PHOTONS AND ELECTRONS 23 



that definite change in the structure of the atom which is entailed by the extrac- 

 tion of one of its constituent parts. ^^ 



In the absorption spectrum of a massive element, such as gold, lead, 

 or uranium, there are several of these continua and absorption edges, and 

 therefore several different values of hpo. Each corresponds to the extrac- 

 tion of an electron belonging to a certain definite class, which we may 

 picture as being normally located in a certain orbit or at a certain distance 

 from the nucleus of the atom. The absorption edge of highest frequency 

 corresponds to the class of electrons of which it takes the greatest amount 

 of energy to dislodge a member. Electrons of this class are therefore 

 conceived as occupying the innermost orbit or the nearest of all available 

 locations to the nucleus itself. They are called K electrons; the corre- 

 sponding absorption edge and its frequency are known as the K absorption 

 edge and the K absorption frequency, and the latter is denoted by vr. 

 The succeeding absorption edges in the sense of decreasing frequency 

 are marked by the symbols Li, Ln, Lm; Mi, Mu, Mm, Miv, Mv; Ni, 

 and others which the reader will seldom encounter. To each there corre- 

 sponds a class of electrons in the atom, and (in the model) an orbit or a 

 location for the electrons of this class; and the symbols are employed 

 for these classes and for the frequencies of their absorption edges. A 

 list of the absorption frequencies of an atom is thus the first step toward 

 a map of the atom itself. 



As we pass along the table of the elements from the heaviest (uraniiuii) 

 down toward the lightest (hydrogen), each absorption edge is shifted step 

 by step toward lower frequencies, and each in turn fades out. The 

 shifting is due to the decrease in nuclear charge; the positive charge at 

 the nucleus of the silver atom, for instance, is only a httle more than one- 

 half that on the nucleus of the uranium atom, and it attracts its K elec- 

 trons correspondingly less strongly, so that much less energy (actually 

 about a quarter as much) is required to tear one of them away. The 

 vanishing of the edge is due to the fact that each atom has one electron 

 fewer than the one next beyond it in the table of the elements, so that one 

 class after another is emptied as we proceed down the list. 



The facts that absorption of X-rays entails extraction of electrons and 

 that these electrons are often endowed with a large kinetic energy, are 

 of the first importance in respect to all the actions and powers of X-rays. 

 Practically all of the electrical effects produced by a beam of X-rays 

 traversing a sheet of matter are due primarily to these electrons — often 

 called "secondary electrons" — which themselves plough through the 

 matter ionizing the atoms very abundantly until they use up all their 



" The reader will see that the quantity [eW, - eWa) should be included in hvo, 

 but it is usually so very small a fraction of hvo that it is commonly ignored. 



