612 Appendix D 



proton. These are both hydrogen nuclei having a positive electronic 

 charge of 1 and the atomic weights of 2 and 1 , respectively. Neither 

 protons nor deuterons are emitted by any naturally occurring atomic 

 nuclei. However, beams of both can be formed in high energy accelera- 

 tors and used to bombard biological cells or layers of proteins. The cell 

 layers must be very thin for, just as alpha particles, both protons and 

 deuterons react very strongly with electrons. The ionization along their 

 path through the cell is very intense. 



Alpha particles, protons, and deuterons are all much heavier than 

 electrons. Accordingly, these heavy, positively charged particles are 

 only slightly deflected when reacting with electrons. The atomic nuclei 

 deflect all these three types of radiation if the particles come close to 

 them, but this happens infrequently. The paths of all three types are 

 fairly straight; given equal starting energies, the pathlengths of each 

 type are distributed quite close to the average value. This is illustrated 

 in Figure 1 of Chapter 10. 



Neutrons have about the same mass as protons but have zero charge. 

 They do not react appreciably with electrons; their primary effect is in 

 interactions with atomic nuclei. In material containing hydrogen, 

 that is, water, proteins,^nucleic acids, and protoplasm in general, one 

 major result of neutron irradiation is the knocking out of hydrogen nuclei 

 (protons) from the molecules by elastic collisions. As a result of these 

 collisions, the path of the neutrons may be very twisted and intertwined. 

 Neutrons can also react with various atomic nuclei. The destructive 

 action of neutrons in living matter is much greater than indicated by 

 their ionizing powers. 



Neutrons with kinetic energies comparable to the molecules about 

 them are called thermal neutrons. These have sufficient energy to enter 

 only a few of the nuclei of the elements found in living tissues. More- 

 over, neutrons are radioactive, decaying to proton and beta particles 

 with a half-life of about 13 minutes. That is, each 13 minutes the 

 number of neutrons is decreased by a factor of two. Neutrons which are 

 not thermal are called high energy neutrons. 



Electrons have a mass about 1/2000 that of a proton. Rapidly 

 moving electrons are called beta rays. The word electron is usually 

 used for negative ones; anti-electrons with an equal mass and a charge 

 of equal magnitude but opposite sign are called positrons, or positive beta 

 particles or positive electrons. Both positive and negative electrons passing 

 through matter interact both with electrons and with atomic nuclei. 

 In addition, a positron can combine with an electron, resulting in their 

 mutual annihilation and the formation of photons (y rays) . The paths 

 of electrons in tissues are very twisted. Both those from radioactive 

 atoms and those from accelerators do not penetrate very deeply into 



