L. H. GRAY 



larger values of R.B.E. are observed under anaerobic than under aerobic 

 conditions. This is exemplified by Table 2 which shows the influence of 

 oxygen tension on the R.B.E. of a radiation relative to X radiation with 

 respect to growth inhibition in Vicia roots. 



Table 2 

 Influence of oxygen tension on R.B.E. of a radiation and X radiation^, b 



* Assuming maximum aerobic to anaprobie sensitivity for neutrons = 1 -4. 



a. Grav, L. H. and Scholes, M. E. Brit. J. Radiol. N.S. 24 (1951) 82 



b. Kihlman, B. A. Exp. Cell Res. 14 (1958) 639 



c. Thodav, J. M. and Read, J. Nature, Land. 160 (1947) 608 



d. Conger, A. D., Randolph, M. L., Sheppard, C. W. and Luippold, H. J. Radiation Res. 9 (1958) 525 



Table 2 also presents published data for growth inhibition of Vicia roots 

 and chromosome damage in Tradescantia microspores, from which it may be 

 seen that according to the level of radiation damage, the dose rate, and the 

 oxygen tension, the R.B.E. of a densely ionizing radiation relative to X 

 radiation for a single type of biological response may have any value between 

 4 • 5 and 33 in the case of roots exposed to a and X radiation, or between 

 4 • 2 and 1 9 in the case of Tradescantia microspores exposed to neutrons and 

 X radiation. 



On a purely empirical basis, the R.B.E. of all types of ionizing radiation 

 may be correlated with the rate of loss of energy (LET) along the tracks of 

 the individual ionizing particles. This ratio varies in the extreme by a factor 

 of a little over 1000 from 0-2 to 250 ekV/ji., as between the fastest and the 

 slowest particles commonly employed in radiobiological experiments. In 

 practice, biological materials cannot readily be exposed to a dose of radiation 



