Accelerators direct measurement

For mechanical wave measurements, notice should be taken of the advances in technology. It is particularly notable that the major advances in materials description have not resulted so much from improved resolution in measurement of displacement and/or time, but in direct measurements of the derivative functions of acceleration, stress rate, and density rate as called for in the theory of structured wave propagation. Future developments, such as can be anticipated with piezoelectric polymers, in which direct measurements are made of rate-of-change of stress or particle velocity should lead to the observation of recognized mechanical effects in more detail, and perhaps the identification of new mechanical phenomena. 

Techniques for the Direct Measurement of Natural Beryllium-10 and Carbon-14 with a Tandem Accelerator... 

Southon, J. R., Nelson, D. E., Korteling, R., Nowikow, I., Hammaren, E., McKay, J., Burke, D., Techniques for the Direct Measurement of Natural 10Be and 14C with a Tandem Accelerator, Chapter 4 in this book. 

Our main motivation to develop the specific transient technique of wavefront analysis, presented in detail in (21, 22, 5), was to make feasible the direct separation and direct measurements of individual relaxation steps. As we will show this objective is feasible, because the elements of this technique correspond to integral (therefore amplified) effects of the initial rate, the initial acceleration and the differential accumulative effect. Unfortunately the implication of the space coordinate makes the general mathematical analysis of the transient responses cumbersome, particularly if one has to take into account the axial dispersion effects. But we will show that the mathematical analysis of the fastest wavefront which only will be considered here, is straight forward, because it is limited to ordinary differential equations dispersion effects are important only for large residence times of wavefronts in the system, i.e. for slow waves. We naturally recognize that this technique requires an additional experimental and theoretical effort, but we believe that it is an effective technique for the study of catalysis under technical operating conditions, where the micro- as well as the macrorelaxations above mentioned are equally important. 

In order to provide AMS analyses to the broad ocean sciences research community, the National Ocean Sciences Accelerator Mass Spectrometry Facility (NOSAMS) was established at Woods Hole Oceanographic Institution (Massachusetts) in 1989. Studies performed there include identification of sources of carbon-bearing materials in the water column and sediment, dating of sedimentary samples, investigations of paleocirculation patterns (e.g., from observations of differences in 14C relative abundances in planktonic and benthic foraminifera, and coral cores and cross sections), as well as studies of modern oceanic carbon cycling and circulation. In fact, much that is known about advective and diffusive processes in the ocean comes from measurements of chemical tracers, such as 14C, rather than from direct measurements of water mass flow. 

Accelerated development/reviewcan be used under two special circumstances when approval is based on evidence of the product s effect on a "surrogate endpoint," and when the FDA determines that safe use of a product depends on restricting its distribution or use. A surrogate endpoint is a laboratory finding or physical sign that may not be a direct measure of how a patient feels, functions, or survives, but is still considered likely to predict therapeutic benefit for the patient. 

If the change took place in the one simple stage 2 C120 = 2 Cl2 + 02 the increase in pressure at any time would be a direct measure of the extent to which the reaction had progressed. When the pressure increase is plotted against time the curve shows that the reaction apparently accelerates as it proceeds, as though the chlorine or the oxygen... 

There are two principal types of ionisation gauge, viz. the hot cathode type in which electrons are emitted by a heated filament, and the cold cathode type in which electrons are released from the cathode by the impact of ions. In both cases the vacuum is measured in terms of the ion current. The electrons are accelerated by a potential difference (usually about 2000 V) across the ionisation tube (see Fig. 6.22). Positively charged ions are formed by the electrons striking gas molecules. The number of positive ions produced is a function of the gas density (i.e. the pressure) and the electron current ie which is normally held constant. The ions are collected at a negatively charged electrode and the resulting ion current it is a direct measure of the gas pressure. The hot cathode version is the most sensitive of the two and can be used to measure vacua down to about 10 10 torr ( 10 8 N/m2). 

Faraday cup is used to collect ions and thus provide a direct measure of the ion current. For higher sensitivity and rapid response, an electron multiplier detector is employed. In this device the accelerated ions impinge on a surface with the emission of secondary electrons. These are multiplied by a cascade along either a channel or set of plates having a large potential gradient. 

AMS directly measures the number of 14C atoms, and the ratio of 14C to 13C and/ or 12C, using a high-energy accelerator as an inlet to a mass spectrometer. The key characteristics of 14C-AMS are the electron stripping and ion acceleration, which allow 14C to be distinguished from isobars and molecules that would confuse a standard mass spectrometer. AMS requires only a fairly small sample of lOOpg to 1 mg of C. In addition, the measurement only takes minutes per sample. 

The acceleration is a direct measure to the dynamic factors. However, there are few reports, if not to say none, about that for meso-scale structures. In a recent attempt, Meng et al. (2009) made use of the multiple sensors of an X-ray computerized tomography (CT) to measure the cluster accelerations. Instead of the conventional use of CT for cross-sectionally scanning the solids distribution, they erected the X-ray fan-beam and the sensors to follow the vertical movement of clusters... 

With the various experimental techniques, the actual measurement concerns product ions after they have been extracted from the source. That is to say, the decomposition occurs in a source before acceleration, but what is actually measured is translational energy after the product ion has been accelerated. To be still more precise, it is, in most cases, the distribution of the component of velocity along the axis of the mass spectrometer (i.e. in the direction in which the ions were accelerated out of the source) which is, in effect, measured. The measured quantity is, therefore, distinct from the translational energy distribution of the product ion (called the laboratory distribution ) as it was upon its formation in the source (i.e. before acceleration). The measured distribution needs to be analysed to obtain the laboratory distribution. Working with means or averages is much simpler, but there are possible pitfalls (see the discussion of the time-of-flight technique below). 

An interesting variation on the deflection method has been reported [668, 893] in which the component of velocity normal to the direction of acceleration is measured by mechanically moving an aperture across the beam. No electrostatic deflection is employed. The aperture is situated some distance from the source in front of the mass spectrometer and, in effect, determines how far off axis ions have moved since they left the source. 

A measurement of/H can be achieved electrochemically (32). Under galva-nostatic (constant current) cathodic conditions, the fractional efficiency of hydrogen absorption is the ratio of the amount of hydrogen produced electrochemically to the amount absorbed by the metal, as is illustrated in Fig. 25. While this appears to be a very simple direct measurement, it has the normal problems associated with an accelerated electrochemical measurement, including the need to demonstrate that the parameter measured electrochemically will retain the same value under the much slower natural corrosion conditions. 

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