Stopping of slow ions

The 50th anniversary of the appearance of a series of seminal papers by O.B. Firsov is taken as an occasion to have a look at some central aspects of the stopping of slow ions in matter. A brief characterization of Firsov’s model of electronic energy loss is given in comparison with alternative views. The main part of the paper is devoted to central problems such as scaling behavior, Z ₁ and Z ₂ structure, deviations from velocity proportionality, threshold behavior, metal-insulator and gas-solid differences, and correlation between nuclear and electronic stopping.     

Evaluation of intracranial stenoses and aneurysms with accelerated 4D flow

The aim of this study was to evaluate intracranial arterial stenoses and aneurysms with accelerated time-resolved three-dimensional (3D) phase-contrast MRI or 4D flow. The 4D flow technique was utilized to image four normal volunteers, two patients with intracranial stenoses and two patients with intracranial aneurysms. In order to reduce scan time, parallel imaging was combined with an acquisition strategy that eliminates the corners of k-space. In the two patients with intracranial stenoses, 4D flow velocity measurements showed that one patient had normal velocity profiles in agreement with a previous magnetic resonance angiogram (MRA), while the second showed increased velocities that indicated a less significant narrowing than suspected on a previous MRA, as confirmed by catheter angiography. This result may have prevented an invasive angiogram. In the two patients with 4-mm intracranial aneurysm, one had a stable helical flow pattern with a large jet, while the other had a temporally unstable flow pattern with a more focal jet possibly indicating that the second aneurysm may have a higher likelihood of rupture. Accelerated 4D flow provides time-resolved 3D velocity data in an 8- to 10-min scan. In the stenosis patients, the addition of 4D flow to a traditional MRA adds the velocity data provided from transcranial Doppler ultrasound (TCD) possibly allowing for more accurate grading of stenoses. In the aneurysm patients, visualization of flow patterns may help to provide prognostic information about future risk of rupture.     

The Mixed Valence Structure of “R–NiF3”

XAS at the nickel K-edge and UV-visible studies of rhombohedral nickel trifluoride indicate that the compound has a mixed valence composition Ni[NiF₆] and is isostructural with Pd[PdF₆]. XAS data are reported for NiF₂, [NiF₆]^2–, [NiF₆]^3–, Pd[PdF₆], CoF₃ and RhF₃ in support of the Ni[NiF₆] study. New data from refinements of X-ray powder diffraction of CoF₃ and IrF₃ are reported in the context of the distortion from hexagonal close packing of rhombohedral trifluorides.