248 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1956 



the coexistence of the bulletlike properties and wavelike properties, 

 but he has not succeeded in presenting them in such a form as to give 

 contentment to the layman. Fortunately for our purpose, these mat- 

 ters need not cause us serious trouble. It will suffice for us to think 

 of these radiations, which in the aggregate we have called photons, as 

 particle-like in nature ; they differ in energy, however, not by the nor- 

 mal differences of velocity, but by some other characteristics custo- 

 marily associated with definite frequencies of vibration. In other 

 words, their velocities are all the same (in a vacuum), and their fre- 

 quency of vibration determines their energy content. 



FURTHER PROPERTIES OF THE FUNDAMENTAL PARTICLES 



Ionization. — Consider the behavior of a charged particle in creating 

 ions as it passes through a gas. It detaches electrons from the atoms 

 which it approaches sufficiently closely, and these electrons form the 

 basis for the creation of ions, as we discussed on page 245. If the 

 charged particle is moving very rapidly, with a speed comparable 

 with but not too nearly equal to the velocity of light, and if it carries 

 the equivalent of one electronic charge, it detaches from atoms about 

 30 electrons per centimeter of path at atmospheric pressure. 



Figure 2. — P is a high-energy charged particle pursuing the line of flight MN. It affects 

 atoms along its path and frees the electrons, E. 



By this act, the particle loses energy as it progresses through the 

 gas, and the more energy it has lost the more rapidly does it spend 

 that which remains. 



Particles moving more nearly with the speed of light, that is, faster 

 than those that begin by detaching about 30 electrons per centimeter 

 of path, are even more active. Thus, the charged particle spends its 

 energy freely when it is very rich (moving nearly with the speed of 

 light) and when it is very poor (near the end of its path). It is 

 most conservative in its expenditures when it is moderately rich. 



The ionization produced by a particle per centimeter of its path 

 depends to a first approximation only upon its velocity and its charge. 

 Doubling the charge increases the ionization per centimeter of path 

 by a factor of four. 



Pair production. — A phenomenon more drastic than ionization oc- 

 curs when very rapidly moving charged particles collide with atoms, 

 and associated with it is a phenomenon resulting from the collision 

 of high-energy photons with atoms. The charged particles concerned 



