Sec. 21.4] THE CYCLOTRON 483 



21.4. Cyclotron Targets, a. Design Features of Targets. A variety of 

 target forms and techniques are employed, depending largely upon the 

 physical properties of the bombarded material, the purpose of the bombard- 

 ment, and whether the material is bombarded in the vacuum chamber of the 

 cyclotron or in an external chamber into which the beam is deflected. 



An internal target requires special consideration in its design and is limited 

 to the comparatively few substances that are stable at elevated temperatures 

 in a vacuum. Although a slightly higher bombardment efficiency can be 

 obtained than is usually possible for external targets, the limitations and 

 operational difficulties make internal targets inconvenient for routine bom- 

 bardments. Nevertheless, internal targets are frequently used in one of two 

 possible forms. In the first, the target material is mounted at the end of a 

 probe and inserted directly into the circulating beam at its maximum radius 

 [3]. In the second form, the target is mounted in a chamber located beyond 

 the deflector. Probe targets are usually less satisfactory for many purposes, 

 but they possess the advantage that considerably greater beam currents 

 can be obtained, and the deflected beam to the target chamber can sometimes 

 be used simultaneously although with some loss of intensity. 



Bombardment in a target chamber into which the beam is deflected is the 

 most widely practiced use of internal targets [4]. Such targets can be 

 changed rapidly by means of a vacuum lock and, in general, engineering and 

 operational factors are more favorable. 



External targets are also bombarded in a target chamber located beyond 

 the beam deflector but are separated from the vacuum tank by a thin alumi- 

 num window through which the beam emerges. Comparatively little 

 difficulty is experienced with these targets since occluded gases, high vapor 

 pressure, and decomposition in no way affect operation of the cyclotron. 

 Such targets can be operated when necessary in an atmosphere of helium 

 at a reduced pressure to prevent oxidation and to avoid excessive beam 

 absorption. In addition to simplicity of operation and design of external 

 targets, gases and liquids can be bombarded as well as most solid substances. 



Practically all substances must be mounted on a water-cooled copper 

 target plate to be bombarded, both for internal and external targets. The 

 details of the design of a target and the choice of target material are, in 

 general, governed by heat transfer, bonding properties, stability of the 

 substance, bombarding efficiency, and the subsequent chemistry. The 

 relative importance of the first three factors in the design difference of internal 

 and external targets is chiefly one of degree, whereas the last two factors apply 

 equally well to both types of targets. 



b. Heat Transfer. Provision for rapid transfer of heat must be made 

 between the target substance and the cooling system. Although as little 

 as 25 watts may be delivered by the beam, the equivalent heat is liberated 



