XV. ELECTRONS, NEUTRONS, AND ALPHA PARTICLES 501 



gations. The overriding consideration in the design of any experi- 

 ment that aims at discovering the processes involved in the action of 

 ionizing radiations must be that the biological structures under in- 

 vestigation should be exposed to a known dose of radiation, expres- 

 sible in terms of ergs of energy absorbed per gram of irradiated tissue. 

 Problems associated with the measurement of dose and the choice of 

 irradiation techniques that enable this condition to be fulfilled will 

 be discussed in subsequent sections. In general there will remain 

 some choice as to the source of radiation to be used. The kind of in- 

 formation that can be derived from an experiment will depend greatly 

 on the exercise of this choice, which governs the manner in which a 

 given total number of ions formed by the radiation are disposed in 

 relation to biological structures of various shapes and sizes. In this 

 section we shall discuss the way in which one type of radiation differs 

 from another in regard to the distribution of the ionization it produces 

 within an irradiated cell. 



2. Classification of Ionizing Radiations According to Mean 

 Spacing of Ions along Tracks of Ionizing Particles 



Referring again to Table I it will be seen that while there are 

 seventeen types of radiation listed these give rise to only four classes 

 of ionizing particle, namely, electrons, protons, a particles, and 

 nuclear particles. The ionization track of positively charged elec- 

 trons, or positrons, is indistinguishable from that of negative elec- 

 trons. Remembering that particles and protons are the positively 

 charged nuclei of He and H atoms, respectively, it is evident that 

 we are really concerned with only two principal classes of particle, 

 the light, negatively charged electrons, and the massive, positively 

 charged nuclei. Very detailed information about the distribution 

 of the ions formed by these radiations in air has been obtained by 

 means of the Wilson cloud chamber. By means of this instrument 

 a supersaturated condition is produced in moist air just prior to 

 the passage of the ionizing particle through the chamber that re- 

 sults in the condensation of water vapor in the form of a single 

 droplet around each ion. The droplets may be photographed to ob- 

 tain a permanent record of the position occupied by the ions at the 

 instant of formation. When scaled down approximately in the ratio 

 of the densities of tissue and air, such pictures enable us to visualize 

 the distribution of ions formed by different types of particle within 

 living cells. A few examples will be given (Figs. 2-8) but it will be 



