66 MUTATION AND PLANT BREEDING 



should be carefully read by anyone contemplating work with ionizing 

 radiation. 



III. Methods of Exposure and Facilities Used 



Methods of exposing suitable material to radiation and the size 

 and nature of the facilities used vary so widely that it is difficult to 

 summarize them. For instance, experimental procedures used with 

 plants vary from the treatment of a few milligrams of pollen or spores 

 with an inexpensive ultraviolet source or small portable X-ray 

 machine to the simultaneous treatment of hundreds or thousands of 

 large plants or trees by gamma sources of many kilocuries in size 

 (159, 167, 171). Space limitation does not allow a full description of 

 all the methods used and facilities available, but representative radi- 

 ation sources are listed in Table 4. A more complete description of a 

 few types is given below. These have been selected largely on the 

 basis of their actual usage or potential significance in cytogenetics or 

 plant breeding. 



A. X-ray Machines 



X-ray machines are probably the most widely available source 

 of ionizing radiation and have as their chief advantages availability, 

 versatility, and, generally, ease of operation. They are also easier to 

 shield than radioisotopes emitting highly energetic gamma rays such 

 as Co co and Cs 137 and do not require such frequent calibration or cal- 

 culations of dose as is necessary with isotopes, especially those with 

 shorter half-lives. 



X-ray machines suitable for biological experiments are usually 

 in the 50- to 300-kv range and their output may exceed 1,000 r per 

 minute for small samples. Newer types may go much higher. As com- 

 pared to Co co gamma sources, however, X-ray machines have as their 

 chief limitations: (a) their requirement for considerable electric 

 power; (b) that the maximum intensities and penetration of their 

 radiation are in most cases much less than that of a Co co source; (c) 

 that the size of the object to be irradiated is definitely limited with 

 most installations, except for very low doses; (d) the fact that X-ray 

 machines cannot be operated economically for continuous long 

 exposures; and (e) maintenance costs are considerable for machines 

 operated near full capacity. 



