Simple Ionization Circuit

Figure 14 illustrates a simple ionization circuit consisting of two parallel plates of metal with an air space between them. The plates are connected to a battery which is connected in series with a highly sensitive ammeter. 

Fluorescent luminaires require a simple electrical circuit to initiate the ionization of the gas in the tube and a device to control the current once the arc is struck and the lamp is illuminated. Such a circuit is shown in Fig. 2.60. 

A later General Electric x-ray photometer26 is noteworthy because it uses current ionization chambers (2.6) as detectors. Improved means of external amplification made it possible to use this type of detector in a satisfactory photometer with the simple circuit shown in Figure 3-9. 

Early electrolytic conductivity detectors operated on the principle of component combustion, which produced simple molecular species that readily ionized, thus altering the conductivity of deionized water. The changes were monitored by a dc bridge circuit and recorded. By varying the conditions, the detector could be made selective for different types of compounds (e.g., chlorine containing, nitrogen containing). 

The conventional TCD is configured with the filaments being connected to form a Wheatstone bridge. A property of the Wheatstone bridge is common mode rejection of the noise which is primarily due to the electronics (l.e. power supply stability and the amplifier circuit). The TCD noise spectrum resembles white (shot) noise rather than the 1/f (flicker) noise of ionization detectors. Modulation techniques for noise rejection of white noise is no better than a simple Wheatstone bridge. 

The DC arc is produced by a direct current, flowing through two electrodes in a shorted circuit. The arc is adjusted with a variable resistor to obtain the desired potential. The sample, which is ionized at the anode, then drifts toward the cathode, forming a cathodic layer of excited atoms and ions. This layer of intense luminosity produces a multitude of emission lines and is the optimum location for the measurement of atomic emission. Because the arc can localize on one part of the electrode, the latter is often rotated rapidly to produce a more even arc. The nonuniformity of the DC arc produces a substantial variation in line intensities, limiting the method to qualitative identification. The apparatus is simple, inexpensive, and sensitive. 

Consider first the simple extrinsic photoconductor. Here the sample is a semiconductor containing a single impurity level, the source of the free electrons (or holes) present in the sample. Thus the fluctuation in the number of the free carriers arises from the fluctuation in the generation and recombination rates through that level. If it is assumed that the temperature is so low that very few of the extrinsic centers are thermally ionized (which is valid for most extrinsic cooled photoconductive infrared detectors), then the short circuit g—r noise current and the open circuit g — r noise voltage which appear only in the presence of a bias current Ig, are given by... 

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