Vol. 43 No.6 - Highlights

A body of work accomplished by the CEA/DAM is reviewed to determine accurate nuclear reaction cross sections for use in neutron transport codes. This work integrates theory and modeling, experiment, computer simulation, and statistical analysis. It involves researchers who thrive on multidisciplinary work, and who are motivated to achieve realistic simulation predictions in nuclear technology applications. Not only has the group succeeded in creating databases of accurate cross sections, but, in every aspect of the work, significant progress has been made in the understanding of the underlying nuclear physics.

This unique analysis of advances in applied nuclear reaction physics, includes, notably: (1) Fission and inelastic scattering using detailed nuclear structure descriptions of actinides; (2) Integral simulations of critical assemblies that reveal compensating errors between different reaction channels (Fig) – this CEA discovery is now motivating the broader community to identify and eliminate deficiencies; (3) Identification of limitations on the applicability of the surrogate method for neutron capture.

CEA/DAM contributes to the Joint Evaluated Fusion and Fission File (JEFF3.1), the database that is used widely in European nuclear technologies. It collaborates closely with related efforts in the USA (ENDF/B-VII) and Japan (JENDL). The authors show useful comparisons of their work against those based on ENDF/B-VII.0 and JENDL3.3-08, demonstrating the quality of these various capabilities. CEA/DAM strengths lie, in particular, in bringing microscopic theoretical insights in fission, coupled-channel optical model, and inelastic scattering to advance the quality of their application nuclear databases.

Plasmas are ionized gases that contain a mixture of electrons, ions, and neutrals. Plasmas are generated by adding some form of energy to a neutral gas. The most common method to generate plasma is to subject a gas to high level of electrical stress, which initiates an electronic avalanche and thus generating electrons, ions, and molecular fragments such as radicals and other reactive species. Low temperature plasmas in particular produce chemical species including reactive oxygen species (ROS) and reactive nitrogen species (RNS). ROS and RNS exhibit strong oxidative properties and can potentially trigger signaling pathways in biological cells. For example oxidation of the lipids and proteins that constitute the membrane of biological cells leads to the loss of their functions. In such an environment, bacterial cells were found to die within minutes or even seconds. In eukaryotes, very low doses of low temperature plasmas were found to help the proliferation of some skin cells and at slightly higher doses, plasmas can induce apoptosis, or programmed cell death, opening the possibility to use plasma technology against cancerous cells.

In this study, we investigate the effect of low temperature atmospheric pressure plasma towards the progression of cancerous human T-cell leukemia cells. The source of low temperature plasma is the plasma pencil, which utilizes short duration high voltage pulses. Our data shows that cell morphology and cell viability was affected in a dose-dependent manner after treatment with low temperature plasma. The outcome of this study revealed that the effect of plasma exposure was not immediate, but had a delayed effect and increasing the time of plasma exposure resulted in increased leukemia cell death.

We propose a novel approach to control the polarization of surface emission from quantum-dot-like objects - an issue of great interest for optoelectronic applications where both fully polarized and polarization-insensitive gain is highly desired. The experimental study of strongly asymmetric In(Ga)As/InP nanostructures in a wide range of geometries verifies our theoretical modelling.

A simple analytical formula describing the dependence of the degree of polarization on the nanostructure height is established, which is of practical importance when aiming at fabricating structures with predictable polarization properties without sophisticated atomistic modelling. Furthermore, it shows that the observed changes in polarization are a consequence of the valence band states mixing. It appears that the surface emission is enhanced for the polarization along the in-plane larger dimension and sensitive to the lateral shape anisotropy. Its saturation for highly asymmetric geometries makes fully polarized emission unachievable solely by the lateral symmetry control.

Our study reveals very high sensitivity of the surface emission polarization to the object height and demonstrates its continuous height-driven enhancement up to 0.9 for so called columnar quantum dashes – a record value for any epitaxial system. It is possible only through the combination of a strong in-plane anisotropy and an enhanced nanostructure height – the condition shown to be essential to fully tailor the polarization properties of the surface emission. Moreover, it opens up a possibility to combine unpolarized edge emission and strongly polarized surface emission in one device via its active region engineering.

A general statistical analysis (classical statistics) is a common experimental procedure to determine the uncertainty of photon statistics in measuring a line shift and width. Given the importance of taking into account the background as well as the measured signal in any photon measurement, the paper describes both the perfect spectrometer measurements with a zero and nonzero background as well as the case of an imperfect spectrometer.

More complex line shapes are reviewed and the problems of their evaluation are discussed. The paper then addresses all situations when, instead of making continuous measurements, modern detection arrays with finite-width wavelength bins (pixels) are employed.

By providing detailed mathematical descriptions for the line width and shapes discussed with either zero or nonzero background subtraction, this work will be of considerable use for many researchers in assessing their experimentally obtained results.

In 1960 Brillouin wrote a footnote in his famous book on ‘Wave Propagation And Group Velocity’ (p.79): “The negative parts of the (theoretical) group velocity have no physical meaning. A negative velocity shows the maximum of the group at the output before it has entered the input of a special medium”. However, since 1985 several physicists measured a negative delay and thus a negative group velocity at a sharp molecular resonance. Recently a giant negative group time delay was observed in a medium of two microwave mode adaptors separated by a 20 m long waveguide when they are not parallel aligned. Instead of +60 ns vacuum time -2.2 µs were measured for the same distance. Such adaptors are used in communication technology to transform rectangular waveguide modes into circular waveguide modes: For instance, in the case of TV reception via satellite. The strange behaviour is based on a 90° shift of the linear polarization of the superimposed right and left circular wave modes in the case of a perpendicular adaptor orientation. Remarkable, the polarization shift of 90° takes place at each reflection and in this way makes the shifted adaptors to reflectors, whenever the distance between the adaptors equals a multiple of half the wavelength.

What can classical chaos do to a quantum system is a fundamental issue, which is highly relevant to a number of branches in physics. The field, named quantum chaos, has been active for at least three decades, where the focus has been on non-relativistic quantum systems described by the Schrodinger equation. With respect to relativistic quantum systems governed by the Dirac equation, Berry and Mondragon were the first to investigate the energy-level statistics of a chaotic neutrino billiard.

The present work presents an astonishing case of how chaos may affect relativistic quantum tunnelling dynamics. By developing an efficient method to solve the Dirac equation in the setting where relativistic quantum particles can tunnel between two symmetric cavities through a potential barrier, it appears that chaotic cavities can mostly suppress the spread in the tunnelling rate. Specifically, when the classical dynamics is integrable, the tunnelling rate for any given energy can assume values in a range that increases with energy (fig. upper panel). However, when the cavities allow fully chaotic dynamics, spread in the tunnelling rate is strongly reduced (lower panel). This suppression can be explained by the emergence of certain class of pointer states (fig.). A remarkable feature, which does not arise in non-relativistic quantum tunnelling systems, is that substantial tunnelling exists even when the particle energy nears zero. This is a consequence of the relativistic quantum phenomenon of Klein tunnelling. The authors found similar results in tunnelling devices made of graphene, implying that the field of relativistic quantum chaos can be highly relevant to the development of such devices.

This article presents a model that describes dividing cells within human tissues from the perspective of physicists could help further the understanding of cancer growth. It explores the relative impact of the mechanical pressure induced by dividing cells in biological tissues. and could have significant implications for the understanding of cancer growth.

The authors create a two-component mathematical model accounting for both the cells and the fluid caught in between. On the one hand, cells are modelled as behaving like a dividing fluid subject to expansion. On the other hand, the interstitial fluid is akin to an ideal fluid that cannot be compressed. This model is designed to elucidate the nature of mechanical pressure exerted upon dividing cells by their surrounding tissues, referred to as homeostatic pressure.

It replaces a previous single-component model they developed last year. Its assumption: the homeostatic pressure is proportional to the fluid pressure within the tissue. If that were the case, very tall organisms such as giraffes could not exist, because the cells in their lower body would die under pressure.

Thanks to the two-component model, the authors find that it is the cells’ pressure and not the interstitial fluid’s pressure that influences the level of cell division. Going one step further, they pinpoint the range of fluid pressure required to drive cell flow within the body.

Such models could help gain a greater understanding of the importance of the disruption of homeostatic pressure in biological tissues caused by cancer cells that are characterised by abnormal levels of cell proliferation.

Controlling the plasma in magnetic fusion experiments remains a major challenge, in particular, with the advent of large-scale facilities such as the ITER tokamak (presently under construction in Cadarache, France). In order to support the operation of the machine, an extensive set of measurements is planned. Passive spectroscopy is a convenient diagnostic tool since it is non-intrusive and quite easy to implement experimentally. For instance, the hydrogen Balmer a line (3 ⇒ 2 transition, visible range) is considered as a way to measure fluxes of the hydrogen isotopes (H, D, T) in the divertor region (see “Progress in the ITER Physics Basis”, Nucl. Fusion, special issue, 2007).

The plasma density in the ITER divertor will be large enough to make Stark broadening observable on the spectral lines of hydrogen isotopes. Such neutral particles survive in the cold and complex edge plasma and their spectra provide invaluable information on its conditions. We have shown that the case where the Stark perturbation can be associated with a series of binary collisions with ions (impact approximation, see the work of Hans Griem, Plasma Spectroscopy) may be adapted to conditions foreseen in ITER. This model is based on an estimation of the S-matrix for atom-perturber collisions using a series expansion for large impact parameters (weak collisions) and, on the other hand, using a cut-off accounting for the oscillating behaviour of the wave-function in the case of small impact parameters (strong collisions). Confrontations with computer simulations indicate that the model is a good candidate for accurate diagnostics in the ITER plasma.

Energy flowing from large-scale to small-scale places may be prevented from flowing freely in specific conditions, similar to those found in disordered solids. In the present article, the author presents an exception he found in a model of turbulence, indicating that there are energy flows from large to small scale in confined space. Indeed, under a specific energy threshold, there are no energy flows, similar to the way electron currents and energy spreading are stopped in disordered solids.

The author relies on numerical simulations to study a kind of turbulence—known as Kolmogorov turbulence—that describes how energy flows from large to small scale in a confined space. According to this concept energy is introduced on large scales, e.g. by wind, and it is absorbed on small scales due to energy dissipation. This approach assumes that a small perturbation will make the system evolution chaotic as energy flows from large to small scales.

However, the author finds that a phenomenon normally observed in disordered metals, called Anderson localisation, which implies that there is no energy flow from one side of the metal to the other, also occurs with the type of turbulences he is focusing on. As a result, energy flow from large scale to small scale does not happen under specific circumstances where the energy level is below a certain threshold level. This result is in keeping with our intuitive experience of a small wind not creating a storm, and that wind needs to reach a certain threshold before a storm can be created.

A new model of background noise present in the nervous system could help better understand neuronal signalling delay in response to a stimulus. The authors present a biologically accurate model of the underlying noise present in the nervous system, which has implications for explaining how noise, modulated by unreliable synaptic transmission, induces a delay in the response of neurons to external stimuli as part of the neurons coding mechanism.

Neurons communicate by means of electrical pulses, called spikes, exchanged via synapses. The time it takes for brain cells to first respond to an external stimulus with an electric signal —commonly referred to as fist-spike latency—is of particular interest to scientists. That is because it is thought to carry much more neural information than subsequent serial spike signals.

The authors analyse the presence of noise in the nervous system detected through changes in first-spike latency. The noise is due to the large number of incoming excitatory and inhibitory spike inputs bombarding synapses. Previous attempts at noise modelling used a Gaussian approximation. Now, the authors have devised a noise model that is closer to the biological reality.

It is shown that there is a relation between the noise and delays in spike signal transmission, caused by unreliable synapses. Yet, synaptic unreliability could be controlled by tuning the incoming excitatory and inhibitory input signalling regime and the coupling strength between inhibitory and excitatory synapses. Ultimately, this could help neurons encode information more accurately.

A new study improves our understanding of plasma sources, a state of matter similar to gas in which a certain portion of the particles are ionised and which are used for example in plasma display panels. Under certain circumstances, plasma tends to form structures such as filaments of electric discharge akin to mini-lightning. The authors investigated the transition from a highly ordered filament pattern, which is arranged hexagonally, to a disordered system due to the reduction of the externally applied voltage.

To analyse the transition in the order of the discharge, Wild and colleagues used two approaches. First, they used a method commonly employed to analyse spatial patterns, called 2D Fourier transformation. Then, for the first time, they applied an analysis tool typically used to evaluate dusty plasma discharges, known as triple correlation function.

The authors observed a pivot point in the voltage at which the decaying order started occurring. This information can ultimately be used to guarantee the quality of applications such as plasma screens. That is because the dissolution of self-organised electric discharge filaments in plasma matter enhances the homogeneity of the matter.