172 



RADIATION BIOLOGY 



source of continuous radiation in the ultraviolet; the visible emission is 

 relatively weak. For this reason the hydrogen arc has been used exten- 

 sively for ultraviolet spectrophotometry. The hydrogen arc has been 

 produced in various forms from low-pressure cold-cathode lamps oper- 

 ating at high voltages to the relatively high-pressure low-voltage hot- 

 cathode type of arc (Finkelstein, 1950; Smith and Fowler, 1936). It is 

 also available commercially as low-wattage hot-cathode sources for spec- 

 trophotometry. The arc described by Smith and Fowler has a hot 

 cathode and operates on direct current at a pressure of 0.2 mm hydro- 

 gen and currents ranging from 0.5 to 20 amp. A voltage of 220 dc is 

 adequate to supply the ballast and arc requirements. 



Even at relatively high currents the hydrogen arc is a rather diffuse 

 low-intensity source. Therefore most of the lamps are of the end-on 

 type, in which the energy comes from a considerable depth of discharge. 

 The high-power lamps require water cooling even when quartz envelopes 

 are used. 



XENON ARC 



The high-pressure compact xenon or Schulz arc (Aldington, 1949; 

 Anderson, 1951; Schulz, 1947) has a spectral energy distribution very 



rich in blue and ultraviolet and 

 somewhat similar to that of the 

 high-intensity carbon arc (Fig. 

 3-13). The arcs described by 

 Anderson contain xenon at a pres- 

 sure of 20-44 atm in a very thick 

 spherical quartz envelope and have 

 tungsten electrodes separated by 

 3.5-6.5 mm. Lamps of 150 1000 w 

 are available which, though not 

 having the brightness of the mer- 

 cury arc (Table 3-14), have a much 

 more uniform spectrum containing 

 a large number of closely spaced 

 lines and a continuum. Brightness 

 values of nearly 200 c mm~- have 

 been obtained, as compared with 

 350 c mm"- for the mercury high-pressure short arc. The electrodes 

 become incandescent and contribute an appreciable amount of infrared 

 and visible energy. The xenon compact arc is an excellent source 

 for use with optical systems because of its high brightness and small 

 dimensions. The xenon arc has considerably higher ultraviolet intensity 

 from 250-350 m^u than does a low-intensity carbon arc (Baum andDunkel- 

 man, 1950), and it is more stable than the carbon arc. 



200 



1800 



600 1000 1400 



WAVE LENGTH, mfj. 



Fig. 3-13. Spectral emission of a 150-\v 

 high-pressure xenon a-c arc at 23 atm and 

 3-mm arc length. (From Anderson, 

 1951.) 



