204 BELL SYSTEM TECHNICAL JOURNAL 



At energy-values below 200 Mev Blackett finds almost no penetrating 

 particles, a singular contrast with the Pasadena observations; he 

 suspects that the penetrating particles become ordinary electrons when 

 they are slowed down into this energy-range. I mention also the meas- 

 urements made on some twenty penetrating corpuscles by Leprince- 

 Ringuet and Crussard, leading to the exceptional conclusion that 

 positives suffer smaller energy-losses than negatives.) 



But granting that there are two sorts of particle with a right to 

 different names: has either a right to the name "electron"? To 

 settle this question, and for several other reasons, it is time to call 

 upon theory. 



It is now some thirty years since there entered into physics a German 

 word, Bremsstrahlung, which can be translated literally into English 

 as "braking radiation," and would no doubt be so translated if 

 "braking" did not sound like another English word of entirely dif- 

 ferent meaning. This is chiefly observed emerging from X-ray tubes, 

 being emitted from their metallic targets when these are struck by 

 the stream of bombarding electrons. It consists of photons or 

 corpuscles of light, each containing at least a part of the kinetic energy 

 of one of the incident electrons. The distribution-in-energy of the 

 photons makes it clear that the electrons frequently lose large fractions 

 of their initial energy en bloc, throwing it off in individual parcels which 

 are these photons (indeed it sometimes happens that the entire kinetic 

 energy of an incident electron is shed in the form of a single corpuscle 

 of light). This radiation forms the so-called "continuous X-ray 

 spectrum" or "X-ray continuum" emerging from targets of X-ray 

 tubes. With the spectrum-lines which are sometimes seen superposed 

 on this continuum we have nothing here to do. *, 



By the classical theory of thirty years ago this continuous spectrum 

 is attributed to the slowing-down of the electrons as they penetrate 

 into the metal, whence the name Bremsstrahlung. By the quantal 

 theory of today it is still ascribed to the slowing-down, which must 

 now be conceived as taking place in instantaneous jerks, occurring 

 probably in the close vicinity of atom nuclei. At each of the jerks, 

 the electron-speed is suddenly reduced and the kinetic energy goes 

 forth in the form of light. The later theory in its quantitative form 

 gives a competent account of the continuous X-ray spectrum as it 

 springs from the tubes of the laboratory, with their bombarding 

 electron-streams energized by voltages of a few tens or hundreds of 

 thousands. For a long time nobody seemingly troubled to extend it 

 to voltages of the order of thousands of millions; a futile extension 

 indeed this would have been, so far as X-ray tubes are concerned. 



