288 ISOTOPIC TRACERS AND NUCLEAR RADIATIONS [Chap. 9 



radius p is given by 



P = 



where y = 



eH 



mc 



l/ 2 &c* 2Ec Y 



Y\ + W ~~H Vo C ° S 6 ) 



v = initial velocity of ion 

 The resolving power for this type of instrument is given by 



m b 

 Am Ab 



Ions of different m/e can be focused on a single collecting electrode by 

 altering the accelerating potential V and the deflecting potential V (= Ed 



CURTATE CYCLOID ION PATH PROTATE CYCLOID ION PATH 



Fig. 66. Trochoidal ion trajectories in crossed electric and magnetic fields. [W. Bleakney 

 and J. A. Hippie, Phys. Rev., 63, 521 (1938).] 



where d is separation of the condenser plates) in direct proportion, i.e., 

 CV = V where the constant C depends upon the design parameters of the 

 instrument. When this condition is fulfilled, ions of any m/e traverse the 

 same path for the proper values of V and V . 



If the design parameters are chosen to make p < a, the ion trajectory 

 is a curtate cycloid. If p > a, the ion trajectory is that of a protate cycloid 

 (see Fig. 66). 



9.8. Ion Sources. The ion sources used in mass spectrographs are either 

 high-voltage discharges or bombardment by low-energy electrons emitted 

 from a hot filament. Both have been widely used, and the choice depends 

 to some extent on the application intended for the mass spectrograph. 

 There is some advantage in using the spark-discharge source for solid mate- 

 rials since the electrodes can be coated with or made of the sample material 

 and little difficulty is encountered in producing ions. Furthermore, the 

 construction and operation of this kind of source is usually simpler than hot- 

 filament sources. On the other hand, filament sources regulated with 

 appropriate circuits give exceedingly stable and reproducible operation over 



