Appendix D 613 



an organism on the average. However, a few will always penetrate 

 much further into the tissues. On a cellular level, both positive and 

 negative electrons have the same types of effects as alpha particles, 

 deuterons, and protons. 



Certain particles with no rest mass are called photons; these particles 

 are also called electromagnetic radiation. The most energetic photons 

 are X rays and y rays. X-ray photons are made by bombarding a 

 metallic surface with electrons, whereas y rays are emitted by atomic 

 nuclei during transitions. Both X-ray and y-ray photons interact 

 comparatively weakly with matter. Their paths through an entire 

 human are quite straight. Both of these types of photons can impart 

 energy to electrons, causing them to be excited or knocked out of their 

 orbitals. 



Slightly lower energy photons are called ultraviolet. Those with a 

 wavelength of around 2,600 A are strongly absorbed by certain com- 

 pounds within the cell called nucleic acids. In this case, the entire photon 

 is absorbed and an electron is raised to a higher energy level. There is 

 a small, but observable, probability that the entire nucleic acid molecule 

 will then be ruptured. 



When a beam of ionizing radiation interacts with a biological cell, it 

 is possible that the ionizations produced throughout the cell are equally 

 effective or that only those in a certain critical volume are important. 

 This critical volume is called the target', the theory necessary to find its 

 size is referred to as target theory or single hit theory. The latter theory is 

 discussed in Chapter 16. 



