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ISOTOPIC TRACERS AND NUCLEAR RADIATIONS [Chap. 21 



on successive transits and continues to execute simple harmonic motion 

 about the equilibrium phase. 



Azimuthal asymmetries in the magnetic field lead to forced oscillations 

 with frequencies equal to or greater than that of the rotational frequency. 

 Small oscillations of this form are usually stable and sometimes must be 

 tolerated. 



21.3. Ion Source. The ion source in a cyclotron is accurately located at 

 the center of the magnetic field and directly above or below the gap between 

 the dees, corresponding to the position in the median plane from which the 



greatest number of ions can be started 

 on stable orbits. In its conventional 

 form the ion source consists of either 

 a helical or U-shaped tungsten fila- 

 ment mounted in a closed copper 

 structure that contains a capillary 

 outlet directed toward the median 

 plane and parallel to the magnetic 

 field, as shown in Fig. 132. The 

 entire structure is supported from the 

 tank wall by an appropriate mecha- 

 nism to allow exact adjustment of its 

 position with respect to the dees for 

 optimum beam output. The gas to 

 be ionized, hydrogen, deuterium, or 

 helium, is piped into the space around 

 the filament, and from there it leaks 

 out through the capillary in which it 

 is ionized by electron bombardment. 

 An ion cloud is then formed in the 

 central region between the dees from which ions are pulled, each half-cycle, 

 into alternate dees by the strong electric field. A tungsten-capped block 

 mounted on the side of the tank opposite the source frequently serves as an 

 anode, operating at several hundred volts positive with respect to the 

 filament structure. Filaments have been successfully operated on direct as 

 well as on low- and high-frequency current. 



Details of the design of ion sources vary somewhat depending upon the 

 particles to be accelerated and the particular cyclotron in which they are 

 used. Usually the same source can be employed for protons and deuterons. 

 Alpha-particle sources, however, are made more massive to withstand the 

 higher rate of deterioration from ion bombardment. Water cooling of the 

 filament leads is essential as is good thermal contact between parts of the 

 source to prevent local melting from ion bombardment. 



Fig. 132. Typical cyclotron ion source. 

 A, support tube enclosing filament struc- 

 ture; B, helical tungsten filament; C, 

 molybdenum or stainless-steel filament 

 shield; D, filament lead insulator; E, fila- 

 ment leads and clamp with squirt tube for 

 water circulation; G, vertical slots in cone 

 facing dees, ionization occurs in hole 

 through cone; H, tungsten cap on copper 

 cone to prevent local melting due to ion 

 bombardment from below. 



