Silicon vidicon

Because vidicon imagers and particularly the silicon vidicon with its various image-intensified derivatives are much more readily available and their behavior and performance are better controlled (even if not always fully understood), a brief description of their principles of operation may be necessary if their spectrometric performance (and that of other imagers as well) is to be properly and intelligently interpreted. 

Figure 1. Diode structure and principles of operation of the silicon vidicon detector...

Two-dimensional (2D) spectroscopy The silicon vidicon is a two-dimensional OID with an area-array target comprising a few hundred thousands dicrete photodiodes. Since these diodes can be randomly read out by the scanning beam, the detector is capable of performing some very useful spectrometric tasks ... 

Because dark current limits the integration times obtainable at room temperature, Peltier cooling (to -150C) is used to reduce thermal population of the conduction band. In contrast to the silicon vidicon and the SIT [vide infra], where the presence of intense radiation may bloom out the entire sensor, blooming is greatly reduced with photodiode arrays even when intense lines saturate individual diodes. 

Vidicon. Although there are several types of vidicon tubes presently available, the most promising of these for spectroscopic work is the silicon vidicon, first conceived at Bell Labs (68). Figure 3 shows a diagram of a silicon vidicon. In contrast to a photomultiplier, which is based on a photoemissive principle, the vidicon television camera tube is based on a conductivity principle, a circumstance which explains its name. 

Response characteristics of the silicon vidicon and image dissector for a variety of scan formats were evaluated and have been presented elsewhere (2, 29 30). Only those data most pertinent to analytical applications with the selected operating conditions are included here. 

Noise characteristics. For repeated measurements of currents between 2 and 15 nA with the silicon vidicon, standard deviations ranged from 0.025 to 0.045 nA with an average value of 0.035 nA. 

To a first approximation, the noise level for the silicon vidicon is independent of the signal level. For repeated measurements of currents between 4 and 4,400 nA with the image dissector, a log-log plot of imprecision vs. signal is linear with a slope of 0.51 0.07 (30) confirming the expected shot-noise behavior. 

Performance data for silicon vidicon used to determine Cr, Cu, Fe, Mn, Ni and Co in synthetic samples by atomic absorption. (With permission, Clin. Chem.,... 

Comparisons with other systems. Data presented in Table VI provide a comparison of results obtained with the image dissector with results reported by others with other systems. Results in the second column represent multielement detection limits observed in this work. Results in the third and fourth columns represent detection limits reported for single element determinations with conventional optics and a silicon vidicon (12J and a commercial atomic absorption instrument (33). 

The multielement detection limits with the echelle/image dissector are comparable to, or better than, single element detection limits reported for a silicon vidicon and conventional optics. Detection limits for Cr, Cu, and Mn with the echelle/ image dissector compare favorably with single element data reported for a conventional atomic absorption instrument with a photomultiplier detector, but detection limits obtained here for Ni and Co are higher by factors of 10 or more than for the conventional instrument. The echelle/image dissector system should be adaptable to a so-called flameless atomizer and be subject to the same improvements in sensitivities and detection limits as conventional detector systems. 

The data in Tables VII and VIII suggest that for emission measurements, the detection limits for the image dissector are lower than those for the silicon vidicon by a factor of about 24. The decrgase in the detection limit ratio at longer wavelengths (> uSOOOA) results from a decrease in the luminous sensitivity of the image dissector relative to the silicon vidicon. The effect is quite apparent in the case of potassium. 

While these data show that the image dissector is superior to the silicon target vidicon in several respects for atomic spectroscopy, the silicon vidicon and other integrating detectors retain significant advantages for molecular absorption (39) and fluorescence spectroscopy (40) where resolution requirements are not so demanding, available radiant fluxes are higher, and a... 

The first detectors to be used in OMA systems were standard TV image tubes. These were silicon vidicons or the more sensitive Silicon Intensified Target (SIT) detectors, which both employed silicon targets to convert optical information into electronic form. More recently, the use of solid state detectors in the form of a diode array (Reticon) has been found to have some advantages over the vidicons and SIT tubes. Current developments in the field of charge coupled devices (CCD) will probably soon provide an even better multielement detector for use in OMA systems. 

Figure 7.16. Schematic diagram of a silicon Vidicon camera tube. An array of photosensitive diodes are grown on a silicon wafer about 15 pm apart. From P. Burke, Research/Development, 24(4), 24 (1973), by permission of the publisher. Copyright 1973 by Technical Publishing Company.

A vidicon detector is a photosensitive device composed of a two-dimensional array of several thousand very small detectors, each capable of responding to incident radiant energy. The most useful vidicon detector for analytical spectroscopy is the silicon vidicon detector. 

The target of a silicon vidicon tube is shown in Figure 6-14. It is composed of an array of p-type semiconductor areas insulated from one another and formed over an n-type silicon base. The spaces between the p-type areas are coated with silicon dioxide to shield the silicon n-type base from the electron beam. 

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