PHOTONS AND ELECTRONS 25 



I have already alluded in this essay, that of the electron literally falling 

 from one place to another in the atom is probably too definite, but there 

 is no reasonable objection to making use of its convenience while remem- 

 bering that a contemporary theorist might prefer a different image or 

 none at all. 



The line which I used as an illustration is called the Ka2 line. It 

 belongs to the K series, which is composed of all the lines due to electrons 

 which fall from various outer shells into a vacancy in the K series. The 

 corresponding transitions thus have in common their initial state, the 

 state of an atom with a vacancy in the K shell; they have different final 

 states, one being the state of an atom with a vacancy in the Ln shell, 

 another the state of an atom with a vacancy in the Lm shell, and so forth. 

 The lines of the K series of a heavy element lie relatively close together 

 and well apart from all the other lines, having considerably greater 

 frequencies (five or ten times as great) than the rest. There is also an 

 Lj series, composed of lines due to electrons which fall from various outer 

 shells into a vacancy in the L^ shell; an Lu series, and series for various 

 other shells. All of these series lie closer together (on the frequency 

 scale) than the K series to any of them, and some overlap, making a 

 very intricate spectrum. The notation for X-ray emission lines is 

 complicated and obscure. 



PRODUCTION OF X-RAYS 



X-rays are produced chiefly by projecting streams of electrons against 

 blocks or "targets" of metal, occasionally by projecting X-rays from 

 another source against the metal from which it is desired to evoke the 

 characteristic rays. The theory outlined in foregoing sections enables us 

 to understand some striking peculiarities of these modes of production. 



Suppose first a stream of electrons, each having kinetic energy U, 

 impinging upon a block of an element having K absorption frequency vk- 

 According to the theory, no line of the K series can be emitted unless 

 there is a vacancy in the K shell, into which an electron may fajl from 

 some outer shell. The lines of the K series cannot therefore be produced, 

 unless K electrons are constantly being knocked out of the atoms. To 

 achieve this the impinging electrons must have at least that energy hvx 

 which we have just identified as the energy needful to extract a K electron 

 from an atom. We must therefore expect that as U is increased, none 

 of the lines of the K series will appear before, and all of them will appear 

 when U attains the value hvx. 



This is verified by experiment. It is a remarkable experience to 

 compare a spectrum obtained with U just below hvK with a spectrum 

 obtained when U is only a little above hvK] the whole K series, entirely 

 absent from the former, is conspicuous in the latter. The corresponding 



