122 BELL SYSTEM TECHNICAL JOURNAL 



alpha-particles.^^ The lines in record {c) were obtained when neutrons 

 were traversing the chamber and a piece of paraffin outside of it; not 

 they, but the charged nuclei which they strike and impel, are producing 

 the record. Those lines which are longer than any in {h) are certainly 

 not due to protons; they must be caused by recoiling nuclei of the 

 atoms of the gas which fills the chamber (air), and which have various 

 speeds because the neutrons strike them more or less glancingly (and 

 probably do not themselves all have the same speed). The shorter 

 lines are due in part to such nuclei, chiefly to protons ejected from the 

 paraffin in such directions that they cross the chamber. Some are 

 very short indeed, half-lost in the dusky haze due to the perpetual 

 wiggling of the oscillograph mirror caused by gamma-rays; they are 

 made by the fastest of the protons. Observations by expansion- 

 chambers and with applied magnetic fields have proved this classifica- 

 tion of the particles. 



All of these methods are available for detecting charged particles 

 which are protons or alpha-particles or corpuscles of a yet greater 

 mass than these. For electrons the problem is harder. 



An electron of given energy — say x thousands of electron-volts — is 

 able to make roughly as many ion-pairs in a gas as could a proton or an 

 alpha-particle of equal energy: that is to say, about 30.Y. Nevertheless 

 it produces much less ionization in an ordinary chamber than either 

 of these" last. This seeming paradox is due to the facts that the ion- 

 pairs produced by the electron are relatively far apart and the loss 

 of energy per centimeter of path is correspondingly low, so that in an 

 ionization-chamber of reasonable dimensions and customary density 

 of gas the traversing electron produces only a few hundreds or perhaps 

 one or two thousands of ion-pairs before it reaches the opposite side 

 of the chamber and plunges into the wall. 



This is made evident by the Wilson method, the tracks of electrons 

 appearing much thinner — less richly peopled with droplets, that is to 

 say — than those of alpha-particles or protons. The expansion- 

 chamber therefore is available for observing fast electrons, and so to a 

 certain extent is the Geiger counter, which skilful observers can adjust 

 so that it will react to these bodies. None of the other methods has 

 yet been used with success. The ions produced by a single electron 

 in an ionization-chamber are apparently too few to observe w^ithout 

 amplification or even to amplify successfully, and the scintillations 

 too faint. If one has neither expansion-chamber nor Geiger counter 

 available, the only thing to be done is to measure the total ionization 



''' The contrast is much more striking than the records suggest, for. the amplifica- 

 tion was fourfold greater when (b) and (c) were made than when (a) was made. 



