Sec. 21.4] 



THE CYCLOTRON 



485 



block with the powder and sealing it in with a cover of thin tantalum foil [6]. 

 It is then bombarded externally, usually in an atmosphere of helium at 

 reduced pressure. 



Both gaseous and liquid substances can be readily bombarded externally 

 in appropriately designed containers. One end of such a container is sealed 

 with an aluminum or tantalum foil through which the beam enters. As in 

 solid targets, proper consideration should be given to the dissipation of heat 

 produced in the target volume. 



Table 44. Fcsable Compounds Used for Cyclotron Targets [4,6] 



d. Target Efficiency. From the point of view of beam utilization, target 

 efficiency is judged by the proportion of the beam producing the desired 

 reaction. An efficiency of 100 per cent is assured, therefore, only in a target 

 consisting wholly of the isotope producing the reaction; the presence of iso- 

 topes of the same species reduces the efficiency by the amount of their total 

 abundance compared to the isotope producing the reaction. When the target 

 material is a compound or mixture, the efficiency is still further reduced and 

 in a more complicated manner since the stopping power of the constituent 

 atoms is different. Obviously, compounds containing only a small per- 

 centage of the reaction element should be avoided when efficient bombard- 

 ment is sought. The volume of target material is also a factor in the effi- 

 ciency when its depth is less than the useful range of the particle and its 

 surface does not completely cover the cross section of the beam. 



So far as bombardment efficiency is concerned, it would appear desirable 

 to make the target more than necessarily massive to ensure complete beam 

 absorption in the target material; however, other considerations make it 

 desirable to reduce the volume of material to the minimum required for 

 complete beam utilization. This is necessary to obtain the highest specific 

 activity and to reduce the volume of material handled in chemical isolation 

 of the active isotope. The surface area should correspond to the beam cross 

 section which is usually known and reasonably stable. The depth should 

 equal the maximum useful range of the particles in the particular target 



