284 



ISOTOPIC TRACERS AND NUCLEAR RADIATIONS 



[Chap. 9 



and the resolving power is given by the expression 



m 



a 



Am 2si 



where S\ = source slit width 



Am = smallest detectable mass difference at mass m 

 9.5. Dempster Double-focusing Spectrograph. A directional and velocity 

 focusing arrangement of electrostatic and magnetic fields was proposed by 

 Dempster [27] in which the ion beam is deflected through an angle of 90 deg 



Fig. 64. Dempster double-focusing mass spectrograph. 



in a cylindrical electric field and through 180 deg in the uniform magnetic 

 field. The geometrical arrangement of the fields and ion paths is shown in 

 Fig. 64. 



Ions of slightly different energies leaving the source 5 through the exit 

 slit at a distance I from the electrostatic analyzer are focused into a velocity 

 spectrum at a distance L after traversing the analyzer. The potential V e 

 across the analyzer plates is related to the energy or accelerating potential V 

 of ions that traverse the central path of radius a by the expression 



V e = 2F log^ 



volts 



Those ions, however, which leave the source at an angle a and a velocity 

 slightly different from v a , i.e., Vi = v (l + P)> move on circular trajectories 

 with radii of curvature given by 



