Methodological possibilities of positron annihilation lifetime (PAL) spectroscopy applied to characterize different types of nanomaterials treated within three-term fitting procedure are critically reconsidered. In contrast to conventional three-term analysis based on admixed positron- and positronium-trapping modes, the process of nanostructurization is considered as substitutional positron-positronium trapping within the same host matrix. Developed formalism allows estimate interfacial void volumes responsible for positron trapping and characteristic bulk positron lifetimes in nanoparticle-affected inhomogeneous media. This algorithm was well justified at the example of thermally induced nanostructurization occurring in 80GeSe2-20Ga2Se3 glass.

No abstract

General-relativistic magnetohydrodynamic (GRMHD) simulations have revolutionized our understanding of black-hole accretion. Here, we present a GPU-accelerated GRMHD code H-AMR with multi-faceted optimizations that, collectively, accelerate computation by 2-5 orders of magnitude for a wide range of applications. Firstly, it involves a novel implementation of a spherical-polar grid with 3D adaptive mesh refinement that operates in each of the 3 dimensions independently. This allows us to circumvent the Courant condition near the polar singularity, which otherwise cripples high-res computational performance. Secondly, we demonstrate that local adaptive time-stepping (LAT) on a logarithmic spherical-polar grid accelerates computation by a factor of 10 compared to traditional hierarchical time-stepping approaches. Jointly, these unique features lead to an effective speed of ∼ 10 9 zone-cyclesper-second-per-node on 5,400 NVIDIA V100 GPUs (i.e., 900 nodes of the OLCF Summit supercomputer). We demonstrate its computational performance by presenting the first GRMHD simulation of a tilted thin accretion disk threaded by a toroidal magnetic field around a rapidly spinning black hole. With an effective resolution of 13,440 × 4,608 × 8,092 cells, and a total of 22 billion cells and ∼ 0.65 × 10 8 timesteps, it is among the largest astrophysical simulations ever performed. We find that frame-dragging by the black hole tears up the disk into two independently precessing sub-disks. The innermost sub-disk rotation axis intermittently aligns with the black hole spin, demonstrating for the first time that such long-sought alignment is possible in the absence of large-scale poloidal magnetic fields.

No abstract

keV) 21,760 cm2 Energy Resolution 85 -175 eV FWHM Time Resolution 100 ns Collimator 4 arcmin FWHM Background Rate 2.2 c/s Count Rate on Crab (0.2-10 keV) 148,000 Large Area Detector (LAD) Energy Range 2-30 keV Effective Area (cm^2 @ 10 keV) 51,000 cm2 Energy Resolution 200 -300 eV FWHM Time Resolution 10 µs Collimator 1° FWHM Count Rate on Crab (2-30 keV) 156,000 Background Rate 822 c/s (5 mcrab)