IONIZATION AND BIOLOGICAL EFFECTS 93 



INTERACTION OF RADIATION AND MATTER 



The transfer of energy from photons to matter takes place through the 

 intermediary of electrons which are set in motion, and then, as we have 

 seen, ionize the material. But there are two processes by which the first 

 step may be brought about (cf. Darrow, Paper I) : By the (a) photoelectric 

 effect, in which case all the energy of the photon is transformed at once 

 and the secondary electron leaves the atom with a kinetic energy less than 

 that of the photon, by the amount necessary just to separate the electron 

 from the atom. To bring about this drastic transformation of a high- 

 energy photon, the latter must "collide" with an electron which figura- 

 tively is capable of "withstanding the full shock before running away." 

 That is, this reaction takes place with tightly bound electrons in the 

 atom, such as those of the K region. If the photon has an amount of 

 energy considerably higher than that required to remove the electron 

 from the atom (varying with the atomic number of the element), the 

 emergent electron travels at a high speed and ionizes a large number of 

 atoms. The energy of the photon which is utilized in just removing the 

 electron from the atom is not lost but reappears as another photon as 

 soon as a new electron takes the place of the one previously dislodged 

 from the atom (cf . Darrow, page 24) . Since this photon does not ionize 

 until it also sets an electron in motion, we need not consider it at this time. 



The elements of which living matter is largely made up have low 

 atomic numbers, and even the electrons in their K regions are not very 

 rigidly bound. Accordingly, when the energy of the photon is very high 

 (as in the case of high-voltage X-rays), the conditions for the immediate 

 and complete transformation of a photon are not favorable and few 

 photoelectric encounters of this type occur. Most of the energy which 

 is spent in ionizing living matter irradiated by high-voltage X-rays is 

 abstracted from the radiation through a different process, that is, (6) the 

 Compton effect (see Darrow, page 39). In this case the transfer of 

 energy from the photon to the electron takes place according to the laws 

 of elastic impact and depends on the angle which the path of the emergent 

 electron makes with the path of the impinging photon. It should be 

 noted that a photon can transmit practically all its energy to an electron 

 in the event of a head-on collision which sends the electron hurtling 

 through space substantially in the direction which the photon would 

 have followed had it not been stopped. If the electron is projected in 

 any other direction, the energy imparted to it is always less than this 

 amount; its speed is lower and the number of ions which it can produce is 

 smaller. No electrons can be emitted backward {i.e., toward the source 

 of radiation) by this process. At most, an electron can be projected at 

 right angles to the path of the photon, but in this case it will abstract 



