Stig Steenstrup

A high-pressure study of thallium

High-pressure experiments on metallic thallium have been carried out using a diamond-anvil cell for pressures up to 32u2005GPa. A hexagonal close packed → face-centred cubic (f.c.c.) phase change takes place around 4u2005GPa. Fitting of the experimental data to the Murnaghan and Birch equations of state has been used to obtain values for the bulk moduli and their pressure derivatives for the two phases. Extrapolation of the high-pressure data for the f.c.c. phase to low pressure has been used to obtain an estimate of the pressure to about 0.23u2005GPa in nanosized metastable f.c.c. thallium inclusions formed in aluminium by ion implantation.

High pressure studies up to 50 GPa of Bi-based high-Tc superconductors

The high-Tc superconductor with nominal composition BiSrCaCu2Ox has been studied at high pressure, i.e. up to 50 GPa. A tetragonal structure was compatible with the measurements at all pressures, and no phase change was observed. The bulk modulus, B0 = 62.5 GPa, obtained has a somewhat smaller value than the one estimated earlier.

The pressure-induced transformation B1 to B2 in actinide compounds

The phase transformation from NaCl structure (B 1) to CsCl structure (B2) in actinide compounds has been studied using X-ray powder diffraction in the pressure range up to about 60u2005GPa. It is shown that the transition is sluggish, has a strong hysteresis and is accompanied by a volume change in the range 8–12%. These features are similar to those of the corresponding transition in the alkali halides and other B1 compounds, indicating a common mechanism for the transformation as concerns the lattice geometry.

High pressure phases of terbium: possibility of a thcp phase

Abstract High pressure phases of trivalent Yb studied by energy dispersive X-ray diffraction with synchrotron radiation exhibits the closed packed sequence (hcp→Sm→dhcp→fcc) typical of the trivalent rare earth metals. Furthermore, a phase consistent with a triple hexagonal closed packed (thcp) structure was observed in a narrow pressure range around 30 GPa.

Correction for escape in X-ray spectra measured using a Ge detector

X-ray spectra measured with a germanium detector can be corrected for escape. A numerical procedure to perform the correction has been implemented. The method employs a point by point subtraction and allows for energy-dependent detector resolution.

A maximum entropy method to compute the 13NH3 pulmonary transit time from right to left ventricle in cardiac PET studies

The distribution function of pulmonary transit times (fPTTs) contains information on the transit time of blood through the lungs and the dispersion in transit times. Most of the previous studies have used specific functional forms with adjustable parameters to characterize the fPTT. It is the purpose here to investigate the possibility of estimating the fPTT in a model-free way. The method employs the maximum entropy principle and is used, in particular, on cardiac positron emission tomographic (PET) studies but is believed to be more generally applicable. Using this principle in a test case, we were able to accurately identify a two-peaked transfer function, which may theoretically be seen in patients with pulmonary disease confined to one lung. Transit time values for [13N]-ammonia were produced by applying the algorithm to PET studies from normal volunteers.