COG ADOLF F. VOIGT 



C. AVAILABILITY OF ISOTOPES 



Radioisotopes can be produced by a number of different nuclear 

 reactions in which one of a few bombarding particles produces a 

 change in the nucleus of the atom, yielding either a new isotope of the 

 same element or of a different element. The former process is termed 

 activation and the latter is true transmutation of elements. In- 

 cluded in the projectiles are the neutron, proton, deuteron, and a par- 

 ticle and in the nuclear reaction one of these particles or a 7 ray will 

 be emitted. A projectile that is positively charged must have high 

 energy to penetrate the positive nucleus of the atom and produce a 

 reaction, but the uncharged neutron gives better yields if its energy is 

 small {1, p. 16; 2, p. 43; etc.). 



1. Radioisotopes Producible by Slow Neutron Activation 



The neutron is undoubtedly the most effective of the various bom- 

 barding particles and the most prolific reaction is that of neutron 

 capture, in which energy in the form of 7 radiation is all that is emit- 

 ted. Since the existence of the neutron is highly transitory it must be 

 produced near the material to be activated. In most instances iso- 

 topes producible by neutron activation will be available from the 

 Atomic Energy Commission {19) although it is possible that some iso- 

 tope of short half-life may need to be made on the spot. If this is 

 necessary someone with proper experience will be available or should 

 be consulted before such production is attempted. 



Although some of the early work employing artificial radioactivity 

 was done with tracers produced by neutrons from a radium-beryllium 

 or similar source, the amounts of activities so producible are quite 

 inadequate for biological appUcation. A vastly higher neutron inten- 

 sity can be obtained from a cyclotron by using it to accelerate deuter- 

 ons and bombarding beryllium with these. Slowing the neutrons 

 down from theix original high energies to the energy of atoms and 

 molecules at room temperature increases their effectiveness in induc- 

 ing the capture (n,7) (neutron in, 7 ray out) process. Such "ther- 

 mal" neutrons are produced by allowing the original high energy neu- 

 trons to traverse a few inches of paraffin "moderator" in order to 

 reduce their kinetic energy by collision. This neutron activation 

 reaction produces nuclides isotopic with the original element and the 

 active atoms are thus diluted by a large number of inactive ones. In 

 other words the "specific activity," in disintegrations per unit time 



