474 



ISOTOPIC TRACERS AND NUCLEAR RADIATIONS [Chap. 20 



length of the tube, and internal drift tubes through which the ion beam 

 passes. 



The drift tubes are essential for their focusing effect on the beam in order to 

 compensate the defocusing caused by the initial angular divergence of ions 

 leaving the ion source, space charge spreading, and scattering by residual 

 gas in the accelerating tube. The focusing arises from the form of the 

 electrostatic field in the gap between two successive drift tubes. As shown 

 in Fig. 125, the bowed field in the first half of the gap tends to deflect all ions 



toward the tube axis. Although the 

 effect in the second half of the gap is 

 reversed because of symmetry of the 

 field, the ions have in the meantime 

 gained axial momentum and the angle 

 by which they are deflected from 

 their path is then smaller than the 

 initial focusing effect. The net effect, 

 therefore, is to maintain a small pencil 

 of ions that emerges with a small 

 cross-sectional area at the target. It 

 is apparent from the principles of the 

 process that the crucial focusing takes 

 place in the first few gaps because the 

 deflections are largest when particles are moving with comparatively low 

 velocity. 



The principal structural features of this type of accelerator are a high order 

 of insulation, shielding, and a good geometrical form for the terminals to 

 reduce heavy charge loss by excessive corona discharge and sparking. Most 

 such generators have been constructed with terminals of nearly spherical 

 form from aluminum sheet, spun or otherwise fabricated to the desired shape. 

 Insulation is provided for by mounting the terminal on sufficiently long 

 tubes of Herkolite, Textolite, or other nonconducting materials which can be 

 fabricated in large structural form. Power for operating the ion source and 

 auxiliary equipment in the terminal is derived from a generator mounted 

 within the terminal and driven by either the charging belt or an auxiliary 

 belt. When possible, humidity control is provided in the space occupied by 

 the charging belt since a relative humidity of less than 50 per cent is generally 

 necessary for consistent and optimum operation. 



An important improvement in the design and operation of electrostatic 

 generators was introduced by enclosing the entire unit in a pressure chamber 

 filled with air, nitrogen, carbon dioxide or freon (CCI2F2) at high pres- 

 sure. The chief advantage lies in the greater insulator strength of gases 

 at pressures above atmospheric, with a corresponding decrease in the break- 



Fig. 125. Electrostatic focusing field be- 

 tween drift tubes (equipotential lines are 

 shown). Trajectories of axial and off- 

 axis particles shown by heavy lines. 



