282 1S0T0PIC TRACERS AND NUCLEAR RADIATIONS [Chap. 9 



ft] 1/ 

 & = 2.07 — ^ X 10 4 cm' 



where m = ion mass, mass units 



e = ion charge, units of electronic charge 

 V = accelerating potential, volts 

 H = magnetic field strength, gauss 

 With a radius of roughly 15 cm and ion source and collecting slit widths of 

 the order of 0.2 mm, resolution of adjacent mass peaks is possible up to 

 mass 400. 



9.4. Mattauch Double-focusing Spectrograph. A comprehensive study of 

 ion optics by Herzog [4] demonstrated that with appropriate combinations 



I 

 A 



Fig. 62. Mass spectrometer using 60-deg magnetic deflection (Nier type). 



of electric and magnetic fields it is possible to achieve both velocity and 

 directional focusing. Where the highest resolution and accuracy are 

 required this is essential since ion sources do not provide ion beams which are 

 strictly homogeneous either in energy or direction. Following these prin- 

 ciples Mattauch and Herzog [5,6] constructed a mass spectrograph that 

 provides directional and velocity focusing over the entire mass range. As 

 shown in Fig. 63, partially collimated ions from the source pass through an 

 electric field between condenser plates E which form an arc of 31° 51'. Again 

 collimated by slit S3, the ions enter the magnetic field where they are deflected 

 through an angle of 90 deg. Directional focusing occurs in the electric 

 field which forms a velocity spectrum at its focal plane, and velocity focusing 

 takes place in the magnetic field. 



The voltage applied across the electrodes of the electrostatic analyzer 

 depends upon both the geometry and the voltage through which the ions 

 are initially accelerated at the source. For ions that traverse the central 

 path C through the analyzer, the required voltage is given by the expression 



