Effect of particle geometry on phase transitions in two-dimensional liquid crystals

Using a version of density-functional theory which combines Onsager approximation and fundamental-measure theory for spatially nonuniform phases, we have studied the phase diagram of freely rotating hard rectangles and hard discorectangles. We find profound differences in the phase behavior of these models, which can be attributed to their different packing properties. Interestingly, bimodal orientational distribution functions are found in the nematic phase of hard rectangles, which cause a certain degree of biaxial order, albeit metastable with respect to spatially ordered phases. This feature is absent in discorectangles, which always show unimodal behavior. This result may be relevant in the light of recent experimental results which have confirmed the existence of biaxial phases. We expect that some perturbation of the particle shapes (either a certain degree of polydispersity or even bimodal dispersity in the aspect ratios) may actually destabilize spatially ordered phases thereby stabilizing the biaxial phase.     

Electron diffraction studies of the molecular structures of iron(II) chloride and iron(II) bromide. Evidence on the structure of dimeric species Fe2Br4

An expression for the extreme values of mean-square amplitudes of vibrations in polyatomic molecules has been derived which permits estimation of the mean-square amplitude without solving the vibrational problem. This expression can be improved for the stretching and scissoring modes when the assignment of frequencies is known. In turn, the corresponding vibrational frequency may be estimated from the experimental value of the mean-square amplitude. The mean-square amplitudes of the butadiene-1,3 molecule are considered as an example.     

The limit of detectability in X-ray electron-probe microanalysis

Summary The definitions given by various authors for the limit of detectability are compared and their relationship to the basic measurements (especially the background value) is explained. A new definition is derived on the assumption that the relative standard deviation must not exceed 33.3% if negative results are to be avoided. Negative results may still appear, but only very rarely. The properties of this limit of detectability are discussed, together with the effect of making multiple measurements of the background and peak.

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Zusammenfassung Die von verschiedenen Autoren angegebenen Definitionen der Nachweisgrenze wurden verglichen und deren Beziehung zu den Bezugsmessungen (besonders des Untergrundes) wurde erklärt. Eine neue Definition wurde von der Annahme abgeleitet, daß die relative Standardabweichung 33,3% nicht überschreiten darf, sollen negative Resultate vermieden werden. Solche können zwar auftreten, aber nur sehr selten. Die Eigenschaften dieser Nachweisgrenze wurden ebenso wie die Auswirkung mehrfacher Messungen des Untergrundes und der Peaks diskutiert.

     

Über die Messung geringer Mengen Radium und Radon

Zusammenfassung Zur Messung geringer²²⁶Radium- und²²²Radongehalte werden auch auf dem Gebiet der Strahlenhygiene verschiedene Verfahren verwendet. Eine zylindrische Szintillationskammer günstigster Abmessungen bietet die Möglichkeit, Personen, die²²⁶Ra inkorporiert haben, ohne vorhergehende Anreicherung des Atem-Radons zu kontrollieren sowie Lebensmittel und andere Proben von niedrigem²²⁶Ra- und²²²Rn-Gehalt zu analysieren.

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Measurement of small amounts of radium and radon

Summary Various procedures employed for the measurement of slight²²⁶radium and²²²radon contents have also been used for the radiation hygiene. A cylindrical scintillation chamber of the most favorable dimensions offers the possibility of checking persons who have incorporated²²⁶Ra, without prior concentration of the breath-radon, and also in the analysis of foods and other samples that contain minute amounts of²²⁶Ra and²²²Ra.