CCD detector

The quantum efficiency of contemporary array detectors can be remarkably high-in excess of 90% in the case of back-iUuminated CCDs. The two main sources of noise for CCD cameras are  

For Raman mapping measurements, the readout time per pixel must be very short. For example, if an image consisting of 128 pixels per line and 128 lines is acquired with an integration time of 1 s per pixel, then the total acquisition time will be a little over 4.5 h. Reducing the readout time to 100 ms per pixel decreases the measurement time to 27 min. A further 10-fold reduction in the readout rate decreases the acquisition time to less than 3 min. The latter situation would allow hundreds of images to be acquired per day, instead of just one or two. Unfortunately, this condition is not readily achievable in practice because the faster the readout time of the detector electronics, the noisier is the readout amplifier. 

Another source of noise in spectra measured using a CCD is caused by the pixel-to-pixel variation in the quantum efficiency of neighboring pixels. This variation can be corrected by illuminating the detector array with a uniform light source and measuring the signal from each pixel. Once the relative response of each pixel has been determined, a correction known as 3l fiat field correction can be applied. 

Several modes of operation are available in state-of-the-art confocal Raman microspectroscopy, including the measurement of samples with a spatial resolution of less than 1 pm, depth profiling, and line mapping. LaPlant and Ben-Amotz have given a detailed description of the design and construction of a confocal Raman microspectrometer [34] and a number of instruments are available commercially. 

The WITec alphaSOO R confocal Raman microscope can be upgraded to perform atomic force microscopy, tip-enhanced Raman spectrometry, and near-field scanning optical microscopy, and is arguably the most versatile instrument for Raman microspectroscopy available today, although the competition is rapidly closing the gap. 

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Figure Cl.5.9. Vibrationally resolved dispersed fluorescence spectra of two different single molecules of terrylene in polyetliylene. The excitation wavelengtli for each molecule is indicated and tlie spectra are plotted as the difference between excitation and emitted wavenumber. Each molecule s spectmm was recorded on a CCD detector at two different settings of tire spectrograph grating to examine two different regions of tlie emission spectmm. Type 1 and type 2 spectra were tentatively attributed to terrylene molecules in very different local environments, although tlie possibility tliat type 2 spectra arise from a chemical impurity could not be mled out. Furtlier details are given in Tchenio [105-1071.

A more recent, and superior, type of detector, which also benefits from the multiplex advantage, is the charge-coupled device (CCD). The CCD, as used for spectroscopy, has been developed from the CCD detector used in a camcorder. 

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. 

Walter, R.L., et al. High resolution macromolecular structure determination using CCD detectors and synchrotron radiation. Structure 3 835-844, 1995. 

The short penetration depth of UV/blue photons is the reason that frontside CCD detectors have very poor QE at the blue end of the spectrum. The frontside of a CCD is the side upon which the polysilicon wires that control charge collection and transfer are deposited. These wires are 0.25 to 0.5 /xm thick and will absorb all UV/blue photons before these photons reach the photosensitive volume of the CCD. For good UV/blue sensitivity, a silicon detector must allow the direct penetration of photons into the photosensitive volume. This is achieved by turning the CCD over and thinning the backside until the photosensitive region (the epitaxial layer) is exposed to incoming radiation. 

CCD detector designers try to increase the signal-to-noise ratio of an amplifier in two ways (1) increase the responsivity, or (2) decrease the random current fluctuation between source and drain. The responsivity can be increased by decreasing the amplifier size. Decreasing the amplifier size decreases the capacitance of the MOSFET. The responsivity of a MOSFET obeys the capacitor equation which relates voltage, V, to the charge Q on capacitance C V = QIC. 

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. 

The CCD detector and the potentiostat were synchronized in an open-loop configuration by starting the experiment with a common trigger. Careful measurements... 

The main detectors used in AES today are photomultiplier tubes (PMTs), photodiode arrays (PDAs), charge-coupled devices (CCDs), and vidicons, image dissectors, and charge-injection detectors (CIDs). An innovative CCD detector for AES has been described [147]. New developments are the array detector AES. With modem multichannel echelle spectral analysers it is possible to analyse any luminous event (flash, spark, laser-induced plasma, discharge) instantly. Considering the complexity of emission spectra, the importance of spectral resolution cannot be overemphasised. Table 8.25 shows some typical spectral emission lines of some common elements. Atomic plasma emission sources can act as chromatographic detectors, e.g. GC-AED (see Chapter 4). 

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]. 

The quality of the intensity data obtainable has been assessed from an experiment on oxalic acid obtained at 100 K with a CCD detector. In this experiment the contamination of 712 to the measured intensities was eliminated by appropriate choice of the generator voltage. Various criteria for judging the quality of the data are discussed below [2],... 

CCD detectors (and also image plates), this contamination is a potential source of systematic errors because these devices cannot discriminate with respect to energy. 

The 27-cm telescope is equipped with four CCD detectors and measures the light curves of bright stars in the wavelength range 370-950 nm. The scientific goals of the mission are ... 

Consider you have forgotten to switch on multi-read 28 with your CCD detector and the raw data are full of cosmic-ray spikes. How do you remove them without spoiling the image ... 

A variety of detectors is used in the field of X-ray scattering. In fact, the proper choice of the detector (as well as the sample thickness) is essential for good quality of the recorded data, whereas the intensity of the synchrotron radiation determines the minimum cycle time between two snapshots - if a modern CCD detector is used. Gas-filled detectors cannot be used to record high-intensity scattering patterns. Image plates need a minimum time of 2 min for read-out and erasure. 

Figure 4.13. mar 185 CCD-detector in operation at beamline BW4, HASYLAB at the rear end of the vacuum tube... 

Several other principles have been used to build X-ray detectors. For instance, ID detectors have been realized by diode arrays. 2D detectors have been realized by conversion of X-rays to visible light, photon amplification, and a television camera (VIDICON). CCD detectors have outperformed both diode arrays and the VIDI-CON. 

In recent years no truly new basic principles have been introduced for the detection of luminescence. However, the technical evolution in the field of microelectronics and optoelectronics, charge coupled device (CCD) detectors, fiberoptics, assembly techniques, and robotics resulted in the introduction on the market of new generations of instruments with increased performance, speed, and ease of handling. In this chapter, some of their typical features will be reviewed. To keep this presentation at a concrete level and to illustrate some specific item, instruments of different makes will be referred to. However, this does not imply they are better than those not cited. It is more a matter of availability of recent documentation at the time of writing. Note that numerical values cited typically relate what can be done today and may vary from one instrument to another from the same company. 

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