14 RADIATION BIOLOGY 



1 curie is regarded as intense, but sources of much greater intensity have 

 been prepared occasionally. 



For a collection of data on radioactivity see Way et al. (1950) ; for data 

 on availability of radioactive materials, see the Isotopes Catalogue (1949). 



l-2d. Neutron Sources. Neutrons become available as by-products of 

 nuclear collisions. They remain available for a short time only since they 

 have a considerable chance of being captured by any nucleus with which 

 they collide. These conditions make it difficult to provide large numbers 

 of neutrons. 



Collisions between ordinary nuclei cannot be produced very easily 

 because nuclei repel one another electrically. Therefore nuclei collide 

 only if they are shot against each other with high energy. Most kinds 

 of nuclear collisions are likely to result in the ejection of one or more 

 neutrons (see Sect. 2-ld). The bombardment of atoms by a beam of fast 

 deuterons is a particularly efficient source of neutrons since the neutron 

 contained in the deuteron is loosely bound and easily splits off. Neu- 

 trons of energy beyond 100 Mev have been produced by this method. 



Collisions between neutrons and other nuclei are not hindered by 

 electrostatic repulsion and can therefore occur in large numbers if only 

 the neutrons are available in the first place. The "fission reaction" of 

 the heaviest nuclei, i.e., their splitting into nearly equal parts, results 

 frequently from the impact of a neutron and liberates on the average 

 about 2.5 neutrons as a by-product. Therefore the fission reaction can 

 take place on a large scale under appropriate conditions, leading to the 

 incidental release of large amounts of neutrons. The principle is that, on 

 the average, one of the neutrons produced by a fission must terminate in 

 production of another fission. If this is so, fissions will follow one another 

 in a self-sustaining chain reaction. The difficulty lies in preventing an 

 excessive waste of neutrons in parasitic processes. 



A device in which a chain reaction of fissions proceeds and is kept at a 

 desired level of operation by controlled absorption of any excess neutron 

 production is usually called a "nuclear reactor" or "pile" (see Halliday, 

 1950, p. 422). The neutrons produced by fission have various energies of 

 the order of magnitude of 1 Mev. A very intense beam of neutrons can 

 be obtained by simply opening a port through the shielding that normally 

 surrounds a pile. 



Neutrons flow in all directions within a pile. A neutron flux of 10^^ 

 neutrons/cm^ sec has been reported. 



1-3. ELECTROMAGNETIC RADIATION 



Light,^ X rays, and radio waves have a common nature. Therefore we 

 may well regard electromagnetic radiation as a single phenomenon. This 



^ The term "light" has been used in this chapter for convenience as inckiding 

 electromagnetic radiations with infrared, visible, and ultraviolet wave lengths. 



