72 MUTATION AND PLANT BREEDING 



D. Nuclear Reactors 



Unfiltered radiation from nuclear reactors is a complex mixture 

 of radiations and is not suitable for well-defined biological experi- 

 ments. Facilities with fairly complicated shielding are required to 

 provide thermal or fast neutron beams with a small amount of gamma 

 contamination (29). 



Some radioactivity is induced in the biological specimens treated 

 with thermal neutrons, but the amount of radioactivity is usually 

 small and of fairly short half-life. The radioactivity induced consti- 

 tutes no serious hazard with reasonable precautions. For most plant 

 material exposure times in the Brookhaven facility are in hours or 

 days. A considerable amount of work has been done in the last decade 

 in the treatment of seeds and other plant material with thermal 

 neutrons for comparison with X- or gamma-ray treatment (35, 141, 

 142, 160, 186, 190, 203). The main rationale behind this is that the 

 relatively high ion density obtained with thermal neutron exposures 

 may give results different from those one would get with the less 

 densely ionizing radiation. 



E. Small Portable Irradiators 

 In addition to the portable X-ray machines and ultraviolet 

 sources, many other relatively inexpensive small sources have been 

 designed and used for various purposes. Beta irradiators of various 

 sizes and types can be used for localized exposures of suitable mate- 

 rial either with small beams through minute pores or by surrounding 

 the material to be irradiated with a radioisotope suitably bound to 

 prevent corrosion or flaking (167). Maximum penetration of biologi- 

 cal material with beta rays is usually only a few millimeters or less, 

 depending upon the radioisotope used. Dosimetry of beta sources is 

 difficult and complex (95). Similar irradiators can be made using 

 alpha emitters and as these penetrate only one or a few cell layers, 

 treatment of very localized areas is possible. Encapsulated radioactive 

 materials in various sizes and shapes are now available commercially 

 and can be used in a great variety of ways for experimental work. 



F. Internal Emitters 



Whenever radioisotopes are located inside cells or tissues, they 

 are called internal emitters. Many internal isotopes have been studied 

 with respect to their cytogenetic effect, but the four most commonly 



