484 ISOTOPIC TRACERS AND NUCLEAR RADIATIONS [Chap. 21 



usually in the very small volume of the target material corresponding to the 

 beam cross section and depth of penetration. The bonding to the water- 

 cooled copper target plate should ensure good thermal contact, and the 

 thickness of the material should be limited to prevent high surface tempera- 

 tures and local overheating due to steep thermal gradients. This is particu- 

 larly important for internal targets when the material has an appreciable 

 vapor pressure or a tendency to decompose at elevated temperatures. 



c. Target Materials and Bonding. The most satisfactory processes for 

 bonding materials to the copper target plate are electroplating, soldering, 

 and brazing. They are applicable, however, only to those few metallic 

 elements and alloys, including beryllium which can be silver soldered, that 

 are stable under bombardment and at elevated temperature, particularly 

 when used with internal targets. Materials that cannot be bonded by these 

 processes are usually prepared in the form of suitable compounds or mixtures. 

 Many of these can be fused and will adhere to a copper target plate on which 

 V-shaped grooves have been milled in a direction parallel to the plane of the 

 beam and to a depth corresponding roughly to the range of the particles in 

 the target material. A thin layer of the material is spread over the grooves 

 and the plate heated in a furnace or from the bottom with a torch until the 

 material fuses into the grooves. Better heat transfer to the plate is obtained 

 if the grooves are not quite filled. 



The choice of compound or mixture used for this type of target is deter- 

 mined mainly by its binding strength and stability. For internal targets 

 this is particularly important; it should not decompose, melt, or have a vapor 

 pressure high enough at elevated temperatures to interfere with operation 

 of the cyclotron or to cause loss of material. These factors are less stringent 

 in external targets, but it is still desirable to use materials with high resistance 

 to thermal shock and very low vapor pressure in order to reduce loss of active 

 material. 



The suitability of a substance for internal targets is difficult to predict 

 beforehand, but several generalizations of the properties of compounds best 

 suited for target materials have been proposed by Reid [4] and Weil [5]: 



1. Ionic compounds of two elements probably will not adhere well. 



2. Ionic compounds in which an ion is diatomic adhere better. If the 

 anion is the metaform, bonding may be good. 



3. Compounds that polymerize may be satisfactory. 



4. Compounds that form a glassy, noncrystalline solid generally form a 

 good bond. 



A list of compounds used for internal targets is given in Table 44. 



Many materials that are stable under target conditions but cannot be 

 fused easily to the target can be bombarded in the form of powder. Targets 

 in this form are prepared by filling a shallow recess in a water-cooled copper 



