Detectors data collection

Table A2.2. Area detector data collection on conventional X-ray sources (Cu Ka sealed tube or rotating anode). 

San Diego, MWPC refers both to the regional/national area detector data collection facility there (for a description of the various systems and the developments over the years see Xuong et al (1978, 1985) and Hamlin et al (1981)) and to the identical type of system supplied commercially (from about 1986) by San Diego Multi-wire Systems . 

Virginia, MWPC refers to the regional/national area detector data collection facility there (for a description of the system and its use see Sobottka et al (1990)). 

PBLG molecular weights were verified by gel permeation chromatography (GPC). Instrumentation (Waters Associates, Inc.) included a dual pump Multi-Solvent Delivery system (Waters 600/600E), 712 WISP Autosampler, a Waters 486 tunable ultraviolet/visible detector, and a Waters 410 differential refractometer detector. Data collection and analysis are facilitated by a Maxima 825 software program. 

In Dynamic Spatial Reconstructor at the expense of use 2D matrix of detectors there was the opportunity to use a divergent cone beam of source emission. This system had a number of lacks. In particular the number of projections is rigidly limited by the number of x-ray sources. The dispersion of source emission results in errors of data collected.. However the system confirmed basic advantages of application of conic beams and 2D matrices of detectors for collecting information about 3D object. 

Typically it takes two to three days to collect a complete data set using a single-reflection detector. The new SMART diffractometer with its CCD detector can collect two or three data sets pet day. 

Position Sensitive Detectors. By replacing the scintillation detector in a conventional powder diffractometer with a Position Sensitive Detector (PSD), it is possible to speed data collection. For each x-ray photon received a PSD records the angle at which it was detected. Typically, a conventional scintillation detector records x-ray photons in a range of a few hundredths of a degree at a time. A PSD can measure many degrees (in 20) of a powder pattern simultaneously. Thus, for small samples, data collection, which could require hours with a conventional detector, could take minutes or even seconds with a PSD. 

Area Detectors. A two-dimensional or area detector attached to a powder diffractometer can gready decrease data collection time. Many diffraction appHcations require so much time with a conventional detector that they are only feasible if an area detector is attached to the iastmment. The Siemens General Area Detector Diffraction System (GADDS) uses a multiwire area detector (Fig. 17). This detector measures an x- and ajy-position for each x-ray photon detected. The appHcations foUow. 

Texture Analysis with GADDS. With a conventional detector, a data collection for a pole figure analysis with a powder diffractometer with a texture attachment could take 12 h or more. With an area detector, it is possible to collect enough data for several pole figures (required for an ODF analysis) ia a few minutes. 

Laue Method for Macromolecule X-Ray Diffraction. As indicated above it is possible to determine the stmctures of macromolecules from x-ray diffraction however, it normally takes a relatively long period of data collection time (even at synchrotrons) to collect all of the data. A new technique, the Laue method, can be used to collect all of the data in a fraction of a second. Instead of using monochromated x-rays, a wide spectmm of incident x-rays is used. In this case, all of the reflections that ate diffracted on to an area detector are recorded at just one setting of the detector and the crystal. By collecting many complete data sets over a short period of time, the Laue method can be used to foUow the reaction of an enzyme with its substrate. This technique caimot be used with conventional x-ray sources. 

The main advantages of EDS are its speed of data collection the detector s efficiency (both analytical and geometrical) the ease of use its portability and the relative ease of interfacing to existing equipment. 

Improvements in technology will shape developments in PL in the near future. PL will be essential for demonstrating the achievement of new low-dimensional quantum microstructures. Data collection will become easier and ter with the continuing development of advanced focusing holographic gratir, array and imaging detectors, sensitive near infiared detectors, and tunable laser sources. 

Photomultipliers are used as detectors in the single-channel instruments. GaAs cathode tubes give a flat frequency response over the visible spectrum to 800 nm in the near IR. Contemporary Raman spectrometers use computers for instrument control, and data collection and storage, and permit versatile displays. 

A third program allows plotting of selected or all data files on the disk. A fourth program is used routinely to watch the responses of the detectors before data collection is started. It plots the latest responses on the screen for a time range selected by the operator. 

The major advances in crystallographic methods were both experimental and theoretical. In experimental terms, there was widespread availability of synchrotron data collection resources and the emergence of CCD detectors that dramatically increased the speed at which data could be collected. A particularly important advance was the development of cryocrystallography methods [39] that revolutionized crystallography by making crystals essentially immortal. 

Analyses were carried out at ambient (23 C) in THF, using a flow rate of 2.0 ml/min. Data from the detectors were collected and processed with a LDS-2 Data System (Chromatix). 

In practice, the GC conditions should be designed to give the shortest analysis time while still providing the necessary selectivity (i.e., separation of both analyt-analyte and matrix-analyte). Selective detectors often have fast data collection rates and improved matrix-analyte selectivity, but analyte-analyte selectivity must be addressed solely by the GC separation. MS can improve both types of selectivity and, by reducing the reliance on the GC separation, faster analysis times can often be achieved in complicated mixtures. 

Sample preparation, injection, calibration, and data collection, must be automated for process analysis. Methods used for flow injection analysis (FLA) are also useful for reliable sampling for process LC systems.1 Dynamic dilution is a technique that is used extensively in FIA.13 In this technique, sample from a loop or slot of a valve is diluted as it is transferred to a HPLC injection valve for analysis. As the diluted sample plug passes through the HPLC valve it is switched and the sample is injected onto the HPLC column for separation. The sample transfer time typically is determined with a refractive index detector and valve switching, which can be controlled by an integrator or computer. The transfer time is very reproducible. Calibration is typically done by external standardization using normalization by response factor. Internal standardization has also been used. To detect upsets or for process optimization, absolute numbers are not always needed. An alternative to... 

The basic function of the spectrometer is to separate the polychromatic beam of radiation coming from the specimen in order that the intensities of each individual characteristic line can be measured. In principle, the wide variety of instruments (WDXRF and EDXRF types) differ only in the type of source used for excitation, the number of elements which they are able to measure at one time and the speed of data collection. Detectors commonly employed in X-ray spectrometers are usually either a gas-flow proportional counter for heavier elements/soft X-rays (useful range E < 6keV 1.5-50 A), a scintillation counter for lighter elements/hard X-rays (E > 6keV 0.2-2 A) or a solid-state detector (0.5-8 A). 

The advent of CCD detectors for X-ray diffraction experiments has raised the possibility of obtaining charge density data sets in a much reduced time compared to that required with traditional point detectors. This opens the door to many more studies and, in particular, comparative studies. In addition, the length of data collection no longer scales with the size of the problem, thus the size of tractable studies has certainly increased but the limit remains unknown. Before embracing this new technology, it is necessary to evaluate the quality of the data obtained and the possible new sources of error. The details of the work summarized below has either been published or submitted for publication elsewhere [1-3]. 

Even in the nominal absence of laser fluctuations or other imagedegrading aberrations, the number of photons that hit the detector during the data collection period of the image (i.e., the exposure time for a CCD image or the pixel dwell time for a confocal image) will contain considerable noise. The photon count x follows a Poisson distribution (Fig. 7.7A) with mean value fi as... 

The secondary electrons emitted from the sample are attracted to the detector by the collector screen. Once near the detector, the secondary electrons are accelerated into the scintillator by a positive potential maintained on the scintillator. Visible light is produced by the reaction of the secondary electrons with the scintillator material. The emitted light is detected by a photomultiplier tube, which is optically coupled to the scintillator via a light pipe. The PMT signal is then transferred to the grid of a cathode ray tube (CRT). Data collection... 

---