280 ISOTOPIC TRACERS AND NUCLEAR RADIATIONS [Chap. 9 



Separation of ions with different values of m/e, as well as focusing of iden- 

 tical values, takes place only in the magnetic field. The electrostatic field 

 serves only as a velocity analyzer to focus the inhomogeneous ion beam into an 

 energy spectrum from which only a small portion is selected by the defining 

 vane and allowed to enter the magnetic field. 



For small deflections, the focal points for different values of m/e lie along 

 the line 



b = a[ — ^~% ) cm 



\<P - 26/ 



which determines the proper location of the photographic plate used for 

 detection. The mass, or more correctly m/e, is determined by the location 

 of the corresponding ion-beam image on the plate with respect to one or 

 more accurately known masses. The relative abundance of isotopes is 

 determined from the comparative intensities of the isotope images. 



This type of mass spectrograph is referred to as velocity focusing since 

 ions with identical m/e but slightly inhomogeneous energies are focused at a 

 point. Ions entering the electric field at slightly different angles, however, 

 cannot be focused, and the ion-beam currents are therefore exceedingly small 

 as a result of the beam attenuation necessary for a high order of collimation. 



9.2. Dempster Mass Spectrograph. With the original Dempster instru- 

 ment [3] the separation of ions with different values of m/e is accomplished 

 by deflection of the ions through an angle of 180 deg in a uniform magnetic 

 field. The ions formed in an ionization chamber / (Fig. 61) by electron 

 bombardment are accelerated and focused by voltages applied to the slits 

 Si, S2, Ss, imparting a final velocity given by 



v = I 2 V— I cm/sec 



where V = voltage difference between slits Si and S 3 

 e = ion charge 

 m = ion mass 

 In the uniform magnetic field the ions follow circular trajectories normal 

 to the field and with uniform linear and angular velocity at a radius of 

 curvature determined by 



(Kj'^TX™) 



R = ±l2V™Y =±-A2.07™X10<? 



where H = magnetic field strength, gauss 



This form of spectrograph possesses some advantage in utilizing slightly 

 divergent ion beams; hence, somewhat greater beam currents are available 

 than in velocity-focusing instruments. It possesses the disadvantage, 



