300 



. THE QUALITY OF THE SECONDARY lONISATION 



DUE TO p Rays. 



By W. H. Bragg, M.A., F.R.S., Elder Professor of Mathe- 

 matics and Physics in the University of Adelaide, and 

 J. P. V. Madsen, B.Sc, B.E., Lecturer on Electrical 

 Engineering. 



[Read October 1, 1907.] 



The so called secondary radiation due to ;8 rays has beea 

 studied by many observers. Quite recently McClelland and 

 H. W. Schmidt have contributed im2>ortant sets of papers 

 upon the subject. 



It is usual to estimate the amount or the energy of the 

 primary and secondary streams by measuring the ionisation 

 produced within an ordinary ionisation-chamber placed in the 

 path of the rays, the principal reason being that such mea- 

 surements can be made with convenience and accuracy. 



But it is only under very special conditions that this 

 procedure is legitimate, because the )8 particle produces more 

 ions in traversing a given space when its velocity is small 

 than when it is great ; in other words, the ionisation 

 increases as the speed diminishes. If two streams of )8 radia- 

 tion are to be compared by this method, it is one of the 

 essential conditions of success that the velocities of the two 

 streams shall be the same, or similarly distributed. This 

 would not be necessary if the ionisation-chamber were made 

 so large that even the fastest ^ particles spent their ionis- 

 ing powers within it, and if the whole ionisation produced 

 by a i8 particle might be taken as a measure of its energy. 

 But it is usual to employ a small chamber, placed compara- 

 tively near to the source of the rays; and in this way an un- 

 fair advantage is given to the particles of smaller velocitv. 



The experiments to be described in this paper show that 

 the secondary radiations returned by different substances 

 struck by the same primary stream are not uniform in qual- 

 ity, but vary from substance to substance. When the ato- 

 mic weight of the substance is small, the radiation is rela- 

 tively soft; it contains a larger proportion of less penetrating 

 rays. If, therefore, the constants of secondary ionisation are 

 measured in the usual way, they are exaggerated in the case 

 of substances of low atomic weight, as compared with those 

 the atoms of which are heavier. 



For the purpose of observing the secondary ionisation due 

 to j8 rays we have adopted the form of apparatus shown 



