XIV. 



X RAYS AND X IRRADIATION 465 



absorptions in the vital spot were assumed to produce death. These 

 data required a Poisson distribution that allowed organisms to sur- 

 vive a large amount of X rays before any deaths were noted. When 

 deaths started they increased rapidly and then later decreased at 

 about the same rate after they had passed the midpoint. The form 

 of the survival curve was explained by the necessity of multiple ab- 

 sorptions of radiation in the sensitive spot before any observed ef- 

 fects were noticed. Figure 7 shows this and other curves plotted on 

 the arith.-log. grid. 



Condon and Terrill (9) extended the theory by pointing out that 

 the size of the X-ray quanta increases with decreasing wavelengths. 

 The energy of the X-ray quanta incident to the tissue is inversely 

 proportional to their wavelength ; on an energy basis a given biologi- 

 cal effect might require several ciuanta of long wavelength X rays 

 in the sensitive spot, where but one quantum would be necessary 

 with short wavelength X rays. 



3. Current Ionization Concepts 



Further developments came with more detailed examination of 

 how X-ray energy is released within tissues and gases. The absorp- 

 tion of a quantum in tissue may generate a photoelectron that carries 

 nearly the full energy of the quantum. This photoelectron progresses 

 through the tissues, occasionally colliding with electrons of atoms in 

 its path, until all of its excess kinetic energy is dissipated in the forma- 

 tion of positive and negative ion pairs. The track or path of the 

 photoelectron is very crooked and irregular. The length of any elec- 

 tron's path is dependent upon its initial energy and on how much 

 energy is lost in each collision. The ion pair density along an elec- 

 tron's path becomes greater as the residual energy of the electron de- 

 creases. With X rays generated at potentials to 10 kv., their ab- 

 sorption by matter gives rise to photoelectrons almost exclusively. 



AVhen the initial energy of the quantum becomes larger, say 50 

 e.kv. (electron kilovolts), the energ}^ distribution takes a somewhat 

 different form. Then about 68% of the energy is distributed as 

 photoelectrons, the rest as recoil electrons (Compton effect, see 4)- 

 A recoil electron is formed when an X-ray quantum collides with an 

 electron in an atom, causing the electron to fly off at an angle from the 

 initial path of the quantum. Recoil electrons ionize in the same 

 manner as the photoelectons, but on the average have much less en- 



