Showing papers by "National Physical Laboratory published in 2008"

A new formalism for reference dosimetry of small and nonstandard fields

Abstract: The use of small fields in radiotherapy techniques has increased substantially, in particular in stereotactic treatments and large uniform or nonuniform fields that are composed of small fields such as for intensity modulated radiation therapy (IMRT). This has been facilitated by the increased availability of standard and add-on multileaf collimators and a variety of new treatment units. For these fields, dosimetric errors have become considerably larger than in conventional beams mostly due to two reasons; (i) the reference conditions recommended by conventional Codes of Practice (CoPs) cannot be established in some machines and (ii) the measurement of absorbed dose to water in composite fields is not standardized. In order to develop standardized recommendations for dosimetry procedures and detectors, an international working group on reference dosimetry of small and nonstandard fields has been established by the International Atomic Energy Agency (IAEA) in cooperation with the American Association of Physicists in Medicine (AAPM) Therapy Physics Committee. This paper outlines a new formalism for the dosimetry of small and composite fields with the intention to extend recommendations given in conventional CoPs for clinical reference dosimetry based on absorbed dose to water. This formalism introduces the concept of two new intermediate calibration fields: (i) a static machine-specific reference field for those modalities that cannot establish conventional reference conditions and (ii) a plan-class specific reference field closer to the patient-specific clinical fields thereby facilitating standardization of composite field dosimetry. Prior to progressing with developing a CoP or other form of recommendation, the members of this IAEA working group welcome comments from the international medical physics community on the formalism presented here.

Direct evidence for multiferroic magnetoelectric coupling in 0.9BiFeO3-0.1BaTiO3.

Abstract: Magnetic, dielectric and calorimetric studies on $0.9{\mathrm{BiFeO}}_{3}--0.1{\mathrm{BaTiO}}_{3}$ indicate strong magnetoelectric coupling. XRD studies reveal a very remarkable change in the rhombohedral distortion angle and a significant shift in the atomic positions at the magnetic ${T}_{c}$ due to an isostructural phase transition. The calculated polarization using Rietveld refined atomic positions scales linearly with magnetization. Our results provide the first unambiguous, atomic level evidence for magnetoelectric coupling of intrinsic multiferroic origin in a ${\mathrm{BiFeO}}_{3}$-based system.

Compensation of Quadrature Imbalance in an Optical QPSK Coherent Receiver

Abstract: This letter explores the Gram-Schmidt orthogonalization procedure (GSOP) for compensation of quadrature imbalance in an optical 90deg hybrid. We present computer simulations for an optical QPSK communication system using a digital coherent receiver and investigate the impact of quadrature imbalance on the required optical signal-to-noise ratio for the receiver and the frequency estimation algorithm. We then demonstrate the improvement which can be achieved using the GSOP, including the impact of quantization in the digital coherent receiver. Finally, we show that the GSOP can equally be applied to polarization-division multiplexed systems, applying the GSOP in conjunction with the constant modulus algorithm to demultiplex a PDM-QPSK signal.

Low field magnetotransport in manganites

Abstract: The perovskite manganites with generic formula RE1?xAExMnO3 (RE = rare earth, AE = Ca, Sr, Ba and Pb) have drawn considerable attention, especially following the discovery of colossal magnetoresistance (CMR). The most fundamental property of these materials is strong correlation between structure, transport and magnetic properties. They exhibit extraordinary large magnetoresistance named CMR in the vicinity of the insulator?metal/paramagnetic?ferromagnetic transition at relatively large applied magnetic fields. However, for applied aspects, occurrence of significant CMR at low applied magnetic fields would be required. This review consists of two sections: in the first section we have extensively reviewed the salient features, e.g.?structure, phase diagram, double-exchange mechanism, Jahn?Teller effect, different types of ordering and phase separation of CMR manganites. The second is devoted to an overview of experimental results on CMR and related magnetotransport characteristics at low magnetic fields for various doped manganites having natural grain boundaries such as polycrystalline, nanocrystalline bulk and films, manganite-based composites and intrinsically layered manganites, and artificial grain boundaries such as bicrystal, step-edge and laser-patterned junctions. Some other potential magnetoresistive materials, e.g.?pyrochlores, chalcogenides, ruthenates, diluted magnetic semiconductors, magnetic tunnel junctions, nanocontacts etc, are also briefly dealt with. The review concludes with an overview of grain-boundary-induced low field magnetotransport behavior and prospects for possible applications.

Nanostructures and nanostructured substrates for surface—enhanced Raman scattering (SERS)

Abstract: We review the performance of various nanoscaled structures needed to support the propagation of the surface plasmons responsible for surface-enhanced Raman scattering (SERS), and assess the potential for the optimisation of the compromise between enhancement and reproducibility that they provide, and hence their utility for relevant applications. We divide these nanostructures into those comprising structured arrays of discrete nanoparticles in two or three dimensions, and those comprising structured or regularly patterned surfaces in two or three dimensions. The most promising in terms of this compromise are those that involve the tethering of functionalised metal nanoparticles to surfaces. They are not only reproducible, but the functionalisation provides a measure of selectivity to relevant target analytes that the majority of SERS applications require. Copyright © 2008 John Wiley & Sons, Ltd.

Surface measurement errors using commercial scanning white light interferometers

Abstract: This paper examines the performance of commercial scanning white light interferometers in a range of measurement tasks. A step height artefact is used to investigate the response of the instruments at a discontinuity, while gratings with sinusoidal and rectangular profiles are used to investigate the effects of surface gradient and spatial frequency. Results are compared with measurements made with tapping mode atomic force microscopy and discrepancies are discussed with reference to error mechanisms put forward in the published literature. As expected, it is found that most instruments report errors when used in regions close to a discontinuity or those with a surface gradient that is large compared to the acceptance angle of the objective lens. Amongst other findings, however, we report systematic errors that are observed when the surface gradient is considerably smaller. Although these errors are typically less than the mean wavelength, they are significant compared to the vertical resolution of the instrument and indicate that current scanning white light interferometers should be used with some caution if sub-wavelength accuracy is required.

Single-parameter nonadiabatic quantized charge pumping

Abstract: Controlled charge pumping in an $\mathrm{Al}\mathrm{Ga}\mathrm{As}∕\mathrm{Ga}\mathrm{As}$ gated nanowire by single-parameter modulation is experimentally and theoretically studied. Transfer of integral multiples of the elementary charge per modulation cycle is clearly demonstrated. A simple theoretical model shows that such a quantized current can be generated via loading and unloading of a dynamic quasibound state. It demonstrates that nonadiabatic blockade of unwanted tunnel events can obliterate the requirement of having at least two phase-shifted periodic signals to realize quantized pumping.

Electrical and mechanical properties of multi-walled carbon nanotubes reinforced PMMA and PS composites

Abstract: The use of multi-walled carbon nanotubes (MWCNT) as reinforcing material for thermoplastic polymer matrices, polymethyl methacrylate (PMMA), and polystyrene (PS) has been studied. MWCNT were synthesized by chemical vapor deposition (CVD) technique using ferrocene-toluene mixture. As-prepared nanotubes were ultrasonically dispersed in toluene and subsequently dispersed in PMMA and PS. Thin polymer composite films were fabricated by solvent casting. The effect of nanotube content on the electrical and mechanical properties of the nanocomposites was investigated. An improvement in electrical conductivity from insulating to conducting with increasing MWCNT content was observed. The carbon nanotube network showed a classical percolating network behavior with a low percolation threshold. Electromagnetic interference (EMI) shielding effectiveness value of about 18 dB was obtained in the frequency range 8.0–12 GHz (X-band), for a 10 vol% CNT loading. An improved composite fabrication process using casting followed by compression molding and use of functionalized MWCNT resulted in increased composites strength. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers

Prospects of Nanomaterials in Biosensors

Abstract: Nanomaterials, the building blocks of nanotechnology, possess important physical properties such as particle aggregation, photoemission, electrical and heat conductivities, and chemical properties like catalytic activities. On the basis of such unique properties, nanomaterials have received much attention from scientists and researchers in different areas of biomedical sciences such as biomolecule detection, sensing applications, gene delivery, clinical diagnostics, etc. This review focuses on different types of nanomaterials including their synthesis, properties, conjugation with biomolecules, and their biosensing applications.

Aspects of scanning force microscope probes and their effects on dimensional measurement

Abstract: The review will describe the various scanning probe microscopy tips and cantilevers used today for scanning force microscopy and magnetic force microscopy. Work undertaken to quantify the properties of cantilevers and tips, e.g. shape and radius, is reviewed together with an overview of the various tip–sample interactions that affect dimensional measurements.

Highly sensitive NH3 sensor using Pt catalyzed silica coating over WO3 thick films

Abstract: Ammonia gas sensing properties of a single layer WO 3 thick film and a double layer sensor structure having a supported catalyst on it have been investigated. The single layer sensor exhibited low response to NH 3 . An enhancement in gas response was achieved by doping WO 3 with Pt, Pd or Au. Ammonia sensing properties of the WO 3 thick film was improved markedly by overcoating a platinum catalyzed silica-niobia layer onto the WO 3 film surface. Such a double layer structure not only increased the gas response, but decreased the response time also.

Thermal-noise-limited optical cavity

Abstract: A pair of optical cavities are designed and set up so as to be insensitive to both temperature fluctuations and mechanical vibrations. With the influence of these perturbations removed, a fundamental limit to the frequency stability of the optical cavity is revealed. The stability of a laser locked to the cavity reaches a floor $l2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15}$ for averaging times in the range $0.5\ensuremath{-}100$ s. This limit is attributed to Brownian motion of the mirror substrates and coatings.

Poly (3,4-ethylenedioxythiophene) γ-Fe2O3 polymer composite–super paramagnetic behavior and variable range hopping 1D conduction mechanism–synthesis and characterization

Abstract: The present paper reports the preparation of poly (3,4-ethylenedioxythiophene) (PEDOT) ferrimagnetic conducting polymer composite by incorporation of ferrite particles in the polymer matrix by emulsion polymerization. Synthesis of PEDOT–γ-Fe2O3 composite was carried out by chemical oxidative polymerization of EDOT with ferrite particles in the presence of dodecylbenzenesulfonic acid (DBSA) that works as dopant as well as surfactant in aqueous medium. The resulting conducting composite possesses saturation magnetization (Ms) value of 20.56 emu/g with a conductivity of 0.4 Scm−1, which was determined by VSM and four probe technique, respectively. B-H curve reveals that ferrimagnetic particles of γ-Fe2O3 show super-paramagnetic behavior at room temperature which was also observed in PEDOT–γ-Fe2O3 composite. The resulting conducting ferrimagnetic composite shows microwave absorption loss of 18.7–22.8 dB in the frequency range of 12.4–18 GHz. Thermogravimetric analysis of the composite revealed that the composite is thermally stable up to 230°C. The characterization of the PEDOT–γ-Fe2O3 composite was carried out using XRD and FTIR spectroscopy. Copyright © 2007 John Wiley & Sons, Ltd.

Influence of Surface Modified MWCNTs on the Mechanical, Electrical and Thermal Properties of Polyimide Nanocomposites

Abstract: Polyamic acid, the precursor of polyimide, was used for the preparation of polyimide/multiwalled carbon nanotubes (MWCNTs) nanocomposite films by solvent casting technique. In order to enhance the chemical compatibility between polyimide matrix and MWCNTs, the latter was surface modified by incorporating acidic and amide groups by chemical treatment with nitric acid and octadecylamine (C18H39N), respectively. While the amide-MWCNT/polyimide composite shows higher mechanical properties at low loadings (<3 wt%), the acid-MWCNT/polyimide composites perform better at higher loadings (5 wt%). The tensile strength (TS) and the Young’s modulus (YM) values of the acid-MWCNT/polyimide composites at 5 wt% MWCNT loadings was 151 and 3360 MPa, respectively, an improvement of 54% in TS and 35% in YM over the neat polyimide film (TS = 98 MPa; YM = 2492 MPa). These MWCNT-reinforced composites show remarkable improvement in terms of thermal stability as compared to that for pure polyimide film. The electrical conductivity of 5 wt% acid modified MWCNTs/polyimide nanocomposites improved to 0.94 S cm−1(6.67 × 10−18 S cm−1for pure polyimide) the maximum achieved so far for MWCNT-polyimide composites.

Quantitative molecular depth profiling of organic delta-layers by C60 ion sputtering and SIMS.

Abstract: Alternating layers of two different organic materials, Irganox1010 and Irganox3114, have been created using vapor deposition. The layers of Irganox3114 were very thin (∼2.5 nm) in comparison to the layers of Irganox1010 (∼55 or ∼90 nm) to create an organic equivalent of the inorganic ‘delta-layers' commonly employed as reference materials in dynamic secondary ion mass spectrometry. Both materials have identical sputtering yields, and we show that organic delta layers may be used to determine some of the important metrological parameters for cluster ion beam depth profiling. We demonstrate, using a C60 ion source, that the sputtering yield, S, diminishes with ion dose and that the depth resolution also degrades. By comparison with atomic force microscopy data for films of pure Irganox1010, we show that the degradation in depth resolution is caused by the development of topography. Secondary ion intensities are a well-behaved function of sputtering yield and may be employed to obtain useful analytical infor...