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white radiation, cannot be raised very high before exciting the characteristic 

 radiation whereas, with a heavy metal target (wave-length of characteristic 

 radiation short) considerable intensity of white radiation can be produced 

 without exciting the characteristic radiation. This is illustrated in Fig. 3.1 

 which shows that a potential of 35,000 volts is high enough to excite the 

 K group radiation from molybdenum, but not high enough to excite the 

 shorter wave-length K radiation from tungsten which, further, gives more 

 intense white radiation at this voltage. Even higher voltages, resulting in 



K<(OfW 



K^ofMo KdOFMo 



Fig. 3.1 — Variation of intensity with wave-length of X-rays from tungsten and 

 molybdenum targets at 35,000 volts 



more intense white radiation, could be used with tungsten without exciting 

 the characteristic radiation (X = .209). 



Figure 3.2 shows the I-\ curve (estimated) for copper, the target metal 

 commonly used in quartz X-ray work, which has a small atomic number 

 and can therefore be used as a source of "monochromatic" X-rays with 

 moderate voltages. (A further advantage of copper for quartz work is 

 pointed out at the end of this section). 



The Kai and Ka^ wave-lengths are so close together that for most uses 

 of "monochromatic" radiation no attempt is made to eliminate the Ka^ 

 racMation. The A'jS radiation, however, gives a distinct intensity peak of 

 shorter wave-length which must be reduced as much as possible by use of 

 a metal filter having a high absorption coefficient for the K^ radiation of 



