734 RADIATION BIOLOGY 



(2) emit a heavy particle immediately to form a stable isotope, e.g., 



B 10 + n-> [B u ]-»Li 7 + a 



or (3) emit a capture radiation immediately, forming a radioactive 

 daughter which then emits /3 or 7 rays at a rate characteristic of the 

 isotope formed, e.g., 



half life 

 N 14 + n -> [N 15 ] -> C 14 + p ca. 6000 years -> N 14 + /3~ 



The radiations of biological importance are the immediate "capture" 

 radiations and the delayed or "decay" radiations, since these produce 

 ionization. Thermal neutrons do not themselves appear to produce any 

 effect, since they are uncharged particles of low energies. 



The reactions outlined indicate that there are two principal classes of 

 radiations arising from slow neutrons which should be of biological sig- 

 nificance, 7 radiation and heavy-particle radiations. As has been shown, 

 exchanges in Tradescantia microspores are linearly related to doses of 

 protons and a particles, but show a geometrical relation to doses of 7 rays. 

 Thus it should be possible, by determining the kind of dosage curve 

 obtained for slow neutron-induced exchanges, to decide the relative bio- 

 logical importance in chromosome aberration production of these two 

 classes of radiations arising from slow neutron exposures. Such experi- 

 ments have been performed, utilizing a special exposure facility in the 

 thermal column of the Oak Ridge reactor (Conger and Giles, 1950). The 

 results obtained indicate that the relationship with doses of slow neutrons 

 is linear for all types of aberrations, giving biological evidence that cap- 

 ture reactions resulting in the emission of particulate radiations are of 

 major consequence in producing aberrations. Physical calculations indi- 

 cate that the boron and nitrogen reactions are responsible for approxi- 

 mately 32 and 52 per cent of the total rep of ionization absorbed in 

 Tradescantia tissue, while the hydrogen reaction accounts for 16 per cent, 

 practically all the remainder. 



Simultaneous X-ray exposures were made in order to compare the 

 relative biological efficiencies of thermal neutrons and X rays for the 

 production of chromatid and isochromatid breaks. After appropriate 

 corrections were made for 7-ray contamination in the neutron exposure 

 chamber, the observed thermal neutron-X-ray ratio for chromatid 

 aberration production was found to be approximately 11:1. If only the 

 nitrogen protons and boron a particles from thermal neutron capture are 

 considered, their efficiency compared with X rays seems to be about 15 : 1. 

 The expected ratio, calculated on the basis of previous radiation results 

 from external sources, was ca. 5.2:1. Thus it appears that particulate 

 radiations originating internally from capture reactions are considerably 

 more efficient (about three times) in break production than are similar 

 radiations from external sources. Since some evidence indicates a non- 



