Bohr radius | EPFL Graph Search

The Bohr radius (a0) is a physical constant, approximately equal to the most probable distance between the nucleus and the electron in a hydrogen atom in its ground state. It is named after Niels Bohr, due to its role in the Bohr model of an atom. Its value is Definition and value The Bohr radius is defined as a_0 = \frac{4 \pi \varepsilon_0 \hbar^2}{e^2 m_{\text{e}}} = \frac{\varepsilon_0 h^2}{\pi e^2 m_{\text{e}}} = \frac{\hbar}{m_{\text{e}} c \alpha} , where

\varepsilon_0 is the permittivity of free space,
\hbar is the reduced Planck constant,
m_{\text{e}} is the mass of an electron,
e is the elementary charge,
c is the speed of light in vacuum, and
\alpha is the fine-structure constant.

The CODATA value of the Bohr radius (in SI units) is History In the Bohr model for atomic structure, put forward by Niels Bohr in 1913, electrons o

Reading and writing single-atom magnets

Donat Fabian Natterer

The single-atom bit represents the ultimate limit of the classical approach to high-density magnetic storage media. So far, the smallest individually addressable bistable magnetic bits have consisted of 3-12 atoms(1-3). Long magnetic relaxation times have been demonstrated for single lanthanide atoms in molecular magnets(4-12), for lanthanides diluted in bulk crystals(13), and recently for ensembles of holmium (Ho) atoms supported on magnesium oxide (MgO)(14). These experiments suggest a path towards data storage at the atomic limit, but the way in which individual magnetic centres are accessed remains unclear. Here we demonstrate the reading and writing of the magnetism of individual Ho atoms on MgO, and show that they independently retain their magnetic information over many hours. We read the Ho states using tunnel magnetoresistance(15,16) and write the states with current pulses using a scanning tunnelling microscope. The magnetic origin of the long-lived states is confirmed by single-atom electron spin resonance(17) on a nearby iron sensor atom, which also shows that Ho has a large out-of-plane moment of 10.1 +/- 0.1 Bohr magnetons on this surface. To demonstrate independent reading and writing, we built an atomic-scale structure with two Ho bits, to which we write the four possible states and which we read out both magnetoresistively and remotely by electron spin resonance. The high magnetic stability combined with electrical reading and writing shows that single-atom magnetic memory is indeed possible.

Nature Publishing Group2017

Nondestructive determination of fresh and spent nuclear fuel rod density distributions through computerised gamma-ray transmission tomography

Stefano Caruso, Rakesh Chawla, Fabian Jatuff

The morphology of single PWR UO2 fuel rods has been investigated by employing computerised gamma-ray transmission tomography. Four highly burnt fuel rods of different burnups (from 52 GWd/t to 126GWd/t) and a fresh rod have been investigated. This paper describes the tomographic station built to acquire the projections together with the related experimental procedure, followed by the description of the image reconstruction techniques implemented. The principal results obtained are presented and analysed at three levels. First, the derived linear attenuation coefficient matrix, which is very useful for the implementation of emission tomography, is discussed. Second, the variation of density as a function of rod radius is presented, showing an interesting morphological change of the fuel with burnup. Particularly for the highest burnup sample, the periphery is characterised by a lower density (a steep decrease of almost 10% from the adjacent region) characteristic of an ultrahigh-burnup high-porosity structure. Finally, the nondestructive derivation of the volume-averaged fuel density was performed, compared with other experimental data, and plotted as a function of burnup, so that an interesting linear relationship between density and burnup was established, providing a potential indicator for the noninvasive determination of nuclear fuel rod burnup. Atomic Energy Society of Japan.

Atomic Energy Society of Japan2008

Kink instability of flux ropes anchored at one end and free at the other

Ivo Furno

The kink instability of a magnetized plasma column (flux rope) is a fundamental process observed in laboratory and in natural plasmas. Previous theoretical, experimental, and observational work has focused either on the case of periodic (infinite) ropes (relevant to toroidal systems) or on finite ropes with both ends tied to a specified boundary (relevant to coronal ropes tied at the photosphere). However, in the Sun's corona and in astrophysical systems there is an abundant presence of finite flux ropes tied at one end but free at the other. Motivated by recent experiments conducted on the RSX device (Furno et al., 2006) and by recent theoretical work (Ryutov et al., 2006), the present paper investigates by simulation the linear and nonlinear evolution of free-ended flux ropes. The approach is based on comparing the classic case of a periodic flux rope with the case of a rope tied at one end and free at the other. In the linear phase, periodic and free ropes behave radically differently. A simulation analysis of the linear phase confirms the experimental and phenomenological findings relative to an increased instability of a free rope: the new stability limit is shown to be just half of the classic limit for periodic ropes. In the nonlinear phase, reconnection is observed to be a fundamental enabler to reach the eventual steady state. The mechanism for saturation of a flux rope is investigated and compared with the classic theory (the so-called bubble state model) by Rosenbluth et al. (1976). A remarkable agreement is found for the classic periodic case. The case of a free rope is again very different. The saturated state is observed to present a outwardly spiraling configuration with the displacement of the plasma column increasing progressively and monotonically from the tied end to the free end. The maximum displacement is observed at the free end where it is consistent with the displacement observed in a periodic rope. The key distinction is that in a periodic rope the same displacement is observed throughout the whole rope to form a helix with constant radius.

2006

Kink instability of flux ropes anchored at one end and free at the other

Ivo Furno

2006