Magnetic Structure Investigations at the Nuclear Center

The magnetic structure of the compounds UOS, ß-CoSO4, YCO5, and HoCO5 is briefly described. UOS is antiferromagnetic. The Néel temperature is Tn=55°K. The magnetic cell is doubled in the c direction with a ++ - - sequence of U moments along c. The apparent spin is S∼1. The negative interaction corresponds to U-O-U links. In ß-CoSO4 (high-temperature modification, space group Pbnm), Co atoms are in 000, 00½, ½½½, ½½0. Here three different antiferromagnetic spin modes, mutually perpendicular, Ax(+ - - +), Gy(+-+-), and Cz(++ - - ), in the Wollan-Koehler notation, are coupled. Direction cosines are 0.71, 0.50, and 0.50, respectively. The Co moment is about 3,84 µB at 4.2°K. A field-induced spin flip to the configuration Fx, Cy, Gz is predicted. YCO5 is ferromagnetic at room temperature with a moment value of Co practically equal to that of metallic Co and moment direction along c, which is conserved down to 4.2°K. In HoCO5 the moment of Ho is opposite to those of the Co atoms. When cooling from room to liquid helium temperature, the direction of easy magnetization changes from near c to a direction in the basal plane and the Ho moment increases from 4 to about 9 µB. The compensation temperature is 70°K

Effects of off great-circle propagation on the phase of long-period surface waves

Surface wave phase corrections for departures from great-circle propagation are computed using two-point ray-tracing through the aspherical earth model M84C of Woodhouse & Dziewonski (1984). For Rayleigh and Love waves with periods in the range 100–250 s, we determine whether these corrections provide significant variance reductions in source determinations compared with corrections calculated assuming great-circle propagation through the heterogeneous structure. For most source-receiver geometries, the off great-circle travel-time effects are small (< 10 s) for second and third orbits (e.g. R2 and R3), and their application in source determinations does not significantly reduce the data variance. This suggests that for the loworder heterogeneous models currently available the geometrical optics approximation is valid for long-period low orbit surface waves. Off great-circle phase anomalies increase quasi-linearly with increasing orbit number, indicating that the geometrical optics approximation degrades for higher orbits, which emphasizes the importance of developing higher order approximations for free-oscillation studies.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/73147/1/j.1365-246X.1987.tb05217.x.pd

Use of a correlation chopper for time of flight neutron scattering - Part II : deconvolution in the experimental case

The deconvolution problem for a correlation chopper in time of flight diffraction experiments has been treated in a previous paper [1] by a matrix formalism. A practical case is now considered since the signal function obtained with a mechanical chopper is not rectangular, but triangular. This observed function Z is deconvoluted by means of either a rectangular inverse sequence (rectangle-triangle deconvolution) or a triangular one (triangle-triangle deconvolution). It is shown that there is a loss of resolution. Perturbations arising from the use of an imperfect mechanical device in elastic scattering, give rise to a negative image of the diffraction function due to variation of the chopper transmission with wavelength. This effect can be corrected a posteriori. The other perturbations give rise to loss of resolution or background modulation.Dans un article précédent [1] nous avons développé un formalisme matriciel permet tant de résoudre facilement, dans le cas idéal, le problème de déconvolution lorsqu'on utilise un chopper statistique dans des expériences de temps de vol. Nous considérons maintenant le cas pratique. La fonction signal émise par un chopper mécanique n'est pas rectangulaire : c'est une fonction triangulaire. Nous déconvoluons la fonction observée Z avec une séquence inverse, soit rectangulaire (déconvolution rectangle-triangle), soit triangulaire (déconvolution triangle-triangle). Nous montrons que dans ce cas il y a une perte de résolution par rapport au cas idéal. Ensuite nous considérons les perturbations introduites par 1'utilisation d'un dispositif mécanique imparfait et nous montrons qu'il apparaîtra, en diffraction élastique, une image negative de la fonction de diffraction due à la variation de la transmission du chopper en fonction de la longueur d'onde. Cet effet peut être corrigé a posteriori. Les autres perturbations induisent des pertes de résolution, ou une modulation du bruit de fond

A time-of-flight spectrometer for elastic neutron scattering on powdered samples

A time-of-flight spectrometer especially built for the study of powdered samples with complex surroundings, such as a high-pressure cell (up to 35 kbars) and a high-temperature furnace (up to 2 500 °C) is described. A correlation chopper is used to decrease the time-independent background due to the high epithermal flux in the incoming beam. The chopper slots are distributed according to a pseudo-random binary feedback shift register sequence of 511 elements, the elementary neutron pulse being 8 μs in width. The geometry has been optimized in order to reach a high resolution : Δd/d values of 0.75 % at d = 1 A and 0.4 % at d = 3 Å are obtained. Structure refinements on test samples with simple structure (UO2 give a reliability factor R of 1.3 % which compares fairly well with the results of the high resolution fixed-wavelength goniometer, D1A at the Institut Laue-Langevin.Nous décrivons un spectromètre à temps de vol spécialement conçu pour des études d'échantillons polycristallins dans un environnement complexe, tel que cellule haute pression (jusqu'à 35 kbars) et four haute température (jusqu'à 2 500 °C). Un chopper statistique est utilisé pour diminuer le bruit de fond independant du temps provenant d'un important flux de neutrons épithermiques dans le faisceau incident. Les fentes du chopper sont réparties selon une séquence pseudo-aléatoire de registre à décalage à contre-réaction binaire de 511 éléments. La bouffée élémentaire de neutrons a 8 μs de large. La géométrie a été optimisée pour obtenir une haute résolution Δd/d de 0,75 % pour d = 1 A et 0,4 % pour d = 3 A. Les afflnements de structure sur des échantillons tests de structure simple (UO2 ont donné des facteurs de confiance R de 1.3 % comparables à ceux obtenus avec le goniomètre à haute résolution (longueur d'onde fixe) D1A de 1'Institut Laue-Langevin

Use of a correlation chopper for time of flight neutron scattering - Part I : Theory of the deconvolution

We recall the principle of the correlation chopper and we develop a matrix formalism changing the deconvolution into the resolution of a linear system of N equations with N unknown roots. We consider the case of an ideal rectangular signal function and we solve the system when the number of analysing channels is equal to, or larger than the number of elements in the slot sequence { ai}. We show that, in practice, the correlation chopper improves the statistical accuracy with respect to a one slot chopper if the mean value of the diffraction function is smaller than one half the time independent background. We show also that an increase of the number of analysing channels, the experiment lasting the same time, improves the definition of the shape of the peaks, without modification of the relative accuracy of the measurement

Use of a correlation chopper for time of flight neutron scattering - Part I : Theory of the deconvolution

We recall the principle of the correlation chopper and we develop a matrix formalism changing the deconvolution into the resolution of a linear system of N equations with N unknown roots. We consider the case of an ideal rectangular signal function and we solve the system when the number of analysing channels is equal to, or larger than the number of elements in the slot sequence { ai}. We show that, in practice, the correlation chopper improves the statistical accuracy with respect to a one slot chopper if the mean value of the diffraction function is smaller than one half the time independent background. We show also that an increase of the number of analysing channels, the experiment lasting the same time, improves the definition of the shape of the peaks, without modification of the relative accuracy of the measurement.Nous rappelons le principe de fonctionnement du chopper statistique et nous développons un formalisme matriciel qui permet de remplacer la déconvolution par la resolution d'un système de N équations à N inconnues. Nous considérons le cas d'une fonction signal rectangulaire idéale, et nous résolvons le système d'équations lorsque le nombre de canaux d'analyse est égal, ou supérieur, au nombre d'éléments de la séquence de fentes {ai}. Nous montrons que, en pratique, le chopper statistique améliore la précision statistique, par rapport à un chopper à une fente, si la valeur moyenne de la fonction de diffraction est inférieure à la moitié du bruit de fond indépendant du temps. Nous montrons aussi que 1'augmentation du nombre de canaux d'analyse, la duree de l'expérience restant constante, permet de mieux définir la forme des pics, sans modifier la precision relative de la mesure

Use of a correlation chopper for time of flight neutron scattering - Part II : deconvolution in the experimental case

The deconvolution problem for a correlation chopper in time of flight diffraction experiments has been treated in a previous paper [1] by a matrix formalism. A practical case is now considered since the signal function obtained with a mechanical chopper is not rectangular, but triangular. This observed function Z is deconvoluted by means of either a rectangular inverse sequence (rectangle-triangle deconvolution) or a triangular one (triangle-triangle deconvolution). It is shown that there is a loss of resolution. Perturbations arising from the use of an imperfect mechanical device in elastic scattering, give rise to a negative image of the diffraction function due to variation of the chopper transmission with wavelength. This effect can be corrected a posteriori. The other perturbations give rise to loss of resolution or background modulation

The-GEOSCOPE Program

he purpose of the GEOSCOPE Program was the installation of 25 stations in the standard configuration defined by the FDSN (VBB 24 bit, continuous recording at 20 samples/s). The installation is almost complete. The effort this year focussed on the accessibility of data, either through the IRIS/GOPHER system for large earthquakes, through CD-ROM production, and on line using the Juke-box JUMBO in the GEOSCOPE Data Center (Paris). This aspect will be stressed as the cooperation between IRIS and GEOSCOPE intensifies

The-GEOSCOPE Program

he purpose of the GEOSCOPE Program was the installation of 25 stations in the standard configuration defined by the FDSN (VBB 24 bit, continuous recording at 20 samples/s). The installation is almost complete. The effort this year focussed on the accessibility of data, either through the IRIS/GOPHER system for large earthquakes, through CD-ROM production, and on line using the Juke-box JUMBO in the GEOSCOPE Data Center (Paris). This aspect will be stressed as the cooperation between IRIS and GEOSCOPE intensifies