Cosmic dust impacts on the Hubble Space Telescope.

Exposure of the Hubble Space Telescope to space in low Earth orbit resulted in numerous hypervelocity impacts by cosmic dust (micrometeoroids) and anthropogenic particles (orbital debris) on the solar arrays and the radiator shield of the Wide Field and Planetary Camera 2, both subsequently returned to Earth. Solar cells preserve residues from smaller cosmic dust (and orbital debris) but give less reliable information from larger particles. Here, we present images and analyses from electron, ion and X-ray fluorescence microscopes for larger impact features (millimetre- to centimetre-scale) on the radiator shield. Validated by laboratory experiments, these allow interpretation of composition, probable origin and likely dimensions of the larger impactors. The majority (~90%) of impacts by grains greater than 50 µm in size were made by micrometeoroids, dominated by magnesium- and iron-rich silicates and iron sulfides, metallic iron-nickel and chromium-rich spinel similar to that in ordinary chondrite meteorites of asteroid origin. Our re-evaluation of the largest impact features shows substantially fewer large orbital debris impacts than reported by earlier authors. Mismatch to the NASA ORDEM and ESA MASTER models of particle populations in orbit may be partly due to model overestimation of orbital debris flux and underestimation of larger micrometeoroid numbers. This article is part of the theme issue 'Dust in the Solar System and beyond'.

The fabrication of aspherical microlenses using focused ion-beam techniques.

Aspheric lenses are the most common method for correcting for spherical aberrations but, in microlens production, highly-controlled lens profiles are hard to achieve. We demonstrate a technique for creating bespoke, highly-accurate aspheric or spherical profile silicon microlens moulds, of almost any footprint, using focused ion-beam milling. Along with this, we present a method of removing induced ion-beam damage in silicon, via a hydrofluoric acid etch, helping to recover the surface's optical and chemical properties. In this paper, we demonstrate that our milled and etched moulds have a roughness of 4.0-4.1 nm, meaning they scatter less than 1% of light, down to wavelengths of 51 nm, showing that the moulds are suitable to make lenses that are able to handle light from UV up to infra-red. Using empirical experiments and computer simulations, we show that increasing the ion-dose when milling increases the amount of gallium a hydrofluoric acid etch can remove, by increasing the degree of amorphisation within the surface. For doses above 3000 µC/cm(2) this restores previous surface properties, reducing adhesion to the mould, allowing for a cleaner release and enabling higher quality lenses to be made. Our technique is used to make aspheric microlenses of down to 3 µm in size, but with a potential to make lenses smaller than 1 µm.

Enhanced imaging of developed fingerprints using mass spectrometry imaging.

Latent fingermarks are invisible to the naked eye and normally require the application of a chemical developer followed by an optical imaging step in order to visualize the ridge detail. If the finger deposition is poor, or the fingermark is aged, it can sometimes be difficult to produce an image of sufficient quality for identification. In this work, we show for the first time how mass spectrometry imaging (in this case time-of-flight secondary ion mass spectrometry, ToF-SIMS) can be used to enhance the quality of partially recovered fingermarks. We show three examples of how chemical imaging can be used to obtain enhanced images of fingermarks deposited on aluminium foil, glass and the handle of a hand grenade compared with conventional development techniques.

Integrated Ion Beam Analysis (IBA) in Gunshot Residue (GSR) characterisation.

Gunshot Residue (GSR) is residual material from the discharge of a firearm, which frequently provides crucial information in criminal investigations. Changes in ammunition manufacturing are gradually phasing out the heavy metals on which current forensic GSR analysis is based, and the latest Heavy Metal Free (HMF) primers urgently demand new forensic solutions. Proton scanning microbeam Ion Beam Analysis (IBA), in conjunction with the Scanning Electron Microscope equipped with an Energy Dispersive X-ray Spectrometer (SEM-EDS), can be introduced into forensic analysis to solve both new and old problems, with a procedure entirely commensurate with current forensic practice. Six cartridges producing GSR particles known to be interesting in casework by both experience and the literature were selected for this study. A standard procedure to relocate the same particles previously analysed by SEM-EDS, based on both secondary electron (SE) and X-ray imaging was developed and tested. Elemental Particle Induced X-ray Emission (PIXE) mapping of the emitted X-rays allowed relocation in a scan of 10 µm × 10 µm of even a 1 µm GSR particle. The comparison between spectra from the same particle obtained by SEM-EDS and IBA-PIXE showed that the latter is much more sensitive at mid-high energies. Results that are very interesting in a forensic context were obtained with particles from a cartridge containing mercury fulminate in the primer. Particle-induced gamma-ray emission (PIGE) maps of a particles from HMF cartridges allowed identification of Boron and Sodium in particles from hands using the (10)B(p,α1γ)(7)Be, (11)B(p,p1γ)(11)B and (23)Na(p,p1γ)(23)Na reactions, which is extraordinary in a forensic context. The capability for quantitative analysis of elements within individual particles by IBA was also demonstrated, giving the opportunity to begin a new chapter in the research on GSR particles. The integrated procedure that was developed, which makes use of all the IBA signals, has unprecedented characterisation and discrimination power for GSR samples.

Phase-sensitive frequency conversion of quadrature modulated signals.

Two mechanisms that can make frequency conversion based on nonlinear mixing dependent on the phase of the input signal are identified. A novel phase-to-polarization converter that converts the orthogonal phase components of an input signal to two orthogonally polarized outputs is proposed. The operation of this scheme and a previously reported scheme at an increased symbol rate are simulated with semiconductor optical amplifiers (SOAs) as the nonlinear devices. Experimental results demonstrate the effectiveness of SOAs for nonlinear mixing over a wide range of wavelengths and difference frequencies and confirm the accuracy of the numerical model.

Phase discrimination and simultaneous frequency conversion of the orthogonal components of an optical signal by four-wave mixing in an SOA.

Simultaneous conversion of the two orthogonal phase components of an optical input to different output frequencies has been demonstrated by simulation and experiment. A single stage of four-wave mixing between the input signal and four pumps derived from a frequency comb was employed. The nonlinear device was a semiconductor optical amplifier, which provided overall signal gain and sufficient contrast for phase sensitive signal processing. The decomposition of a quadrature phase-shift keyed signal into a pair of binary phase-shift keyed outputs at different frequencies was also demonstrated by simulation.

Generation of 21.3 Gbaud 8PSK signal using an SOA-based all-optical phase modulator.

We describe a novel SOA-based all-optical pure-phase modulator, and show how deleterious cross-gain modulation from the SOAs can be suppressed by utilizing an integrated interferometer structure. We experimentally demonstrate the use of the optical gate as a π/4 phase modulator producing 21.3 Gbaud 8PSK from 21.3 Gbit/s OOK and 21.3 Gbaud QPSK inputs. The modulator produces 3 dB of gain and coherent detection-based bit error rate measurements indicate a 2.4 dB excess penalty.

All-optical technique for modulation format conversion from on-off-keying to alternate-mark-inversion.

We propose and numerically investigate for the first time a novel all-optical on-off-keying to alternate-mark-inversion modulation format converter operating at 40 Gbps employing a semiconductor optical amplifier (SOA)-based Mach-Zehnder interferometer (MZI). We demonstrate that this SOA-MZI operates as a pulse subtractor, and in the absence of patterning will produce perfectly phase inverted pulses regardless of the individual SOA phase excursions. We use a comprehensive computer model to illustrate the impact of patterning on the output phase modulation, which is quantified through the definition of the phase compression factor.

Pattern compensation in SOA-based gates.

We propose a novel scheme employing complementary data inputs to overcome the patterning normally associated with semiconductor optical amplifier based gates and demonstrate the scheme experimentally at 42.6Gb/s. The scheme not only avoids introducing patterning during switching, but also compensates for much of the patterning present on the input data. A novel gate was developed for the experiment to provide the complementary signals required for the scheme.

A computer model of the Bystander effect: effects of individual behaviours on the population response.

A computer program was written to test assumptions on the mechanisms of response of cells to radiation. Current assumptions were implemented in the model and simulations were run to predict the survival of the hamster cell line V79 to irradiation with focused C(K) X-rays, 250 keV X-rays or 3.2 MeV protons. The model could reproduce the different types of response and support the idea that the response at low doses is non-linear and may be independent of LET.

Cellular automaton model of cell response to targeted radiation.

It has been shown that the response of cells to low doses of radiation is not linear and cannot be accurately extrapolated from the high dose response. To investigate possible mechanisms involved in the behaviour of cells under very low doses of radiation, a cellular automaton (CA) model was created. The diffusion and consumption of glucose in the culture dish were computed in parallel to the growth of cells. A new model for calculating survival probability was introduced; the communication between targeted and non-targeted cells was also included. Early results on the response of non-confluent cells to targeted irradiation showed the capability of the model to take account for the non-linear response in the low-dose domain.

Optoelectronic neural-network scheduler for packet switches.

A novel, to our knowledge, type of packet scheduler that could significantly outperform current state-of-the-art schedulers is presented. The operation and the design of such a scheduler are discussed, and a fully operational experimental implementation is described. The scheduler uses a neural network in a winner-take-all strategy to optimize decisions on the throughput of both a crossbar and a banyan switching fabric. The problems of high interconnection density are solved by use of a free-space optical interconnect that exploits diffractive optical techniques to generate the required interconnection patterns and weights.

Use of transgenic plants to study antioxidant defenses.

The abundance of O2 and the highly energetic electron transfer reactions associated with thylakoid membranes make chloroplasts a major source of reactive oxygen intermediates (ROI) in photosynthetic tissues of plants. Attempts to reduce oxidative damage in chloroplasts have included the manipulation of ROI-scavenging enzymes by gene transfer technology. Much of this work has focused on chloroplast-localized superoxide dismutase (SOD), both chloroplast-targeted and cytosolic ascorbate peroxidase (APX) and glutathione reductase (GR). Increased activity of SOD in chloroplasts of transgenic tobacco plants generally leads to increased protection from membrane damage caused by exposure to methyl viologen (MV). In addition, overexpression of chloroplastic Cu/Zn SOD can lead to increased protection from photooxidative damage caused by high light intensity and low temperatures. Transgenic tobacco plants that overexpress APX either in the cytosol or chloroplastic compartments also show reduced damage following either MV exposure or photooxidative treatment and transgenic plants that express increased levels of GR have elevated pools of ascorbate and GSH. Though still preliminary, these results clearly indicate that alterations in the expression of enzymes involved in ROI-scavenging can have significant metabolic effects and provide the hope that this strategy can be used to develop crop plants with increased stress tolerance.

Postoperative management of cerebral air embolism: gas physiology for surgeons.

Cerebral gaseous microemboli are present in most, if not all, cardiopulmonary bypass-assisted operations. Fortunately, the great majority are subclinical. Clinically significant cases of cerebral air embolism are largely underdiagnosed, undertreated, and underreported. The management of cerebral air embolism has been challenged due to the lack of prospective, randomized studies. Preventive measures that have been implemented throughout the years, resulting from empirically acquired knowledge, have avoided frequent major mishaps. Perfusion accidents, in which massive amounts of gas are pumped into patients, are managed intraoperatively by common-sense heroic measures which, at best, remove 50% of the embolized gas. Postoperative confirmation of a neurologic insult after a cardiopulmonary bypass-assisted operation, in which a cerebral air embolism is likely the source, is one of the most distressing situations a surgical team has to confront, due in part to the lack of pathognomonic diagnostic tools and to the absence of a "scientifically proven" (supported by prospective, randomized studies) therapeutic regimen. In lieu of the latter, we present the physical and physiologic bases that will justify the use of several therapeutic tools when facing a suspected CAE. These tools, when applied rationally, will represent some of the most innocuous modalities in the medical armamentarium.

Performance of an optoelectronic neural network in the presence of noise.

Optoelectronic neural networks must not only be highly parallel but also fast to compete with electrical systems. Receiver noise becomes an important consideration at high data rates; so the limits set by noise to network size and speed are analyzed. A network incorporating an array of high-speed multi-quantum-well modulators was constructed. It employed a general method for optical representation of bipolar values, which required only a minimal increase in network dimensions and gave the network immunity to common-mode parameter variations. Different ways of partitioning pattern-recognition problems were compared, and the accuracy of one configuration was tested with the experimental network over a range of noise levels.

Optoelectronic implementation of neural networks.

The performance of electronic neural networks will eventually be limited by the capacity of their interconnections as operating speeds are increased. The use of optical connections to link electronic elements offers a way to overcome this problem. In particular, spatial optic techniques are especially suited to neural systems because of their parallel structure. Three experimental optoelectronic networks are described that employ holographic components and arrays of high-speed optical modulators. These of a network to 64 x 8 connections with optical fan-out. A hybrid networks have been used to demonstrate operation at 50 MHz, novel training algorithms and the expansion technique for integrating optoelectronic systems is also being developed.

Overexpression of Superoxide Dismutase Protects Plants from Oxidative Stress (Induction of Ascorbate Peroxidase in Superoxide Dismutase-Overexpressing Plants).

Photosynthesis of leaf discs from transgenic tobacco plants (Nicotiana tabacum) that express a chimeric gene that encodes chloroplast-localized Cu/Zn superoxide dismutase (SOD+) was protected from oxidative stress caused by exposure to high light intensity and low temperature. Under the same conditions, leaf discs of plants that did not express the pea SOD isoform (SOD-) had substantially lower photosynthetic rates. Young plants of both genotypes were more sensitive to oxidative stress than mature plants, but SOD+ plants retained higher photosynthetic rates than SOD- plants at all developmental stages tested. Not surprisingly, SOD+ plants had approximately 3-fold higher SOD specific activity than SOD- plants. However, SOD+ plants also exhibited a 3- to 4-fold increase in ascorbate peroxidase (APX) specific activity and had a corresponding increase in levels of APX mRNA. Dehydroascorbate reductase and glutathione reductase specific activities were the same in both SOD+ and SOD- plants. These results indicate that transgenic tobacco plants that overexpress pea Cu/Zn SOD II can compensate for the increased levels of SOD with increased expression of the H2O2-scavenging enzyme APX. Therefore, the enhancement of the active oxygen-scavenging system that leads to increased oxidative stress protection in SOD+ plants could result not only from increased SOD levels but from the combined increases in SOD and APX activity.

Effects of mating system in Japanese quail : 1. Inbreeding and fitness.

A selection experiment, conducted for 17 generations, compared a selected population mated in a cyclic manner with alternating generations of inbreeding and outbreeding, a selected population mated at random and a randomly mated, randomly selected control population. Trends in the number of birds available for selection and number selected are presented for each population. Actual and theoretical levels of inbreeding for each population were similar. Samples of all populations were subjected to continuous full-sib inbreeding at generation 12. All populations were sensitive to inbreeding depression. However, the cyclic mated population had greater reproductive fitness after continued intensive breeding.