Cathode efficiency

Several different definitions are used to speeify the effieieney of a PMT eathode. Often the sensitivity of a PMT is speeified in units of eathode luminous sensitivity . This is the cathode eurrent per lumen incident light from a tungsten lamp operated at a temperature of 2,856 K. Beeause the intensity maximum of the lamp is at about 1,000 nm, the luminous sensitivity may not represent the efficiency at a given wavelength. Photocathodes of different spectral sensitivity are therefore not directly comparable. Moreover, the cathode luminous sensitivity does not include the effieieney of the eleetron transfer from the cathode into the dynode system and the possible loss of photon pulses due to incomplete resolution of the pulse height distribution (see Fig. 6.27, page 241). 

In the test sheets of PMTs, the manufacturers occasionally specify the measured anode luminous sensitivity instead of the cathode sensitivity. The anode sensitivity is the cathode sensitivity (including the electron transfer efficiency) multiplied by the gain of the tube. Because almost any gain can be obtained by increasing the supply voltage, the anode luminous sensitivity cannot be used to compare the photon counting performance of PMTs. 

The most useful parameter for characterising the efficiency of a photoncounting detector is the quantum efficiency. The quantum efficiency, QE, of a photocathode is the probability of the emission of a photoelectron per incident photon. It is directly related to the radiant sensitivity, S  

Usually quantum efficiencies given for detectors refer to the emission of a photoelectron or, in avalanche photodiodes, the generation of an electron-hole pair. The detection efficiency in PMTs is smaller for the reasons mentioned above Not all photoelectrons cause a detectable anode current pulse in a PMT, and not all electron-hole pairs trigger an avalanche in a SPAD. 

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There are two reactions that influence the cathodic efficiency, namely the reduction of OCl and of CIO3... 

Plate Thickness. In plating processes, plate thickness can be predicted knowing the cathode efficiency of a particular plating solution, the current density, and time of plating. 

Cathode Efficiency. Faraday s law relates the passage of current to the amount of a particular metal being deposited ie, 96,485 coulombs, equal to one Faraday, deposits one gram-equivalent weight of a metal at 100% efficiency. The cathode efficiency, an important factor in commercial electroplating, is the ratio of the actual amount of metal deposited to that theoretically calculated multipHed by 100%. 

RoUs and other relatively simple shapes make use of inert shields and thieves to avoid edge buildup and produce a more even plate thickness. For more compUcated shapes having deeper recesses thicker deposits from cyanide copper baths have been used as an undercoat to the copper sulfate deposit. Acid copper baths operate near 100% efficient over a wide current density range. The cathode efficiency is usuaUy slightly less than the anode efficiency, bringing about a slow increase in copper unless drag-out losses are high. 

AE = total potential difference caused by polarisation (anode and cathode) on the cathode area indicated by the subscript and c = cathode efficiency as indicated by the subscript. 

As A will be a function of current density, T will be a function of electrode area, and comparisons should therefore be made with cells of standard size. Equation 12.12 shows that high throwing indices will result when polarisation rises steeply with current (AE, AEj) and cathode efficiency falls steeply (cj >> f i)- The primary current ratio, P = affects the result because... 

Many baths in which metal is reduced from complex anions (e.g. cyanide baths, stannate baths) give high throwing indices because both polarisation and cathode efficiency variation favour a low value of M. The cathode efficiency for a typical copper cyanide bath (40°C) was ... 

The polarisation and cathode efficiency terms in equation 12.12 cannot be altered in practice to improve thickness distribution, as they tend to be decided by overriding considerations. It is usual to accept the distribution obtained without special precautions as being the best commercial solution, although the average thickness needed to achieve the necessary minimum... 

Several high-efficiency hard chromium plating baths are now available commercially. A solution which does not contain fluoride, and does not therefore attack steel or aluminium, has been described by Schwartz . At 50 A/dm and 53°C the cathode efficiency is about 25%, enabling deposition to be carried out at the rate of I m/min, with a consequent substantial saving in power and time. The deposit is bright, and has a hardness of about 1 050 Hy. 

For ruthenium, electrolytes based on ruthenium sulphamate or nitrosyl-sulphamate have been described, but the most useful solutions currently available are based on the anionic complex (H2 0 Cl4 Ru N Ru-Cl4-OH2) . The latter solutions operate with relatively high cathode efficiency to furnish bright deposits up to a thickness of about 0-005 0 mm, which are similar in physical characteristics to electrodeposited rhodium and have shown promise in applications for which the latter more costly metal is commonly employed. Particularly interesting is the potential application of ruthenium as an alternative to gold or rhodium plating on the contact members of sealed-reed relay switches. 

The major disadvantage to electrolytic recovery is high energy cost. Energy costs will vary, of course, with cathode efficiencies and local utility rates.22... 

In a complex reaction system occurring in a flow electrolyzer, one of the anode reaction products reacts with the principal constituent of the composite electrolyte, yielding an anionic species whose parasitic reaction at the cathode reduces the cathode efficiency. 

Chiba [43] has found that the electrodeposition of copper from a cupric-EDTA bath, in the presence of ultrasound, gave increased limiting current densities and increased cathodic efficiences while at the same time reducing the grain size. Walker [44,45] has shown that deposits obtained from sulphate baths in the presence of ultrasound show increased hardness. 

Notvack and Habermehl [47] observed a significant drop in cathodic efficiency (90 % to 20 %) as the current density in a silent system was changed from 100 to 200 A m . In contrast ultrasound was highly beneficial and the efficiency remained very high (70 %) even at a current density of 800 A m . ... 

Cathode efficiency in plating, in general, depends on a number of key parameters of the electrolyte or bath, such as chemical component concentrations, pH, agitation, and (last but not least) current density. 

Finally, cathode efficiencies are seldom determined directly. When questions arise in the plating shop, the experienced plater may adjust current and/or bath composition and/or temperature and other parameters to give a visibly normal amount of gas evolution. 

Figure 12.4. Effect of cyanide/zinc ratio on cathode efficiency. (From Ref. 4, with permission from Wiley.)...

The effect of potential on the cathodic efficiency of a zinc electrodeposition in a cyanide-free alkaline bath was also investigated [416]. 

The most important problem is that of contamination of process solutions. Just to take one example, that of nickel, sodium and calcium concentrations have been shown to increase when dragout is returned to the process solution, likely sources being the water used for rinsing. These contaminants interfere with the plating process. Organics, chlorides, and heavy metals, from sources including the process solution itself and the work being processed, can also accumulate and pose problems. And finally, nickel metal can rise to undesirable concentrations because of the difference in anode and cathode efficiencies. While these problems may take years to manifest themselves in a low-volume operation, eventually treatment and purification is required.[20]... 

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