480 



ISOTOP1C TRACERS AND NUCLEAR RADIATIONS [Chap. 21 



a high order of stability is apparent from the fact that in large cyclotrons the 

 ions will have traveled a total path length of hundreds of meters before 

 arriving at the target where the beam should have a cross-sectional area no 

 greater than a few square centimeters. Consequently small deviations from 

 the normal orbit and from the correct phase with respect to the dee voltage 

 must either be corrected while the ions are in transit or must lead to stable 

 oscillations about the normal orbit. The reasons for the high degree of 

 stability in cyclotron orbits is to be found in the ion focusing properties of the 

 electric and magnetic fields and the varying amounts of energy that ions 



Ions enter dee no. 2 



^- Dee no. 2 voltage 



\ 



Fig. 129. Starting phase of particles in the cyclotron with respect to the radio-frequency 

 dee voltage. 



with different phase angles derive from the electric field in traversing the 

 gaps. 



Electrostatic focusing alone is important during the first part of the 

 acceleration, before the ions have reached large radii and high energies. 

 When an ion, that is not in or parallel to the median plane traverses the 

 first half of the gap between dees, the electric field, because of its lenslike 

 contour, focuses the ion toward the median plane, as shown in Fig. 130. 

 In the second half of the gap, the symmetry in the field causes an opposite 

 or defocusing effect. The ion, however, has increased its momentum, and 

 the angle of deflection is then less. The deflection toward the median plane 

 is always greater, and the net effect, therefore, is focusing. If the ion trav- 

 erses the gap shortly after the peak dee voltage has been reached, the focusing 

 effect is further augmented since the field weakens during transit and the 

 particle is focused both by its momentum increase and also by the asym- 

 metrical field it now experiences. The reverse process follows when transit 

 occurs during the part of the cycle when the voltage is increasing rapidly; 

 defocusing is enhanced and may exceed all focusing effects. The ion is then 

 lost by collision with the dee or other structural parts after a sufficient 

 number of periods. 



Magnetic focusing fortunately becomes predominant at large radii and 

 high energies where electrostatic focusing becomes ineffective. This effect 



