Showing papers by "National Physical Laboratory published in 1996"

R50 as a beam quality specifier for selecting stopping-power ratios and reference depths for electron dosimetry.

Abstract: For electron beam reference dosimetry in radiotherapy, it is shown that by choosing the reference depth as dref = 0.6R(50)-0.1 cm, where R50 is the half-value depth in centimeters, the Spencer-Attix water-to-air stopping-power ratio at dref is given by (Llp)airw = 1.2534 - 0.1487 (R50)0.2144. This is derived from data for (Llp)airw obtained from realistic Monte Carlo simulations for 24 clinical beams. The rms deviation of this expression from the Monte Carlo calculations is 0.16%, with a maximum deviation of 0.26%. This approach fully takes into account the spectral differences between real electron beams of the same R50 and allows an absorbed-dose calibration at a standards laboratory to be easily and accurately transferred to a reference clinical beam. Using a single parameter to specify (Llp)airw, rather than the two parameters (R50 and depth) needed when the reference depth is chosen as the depth of dose maximum, has the potential to greatly simplify electron beam dosimetry protocols and allows the use of a similar formalism for photon and electron beam dosimetry. For use in converting a depth-ionization curve into a depth-dose curve, a somewhat less accurate but general expression for (Llp)w(air) as a function of R50 and depth is presented.

Reference software for finding Chebyshev best-fit geometric elements

Abstract: This paper summarizes work carried out in a project supported by the European Communities' Bureau of Reference (BCR) to develop reference software for finding best-fit geometric elements, under specified criteria, to coordinate data. The geometric elements considered are the line, plane, circle, sphere, cylinder, and cone. The criteria for determining the elements are, generally, minimum zone (MZ) and, where appropriate, minimum circumscribed (MC) and maximum inscribed (Ml). The software developed implements methods founded on optimization theory. Two approaches are described. the first implements mathematical programming methods that exploit the particular structure of the problems considered and provide a unified approach to their solution; the second, applicable to the MZ and, where appropriate, MC problems for the line (in two-dimensions (2-D)], plane, circle (in 2-D) and sphere, implements a combinatorial method that returns all global solutions.

Modelling of the hydrogen distribution at a crack tip

Abstract: A model has been developed to predict the distribution of hydrogen atoms at a crack tip with the unique feature of incorporation of generalised boundary conditions which account realistically for the electrochemistry-diffusion interface. The use of boundary conditions involving expressions for the flux allows identification of the conditions for diffusion and surface reaction control of hydrogen transport. For ferritic steels, which have a relatively large diffusivity compared with face centred cubic alloys, the model predicts that hydrogen transport is controlled by the kinetics of cathodic generation of hydrogen atoms. The crack-tip concentration is approximately an order of magnitude less than that predicted assuming the conventional constant concentration boundary conditions derived from diffusion controlled transport. The results imply that crack growth rates will be limited by the kinetics of surface reactions and that predictive models of crack growth rates and thresholds for cracking based on diffusion control should be reassessed. The use of generalised boundary conditions also enables the calculation of the time evolution of the hydrogen distribution in initially precharged metals. It is shown that the crack tip and crack walls act as sinks for hydrogen atoms in these circumstances, depleting the hydrogen atoms in the crack-tip region. The loss of hydrogen through the crack tip in precharged samples may affect the location of hydrogen cracking and is an additional factor to be considerred in comparing internal and external hydrogen embrittlement. It also raises questions concerning the meaning of tests on cadmium plated samples because of the loss of hydrogen through the crack tip once the coating has been fractured due to dynamic straining or crack advance.

Static SIMS: A Study of Damage Using Polymers†

Abstract: As a result of work to establish the surface potential of insulators accurately in a quadrupole static SIMS system of high sensitivity, we have been able to study the effects of increasing dose-related damage on the intensities of the mass spectral peaks in the two archetypal bulk polymers PET and PTFE, as well as thin hydrocarbon contamination layers, with high accuracy. It is shown that the intensities follow very well-defined functions which give damage cross-sections whose values reflect the fragmentation behaviour of the polymers. The effects reflect the number of bonds to be broken to liberate the fragment, the internal complexity of that fragment and the typical damage zone of the ion impact. These concepts show that a static SIMS limit of below 3 x 10 15 ions m -2 exists for changes of <10% in the intensities of the most easily damaged species but that some larger fragments may require a dose of 2 x 10 17 ions m -2 to maximize their intensity This work has three main conclusions. Firstly, by defining a figure of merit factor, F, equal to the ratio of the absolute intensity of a peak to the fractional rate of change of that peak with the ion dose, it is possible to define the optimal beam parameters for static SIMS measurements. The higher the value of F, the more the intensity per unit of onset of damage. The highest F values occur at higher beam energies and, in general, xenon gives greater efficiency than argon. Secondly, the development of damage may be described by simple bond breaking. Thirdly, a study of the damage process gives quite detailed structural bonding information not directly available from the traditional static SIMS spectrum.

Constituent phase nanoindentation of WC/Co and Ti(C,N) hard metals

Abstract: Nanoindentational experiments were carried out on WC/Co and Ti(C,N) hard metals with the aim of evaluating the feasibility of using the technique as a way of determining the mechanical properties of the constituent phases on a local scale. Clear differences were found between the load-displacement curves for the hard and soft binder phases in both materials. There were corresponding differences in the hardness of the two phases. However, the magnitude of the hardness and modulus values that were calculated were always much larger than those expected from macroscopic hardness tests, owing in part to the large effect on results of uncertainties in tip shape at the small indentation depths that had to be used to give information on a scale relevant to the microstructure of the samples. For the WC/Co arrays of indentations were carried out to produce maps of hardness which could be clearly related to the details of the microstructure. With the Ti(C,N) cermet sample, the mapping was not successful, probably because of the complexity of the microstructure in these materials.

Magnetic moment (saturation) measurements on hardmetals

Abstract: This note summarises the background to the use of appropriate units for the measurement of magnetic moment (saturation) in hardmetals. It also discusses the methods used for correlating composition/structure with magnetic properties and indicates the theoretical ranges likely to be obtained in typical industrial products. It includes comparative data obtained on two different measurement systems on both hardmetals and synthetic Co, Co-Ni and Ni alloys containing W and C in solution.

Electrooptic sampling of low temperature GaAs pulse generators for oscilloscope calibration

Abstract: Optoelectronic generation of well characterized ultrashort electrical pulses plays an important role in the calibration of fast-sampling oscilloscopes. In this paper the authors describe the National Physical Laboratory (NPL) pulse generator, comprising a femtosecond laser and an ultrafast photoconductive switch which can generate electrical pulses as short as 650 fs. The photoconductive switch consists of a GaAs substrate with a top GaAs layer grown under low temperature conditions to ensure a subpicosecond recombination rate. The technique of electrooptic sampling is used to measure pulses on planar transmission lines, such as coplanar waveguide and coplanar stripline. Good agreement is shown between electrooptic sampling measurement and the modelling of pulse propagation along a coplanar waveguide, enabling one to optimize a design of a calibration test source. The use of a pulse generator to calibrate a 50 GHz sampling oscilloscope is described.

pH-metric determination of acid values in vegetable oils without titration.

Abstract: A new method for determining acid values in vegetable oils has been developed for a nontitration pH-metric technique with a reagent consisting of 0.20 M triethanolamine in a 1∶1 solution of water and isopropanol. The emulsive properties of this reagent provide rapid (within a minute) and quantitative extraction of free fatty acids from an oil into the solvent phase. Acid values were determined by measuring conditional pH of the emulsified reagent in a pH-meter with an aqueous reference electrode. Three different applications of this technique are discussed.

The UK primary standard calorimeter for photon-beam absorbed dose measurement

Abstract: A description is given of the UK primary standard graphite calorimeter system. The calorimeter measures absorbed dose to graphite for photon radiations from 60Co to 19 MV x-rays, and is the basis of the NPL therapy-level absorbed dose to water calibration service. Absorbed dose to graphite from the photon calorimeter has been compared with three other standards: an ionization chamber and cavity theory, for 60Co gamma radiation; the NPL electron calorimeter, for 12-14 MeV electron beams; and the BIPM 60Co absorbed dose standard. The three standards agreed within 0.5% which is similar to the measurement uncertainties.

Low frequency alternating current conduction and dielectric relaxation in polypyrrole, poly(N‐methyl pyrrole), and their copolymers

Abstract: The alternating current (ac) conductivity [σm(ω)], dielectric constant (e′) and loss (e″) of polypyrrole (PPY), poly(N‐methyl pyrrole) [P(NMPY)] and their copolymers; poly(N‐methyl pyrrole–pyrrole [P(NMPY–PY)] have been measured in the frequency range 102–106 Hz and in the temperature range 77–350 K At 77 K, the ac conductivity can be expressed by the relation; σac=Aωs where the slope s lies in the range 072–081 for these three polymers and decreases with increase in temperature The well‐defined loss peaks, whose magnitude decreases with the increase in frequency, have been observed in the temperature region where the measured ac conductivity approaches the direct current (dc) conductivity These loss peaks have been associated with the movement of charge carriers in these polymeric films The dc conductivity has also been measured in the temperature range 77–350 K and an attempt has been made to correlate it with dielectric data

Inter-personal awareness and synchronization: assessing the value of communication technologies

Abstract: How may we discriminate between the multitude of point-to-point communication facilities currently available? To take just one aspect of communication, how can we assess the fluency of coordination that results from using some communication technology? This paper describes two groups of measures with this general purpose. The measures described have been devised to be used in a particular approach to evaluation for the design of communication systems that borrows from experimental and ethnographic methods. This approach is promoted as a practical and rigorous way of assessing design alternatives. The first group of measures are subjective ratings that assess someone's awareness of the attentional status of their conversational partner, such awareness is necessary for the successful coordination of conversation. The rating scales are shown to be sensitive in that they distinguish between video and audio mediated conversation in a short experiment. The second group are measures derived from video records of communicative behaviour using “activity set” analysis. This can be used to assess coordination in communication directly. An activity set is a mutually exclusive and exhaustive set of behavioural states. A publicly available tool, Action Recorder, makes it possible to score the tapes in near real time. “Simple statistics” are extracted from a single activity set, examples are: the proportion of time spent looking towards the video monitor and the average duration of these glances. “Contingent statistics” are extracted from two or more activity sets, for example, the proportion of time both members of a pair are looking towards their video monitors. A way of assessing the synchronization evident in two people's behaviour is presented that makes use of these contingent statistics. Inter-observer reliabilities are given for all the measures generated.

First VHF radar observations of tropical latitude E-region field aligned irregularities

Abstract: Indian MST radar at Gadanki (13.5°N, 79.2°E, 12.5° dip) was operated during July/August 1994, to observe the 3-m scale size field aligned irregularities associated with the lower E region. Irregularity structure was studied by using height-time variation of the echo intensity and weighted mean Doppler velocity. In this paper results of three diurnal cycles of observation are presented. Field perpendicular echoes were observed both during daytime and nighttime. A layered irregularity structure extending down to altitude below 86 Km was seen during the nighttime. The daytime structure showed a narrow echoing region with significant downward slope. Doppler velocity was in the range of 20–50 ms−1, both during day and night and was, in general, consistent with the slope of scattering structure observed in the height-time-intensity plots.

Recent advances in planetary ionospheres

Abstract: Recent results on the ionospheres of various planets are reviewed. In view of the large amount of data now available on Venus, much of the work relates to this planet. The outer planets received somewhat less attention partly due to lack of measurements in the post-Voyager era. Two outer planetary satellites, that of Titan and Triton present very interesting ionospheres and have been included in the review.

The physical properties of alloys in the liquid and ``mushy'' states

Abstract: Data on the physical properties of alloys at high temperatures are urgently needed for the mathematical modeling of high-temperature processes such as casting, welding, secondary refining, dip melting, spray forming, and metal powder production. Data are required for those properties which are involved in heat and fluid flow in high-temperature processes. Levitated drop methods have been adopted to measure the surface tensions, densities, and enthalpies of commercial alloys, and rapid, transient methods have been developed to measure thermal conductivities to avoid the problem of convection. The results obtained for the properties of commercial alloys for the liquid and “mushy” states are discussed.

Thermal evolution of sol-gel derived zirconia and binary oxides of zirconia-silica

Abstract: This study is centred around the evolution of the zirconia polymorph in zirconia and in binary oxides of zirconia-silica derived by the sol-gel technique that can help alter the microstructure by changing the experimental parameters

Dielectric spectroscopic studies on polypyrrole glucose oxidase films

Abstract: An investigation was made into the dielectric spectroscopic characteristics of p-toluene sulfonate (PTS) doped polypyrrole (PPY) films in the presence and absence of immobilized glucose oxidase (GOX) in three different configurations : Al-PTS-PPY-Al, Al-PTS-PPY/GOX-Al, and Al-PTS-PPY/GOX/β-D-glucose-Al, respectively. Measurement of dielectric loss and capacitance yielded valuable information about the dielectric properties of GOX immobilized in PTS doped PPY films. The effect of both the temperature and varying β-D-glucose concentrations on the mobility of the charge carriers in these films was also systematically studied.