170 RADIATION BIOLOGY 



high-intensity arc is used extensively with optical systems such as motion- 

 picture projectors, theatrical spotlights, and searchlights and for experi- 

 mental purposes requiring the highest radiation intensities. The two 

 most common types are the reflector lamps for medium power, with 

 automatic-feed and nonrotating carbon electrodes mounted on the reflec- 

 tor axis, and high-power condenser-lens eciuipment with rotating positive 

 electrodes, with the negative carbon displaced about 60° from the axis. 

 The condenser-lens type of equipment collects the flux without intro- 

 ducing shadows in the beam, as "reflecting optical systems do. The most 

 stable operation is obtained with the automatic-feed rotating positive 

 carbon lamp. This source is probably the most satisfactory for optical 

 systems requiring intense irradiation of small areas with uniform flux. 

 as in the irradiation of a monochromator slit. 



Flame Arc. The cores of flame-arc carbons contain large quantities of 

 rare-earth salts and other materials to accentuate certain regions of the 

 spectrum. In the white-flame arc the salts are principally cerium. Cal- 

 cium produces a yellow flame; strontium, a reddish flame; and the poly- 

 metallic cores containing iron, nickel, cobalt, and aluminum produce 

 flames especially rich in ultraviolet energy in the region of 300 m/x (Greider 

 and Downes, 1932; National Carbon Company, 1944). 



Owing to the activity of the core material, the arc produces a large 

 flame which is responsible for most of the radiant energy and luminosity. 

 The emission spectrum depends chiefly on the composition of the flame- 

 producing materials. The flame arc is the most flexible of all carbon arcs 

 in regard to spectral characteristics. 



Since most of the luminosity is in the flame, the intensity of the flame 

 arc is much less than that of the other two types (see Table 3-14). Low 

 current densities are used, and alternating current is a common source 

 of power, since high intensity is not the principal objective. These lamps 

 are used principally for general irradiation in photochemical industrial 

 processing and as therapeutic sources for artificial sunlight. 



ZIRCONIUM OR CONCENTRATED ARC 



This source consists of a d-c arc between a cathode of a metallic film 

 of zirconium or zirconium oxide and an anode of refractory metal (Buck- 

 ingham and Deibert, 1946a, b). The zirconium oxide is packed into a 

 tube of refractory metal such as tantalum, and in the enclosed arc the 

 discharge takes place to a refractory metal ring, as shown in Fig. 3-12. 

 Zirconium oxide has a melting point in the vicinity of 3000°K and is 

 maintained in the molten state by the discharge. Some of the oxide is 

 reduced to zirconium metal as a thin film on the molten surface. The 

 enclosed lamps are operated in an atmosphere of argon, and the arc is 

 initiated by a high-voltage discharge and is sustained as a low-voltage arc. 



Zirconium enclosed arcs are manufactured in power ratings of 2-1000 w. 



