Interdiffusion studies in Bi-based layered systems with nanosecond laser pulses

Interdiffusion processes are induced by nanosecond laser pulses from an excimer laser. The Bi-based systems studied are formed by a Bi layer and a Sb or Ge layer. Configurations with Bi at the surface layer or at the innermost layer are both studied. Real-time reflectivity measurements are performed during the irradiation to determine the process kinetics and times and Rutherford backscattering spectrometry is used to obtain the concentration depth profiles. It will be shown that there is an interfacially initiated diffusion process in the Bi-Sb system and that the diffusion coefficients of this system within the liquid phase are in the 10^–5–10^–6 cm²/s range. The Bi-Ge system shows instead little mixing, the diffusion coefficients of the system within the liquid phase being at least two orders of magnitude lower. The differences observed when Bi is the surface layer or the innermost one are related to the different thermal responses of the system.     

On the cube problem of Las Vergnas

We prove a conjecture of Las Vergnas in dimensions d7: The matroid of the d-dimensional cube C ^d has a unique reorientation class. This extends a result of Las Vergnas, Roudneff and Salaün in dimension 4. Moreover, we determine the automorphism group G ^d of the matroid of the d-cube C ^d for arbitrary dimension d, and we discuss its relation to the Coxeter group of C ^d . We introduce matroid facets of the matroid of the d-cube in order to evaluate the order of G ^d . These matroid facets turn out to be arbitrary pairs of parallel subfacets of the cube. We show that the Euclidean automorphism group W ^d is a proper subgroup of the group G ^d of all matroid symmetries of the d-cube by describing genuine matroid symmetries for each Euclidean facet. A main theorem asserts that any one of these matroid symmetries together with the Euclidean Coxeter symmetries generate the full automorphism group G ^d . For the proof of Las Vergnas' conjecture we use essentially these symmetry results together with the fact that the reorientation class of an oriented matroid is determined by the labeled lower rank contractions of the oriented matroid. We also describe the Folkman-Lawrence representation of the vertex figure of the d-cube and a contraction of it. Finally, we apply our method of proof to show a result of Las Vergnas, Roudneff, and Salaün that the matroid of the 24-cell has a unique reorientation class, too.     

Quality control of201TlCl solution obtained at IPEN-CNEN/SP

The radiopharmaceutical²⁰¹TlCl(thallium-201 chloride) is used in nuclear medicine for myocardial visualization. The solution of²⁰¹TlCl was prepared using²⁰¹Tl obtained by irradiating a natural mercury target with protons from the CV-28 cyclotron installed at IPEN-CNEN/SP. This solution was subjected to different quality control processes required for its use in medicine. Some of these controls concerned the determination of the radionuclidic impurities:²⁰⁰Tl,²⁰²Tl and²⁰³Hg; the chemical identification of²⁰¹Tl⁺; the hydrazine concentration, mercury contamination and the presence of phosphate. Furthermore. the biological distribution in Wistar rats and tests for sterility, pyrogenicity and toxicity were carried out. It was verified that the solution obtained was in the form of thallous chloride. This radiopharmaceutical gave good heart images in animals but due to the high levels of²⁰⁰Tl and²⁰²Tl its use in humans is not possible unless enriched²⁰²Hg is used as target in the irradiation.