XVJI. RADIOACTIVE TRACERS 607 



per unit weight or millicuries per unit weight, will be quite low. One 

 way of getting around this is to use the so-called Szilard-C'halmers 

 process. In nuclear processes such as radioactive decay and the 

 emission of 7 rays after neutron capture, the atom itself recoils and 

 chemical bonds holding it in some molecule are broken. If these 

 bonds are covalent or of other type not permitting exchange with free 

 atoms, it may be possible to separate the atoms that have been acti- 

 vated and have recoiled from the bulk of inactive ones. The classic 

 example of this is the production of radioactive bromine by irradia- 

 tion of ethyl bromide with neutrons. The bromine atoms recoiling 

 after activation break the C-Br bonds, and the liberated atoms do not 

 re-enter the organic molecules (30). 



2. Other Important Radioisotopes 



Except for short-lived isotopes all those producible by the neutron 

 capture process are available from the Atomic Energy Commission 

 {19; 2, p. 56; 3, p. 43). In spite of the fact that the neutron inten- 

 sity in the neutron chain reactor or pile is much greater than that avail- 

 able from cyclotrons the specific activities of some radioisotopes from 

 the pile are not sufficient for some puposes. This is particularly true 

 if the parent isotope is present to only a small percentage. For ex- 

 ample the 44 day Fe^^ is produced by neutron capture in Fe^^ the 

 abundance of which is only 0.33% and the important 180 day Ca** 

 comes from the 2.1% abundant Ca^''. Unfortunately the Szilard- 

 Chalmers process does not work in a pile since the radiation intensity 

 is so great that the covalent compounds necessary for the process are 

 destroyed. The radioactive atoms are thus diluted by many others 

 from the decomposed molecules and the expected enrichment is not 

 obtained. 



Certain other isotopes that can be made by the n,p reaction are 

 also available {19). Among these are Q,^\ P3^ and S^^ In this case 

 the target and product are not isotopic and production methods have 

 been devised for some of these to yield high activities undiluted by 

 inactive isotopes, i.e., "carrier-free." If this were actually attained 

 the specific activity would be that of the pure isotope, but usually 

 there will be some dilution by inactive isotopes occurring as impuri- 

 ties. The products of the fission of uranium are, of course, highly 

 radioactive and some of them arc quite useful as tracers, though they 

 are not of greatest interest biologically. In this case also the produc- 



