70 



MUTATION AND PLANT BREEDING 



carefully protected open fields for chronic or acute treatment of 

 plants of almost any desired size. Some uses of gamma sources in 

 botanical research have been summarized by Sparrow (159). While 

 much has been written about the value of these large gamma sources 

 in plant breeding (146, 147, 167), their usefulness in this respect seems 

 to be generally no greater than the more conventional radiation 

 sources such as X-ray machines or shielded radioisotopes used under 

 laboratory conditions. They are, however, very versatile and may be 

 especially suited for certain specialized crops which are ordinarily 

 reproduced by vegetative methods. The main advantage of these large 

 eamma sources is the ease with which chronic treatments can be made 

 at any desired dose rate and the very high intensities of radiation 

 available at least in a restricted area near the source. Daily dose rates 

 available at various distances from three Co 60 sources are given in 

 Table 5. As shown in Figure 4, dose rates from a given size of source 



Table 5. — Dose-distance Relationship for Three Cobalt-60 Sources in Use in the 

 Biology Department at Brookhaven National Laboratory. * 



varies with the energy of the radiation. It is the opinion of the author 

 that these sources should be regarded as versatile radiobiological tools 

 and not primarily as facilities for the average plant breeder. 



While most installations to date have used Co 60 because of its 

 availability, it would seem that Cs 137 would be more suitable for 

 future installations. The main advantage of Cs 137 is its long half-life 

 (30 years) compared to the 5.3 year half-life of Co 60 . Another advan- 



