiotherapy had lower survival rates than patients in the same area who had similar operations without radiotherapy. In each area, the survival rate was higher for patients who had radical mastectomy than for those who had simple mastectomy. The Japenese patients had the highest survival rate within nearly every treatment and extent-treatment category. Thus, the high survival rate of these patients was not explained by the variables considered. Survival differences between Boston, Glamorgan and Slovenia were largely explained by differences in extent of disease and type of treatment. As the nature of the treatment—survival trends was consistent with the interpretation that treatment tended to be selected according to apparent prognosis, the degree to which treatment customs were determinants of the differences in survival rate among the three areas is uncertain. Differences Internationales Dans Le Traitement Du Cancer Du Sein Et Le Taux De Survie Ment suivi un traitement radiotherapique avaient des taux de survie inferieurs a ceux des patientes de la měme region qui avaient subi des operations analogues sans radiotherapie. Dans chaque region, le taux de survie etait plus eleve pour les patientes qui avaient eu une mastectomie radicale que pour celles qui avaient eu une mastectomie simple. Comme le taux de survie des patientes japonaises etait le plus eleve, quel qu'ait ete le traitement et la categorie (extension de la maladie/traitement), ou presque, il ne s'expliquait pas par les variables retenues. Les differences entre les taux de survie a Boston, dans le comte de Glamorgan et en Slovenie s'expliquaient en grande partie par les differences dans l'extension de l'affection et le type du traitement. Comme la nature des courbes traitement-survie concordait avec l'interpretation selon laquelle le traitement est choisi en fonction du pronostic apparent, il y a incertitude en ce qui concerne la mesure dans laquelle les differences de taux de survie entre les trois regions considerees sont imputables a la nature du traitement.

Iodide, thiocyanate and cyanide ions as capturing reagents in one-step copper-thiocholine method for cytochemical localization of cholinesterase activity.

Abstract: The necessity of the presence of iodide in Cu-ThCh reaction was investigated by following the precipitate formation "in vitro" and by evaluating the ultrastructural localization of the precipitate in sympathetic ganglion cells of the frog and in the end-plate regions of the rat diaphragm. It was found that thiocyanate or cyanide is the only anion that can be substituted for iodide as the capturing agent in precipitation. The optimal concentration in the preincubation and incubation media of any one of the three anions is from 2 to 5 mM. At a concentration below 1 mM precipitation "in vitro" is considerably delayed as a result of which in electron microscopy diffusion artefacts appear in tissue sections. The unconverted primary precipitate obtained in the presence of iodide had been used for ultrastructural localization of ChE activity and now this use has been extended to precipitates obtained in the presence of CN- or CNS-. Better-quality localization in the presence of either one of the latter anions suggests that they, and particularly CN-, should be substituted for I- in the one-step Cu-ThCh method for the cytochemistry of cholinesterases.

Ab initio study of a linear chain of H atoms using different orbitals for different spins

Abstract: The variational form of the ab initio crystal-orbital method using different orbitals for different spins is applied to an infinite linear chain of H atoms. The cohesive energy of the optimized arrangement of the H-atom chain is 0.039 a.u. The correlation energy obtained is 0.11 eV/atom.

Soft-mode dynamics of partially deuterated KH 2 P O 4 -type crystals

Abstract: The dynamics of isotopically disordered ferroelectric crystals of $\mathrm{K}{({\mathrm{H}}_{1\ensuremath{-}x}{\mathrm{D}}_{x})}_{2}\mathrm{P}{\mathrm{O}}_{4}$ type is discussed on the basis of the coupled proton-deuteron tunneling-lattice optic-mode model. It is shown that one of the collective modes of the disordered system exhibits a soft-mode behavior at the ferroelectric transition temperature. The collective mode frequencies and their pressure derivatives are calculated by a modified random-phase approximation as a function of deuteron concentration and compared with recent low-temperature Raman data of Peercy. The theory allows an estimate of the various parameters of the model and provides strong support for the proton tunneling picture of hydrogen-bonded ferroelectrics.

Ab initio crystal orbital studies on linear chains of H atoms

Abstract: Anab initio crystal orbital method is used to calculate the energies of an infinite chain of H atoms and of linear arrangements of H2 molecules with different interatomic distances. The H2 arrangements are not stable in respect to isolated molecules. The cohesive energy of an optimized arrangement of H atoms chain is 0.0354 a.u.

Study of the arrangement of crystallites in gamma-irradiated human enamel by electron paramagnetic resonance.

Abstract: The arrangement of tooth enamel microcrystals has been studied on CO3-3 bound electrons by electron paramagnetic resonance. It was found that noncarious human maxillary central incisors have a greater degree of alignment of tooth enamel microcrystals than the carious ones. The outermost surface layer of enamel showed a greater crystallite degree of alignment than other parts.

Dielectric properties of the 250 K phase transition in BaMnF4 and of CsH2PO4 and CsD2PO4

Abstract: The dielectric constant of BaMnF4 has been measured close to the 250 K phase transition along a, b and c crystal-lographic axes. The dielectric constant along the a axis shows a small peak at -23.3°C (∼250 K), whereas along the other two axes the dielectric constant changes monotonically with the temperature.Orthorhombic CsH2PO4 undergoes a ferroelectric transition at Tc = -119.5°C, whereas the ferroelectric transition temperature in isomorphous CsU2PO4 is Tc = -5.55°C. The transitions are of fust order in both cases. The rather large isotope effect demonstrates the importance of the O-H-O bonds in the transition mechanism.

Preparative and structural studies of halodimolybdates(II). I. Preparation and crystal structure of (picH)2Mo2Br6(H2O)2

Abstract: (picH)2Mo2Br6(H2O)2 (picH = 4-methylpyridinium cation) was prepared from (NH4)3Mo2CI9 · H2O and picHBr in 4 M aqueous HBr at −20°C. The compound crystallizes in the triclinic P1 space group. The cell dimensions are: a = 7.822(5); b = 7.937(2), c = 10.163(7) A α = 73.08(5)°; β = 85.46(3)°; γ = 74.25(1)°. For z = 1, d(calcd) = 2.55 gcm−3, d(obsd) = 2.53(2) gcm−3. The structure has been solved from 1191 independent reflections within the sphere bounded by θ = 30°, collected on a counter diffractometer, using MoKα radiation. Refinement led to final unweighted and weighted residuals of 0.035 and 0.041. Hydrogen atoms were not included. Structure consists of Mo2Br6(H20)2− of crystallographic Ci symmetry and two picH+ ions. Br atoms and H2O molecules surround Mo2 pair forming eclipsed configuration. The distance MoMo is 2.122(2) A three independent MoBr bond lengths 2.589(2), 2.568(2) and 2.593(2) A. H2O molecule is coordinated to Mo at 2.185(10) A. Bond lengths and angles within the aromatic ring are normal. The compound is diamagnetic. Two water molecules are evolved when the substance is heated to 160°C in the inert atmosphere. The reaction with pyridine gives Mo2Br4Py4. Praparation und Strukturuntersuchungen an Halogendirnolybdaten(I1). I. Synthese und Kristallstruktur von (picH)2Mo2Br6(H2O)2 (picH)2Mo2Br6(H2O)2 (picH = 4-methylpyridinium-Kation) wurde aus (NH4)5Mo2Cl9 · H2O und picHBr in 4M wasriger HBr beim −20°C hergestellt. Die Verbindung kristallisiert in der Raumgruppe P1 mit a = 7,822(5), b = 7,937(2); c = 10,163(7) A; α: = 73,08(5)°; β = 85,46(3)°; γ = 74,25(1)°. Die gemessene Dichte betragt 2,53(2) gcm−3; damit ergibt sich z = 1 und drōntg. = 2,56 gcm−3. Die Struktur wurde aus 1191 unabhangigen Reflexen bestimmt, die mit Hilfe eines automatischen Diffraktometers (θ < 30°) mit MoKα-Strahlung gemessen wurden. Die Verfeinerung fuhrt zu den Gutefaktoren von 0,036 bzw. 0,041. Die Wasserstoffatome wurden nicht berucksichtigt. Die Struktur ist aus Mo2Br6(H2O)2−-Einheiten mit der Ci Lagesymmetrie und picH+-Ionen aufgebaut. Das Mo2-Paar ist von Br-Atomen und H2O-Molekeln in einer „eclipsed-Form” umgeben. Die MoMo-Abstande betragen 2,122(2) a, die drei unabhangigen MoBr. Abstande sind 2,589(2), 2,568(2), 2,593(2) A. Die H2O-Molekeln sind zum Molybdan mit einem Abstand von 2,185(10) A koordiniert. Die Bindungsabstande nnd -winkel innerhalb der aromatischen Pyridinium-Ringe sind normal. Die Verbindung ist diamagnetisch. Beim Erhitzen bis 150°C in Inertatmosphare werden zwei Wassermolekeln abgegeben. Die Reaktion rnit Pyridin liefert Mo2Br4Py4.

Acoustic emission of a crazing polymer

Abstract: The acoustic emission from a crazing polyvinyltoluene in a tensile and bending experiment is described Acoustic emission appears as a series of bursts which most likely correspond to the initiation and growth of crazes The emission intensity is characterised by acoustic activity (pulse rate) measured by the ring-down technique The average activity increases with strain During repeated loading the acoustic activity shows a measurable intensity and significant rise only beyond the maximum strain of the former runs This is equivalent to Kaiser's effect in metals Acoustic emission during the creep experiment occurs in three characteristic periods They are characterized as the relaxation, fatigue, and breakdown periods Visual observations indicate that the relaxation period corresponds to the initiation, and the fatigue period to the growth of crazes In the breakdown period a macroscopic crack develops and the sample fails

Frequency dispersion of the proton zeeman spin-lattice relaxation in the smectic phases of tbba

Abstract: Using a field cycling technique, the Larmor frequency (VL) dependence of the proton Zeeman spin-lattice relaxation time TI has been measured in the smectic A, smectic C, smectic B and smectic VII phases of terephtal-bis-butylaniline (TBBA) between VL = 90 MHz and VL = 0.14 MHz and a significant dispersion has been found. The observed coiJ2 dispersion law in the smectic A phase and the deviations from it at low frequencies can be explained by nematic director fluctuations in the smectic A phase and the coupling between nematic and smectic order.

Photonuclear reactions in Zr-90

Abstract: Photonuclear reactions in $^{90}\mathrm{Zr}$ were studied in the excitation region of the giant dipole resonance. Experimental results which include all the most important reaction channels consist of cross sections integrated over angles, and for photoprotons, of additional data on angular distributions. Photoproton cross sections were determined by a least-squares-fit analysis of proton spectra resulting from a series of irradiations with bremsstrahlung of different end-point energies. In addition to the ground state $^{90}\mathrm{Zr}(\ensuremath{\gamma},{p}_{0})^{89}\mathrm{Y}_{\mathrm{g}.\mathrm{s}.}$, photoproton cross-section data for three groups of $^{89}\mathrm{Y}$ excited states are presented. Other results which consist of cross sections for reactions ($\ensuremath{\gamma},{p}_{1}$), ($\ensuremath{\gamma},{n}_{1}$), ($\ensuremath{\gamma}, 2n$), ($\ensuremath{\gamma},\mathrm{np}$), and ($\ensuremath{\gamma},n$) were obtained through different techniques based on the measurement of activation yields. It is shown that 70% of the total photoproton cross section is due to the transition to the ground, the 1.74 MeV, and possibly the 1.51 MeV states of $^{89}\mathrm{Y}$. The photoneutron cross section is dominated by the contribution involving the first excited state and all those states of $^{89}\mathrm{Zr}$ which subsequently decay to it. Experimental data are interpreted in terms of isospin splitting of the giant dipole resonance. In the total photoproton cross section, apart from a broad resonance at 16 MeV similar to the dominant structure of the ($\ensuremath{\gamma},n$) reaction, an additional resonance of approximately the same magnitude appears at 20.5 MeV. The latter resonance is attributed to the ${T}_{g}$ part of the giant dipole resonance. Its integrated cross section of about 8% of the total photonuclear cross section accounts for the main part of the expected ${T}_{g}$ strength. Neutron channels do not seem to contribute significantly to the decay of the ${T}_{g}$ resonance. It is concluded from ($\ensuremath{\gamma},\mathrm{np}$) and ($\ensuremath{\gamma},2n$) reactions that the ${T}_{g}$ resonance is superimposed on a ${T}_{l}$ background extending to higher energies.NUCLEAR REACTIONS $^{90}\mathrm{Zr}(\ensuremath{\gamma},p)$, ($\ensuremath{\gamma},n$), ($\ensuremath{\gamma},np$) and ($\ensuremath{\gamma},2n$), ${E}_{\ensuremath{\gamma}}=12\ensuremath{-}30$ MeV; measured photoproton spectra and bremsstrahlung activation yields; deduced $\ensuremath{\sigma}(E,\ensuremath{\vartheta})$ and $\ensuremath{\sigma}(E)$.