24 BIOLOGICAL EFFECTS OF RADIATION 



energy and are brought to a stop; and the biological effects must be due 

 either to these electrical effects or to something as yet unknown. 



It must be mentioned at this point that the so-called "absorption 

 coefficient" of a given substance for a given beam of X-rays is frequently 

 so defined as to take account not only of absorption in the foregoing 

 sense, but also of scattering— a phenomenon to be discussed in a later 

 section. 



X-RAY EMISSION SPECTRA 



Emission of light is the converse phenomenon to absorption. While 

 I began the discussion of absorption by speaking of the optical spectrum, 

 there are reasons for reversing the procedure in dealing with emission, and 

 beginning with that of X-rays. 



X-ray emission spectra are Hne spectra ; and each line (with the excep- 

 tion of some extremely faint ones) has a frequency which is the difference 

 between the frequencies of two absorption edges. 



This principle has an important meaning. For definiteness let us 

 take the line, in the spectrum of some heavy element, of which the 

 frequency is the difference between that of the K absorption edge and that 

 of the Lii absorption edge. The photons of the line have each an energy 

 which is the difference between the energy required to extract a K electron 

 and the energy required to extract an L^ electron. We now imitate the 

 usage introduced in dealing with absorption series in optical spectra. 

 The energy required to extract a K electron is the difference between the 

 energy of the atom in its normal state with all of its electrons present, 

 and the energy of the atom in a particular abnormal state with one 

 K electron missing — one vacancy in the K class, let me say. The energy 

 required to extract an L^ electron is the difference between the energy 

 of the atom in its normal state aforesaid and the energy of the atom in 

 another particular abnormal state with a vacancy in the L„ class. Sub- 

 tract the latter from the former: the result is the difference between the 

 energy of the atom with a vacancy in the K class, and the energy of the 

 atom with a vacancy in the L„ class. This becomes much more concrete 

 if I introduce a metaphor. A photon of the line in question will he emitted, 

 when an electron falls from its place in the L,, class to a vacancy existing 

 in the K class, and the atom releases the energy thus made free. 



Each of the principal lines of an X-ray emission spectrum is correlated 

 in this manner with a transition, in which an electron passes from one class 

 —or, to use a commoner word, from one "shell"— to another, leaving a 

 vacancy in the shell whence it comes and filling a vacancy in the shell 

 to which it goes. We may also say that the atom makes a transition 

 between a state distinguished by a vacancy in one shell, and a state 

 distinguished by a vacancy in another shell. Like other pictures to which 



