PRINCIPLES OF RADIOLOGICAL PHYSICS 6 



practical rather than intrinsic significance (see Sect. l-3c). Following 

 are some notes on other radiations of lesser biological importance: 



Cosmic rays consist of a complex of radiations which flow constantly through 

 the earth's atmosphere in a general downward direction and achieve some pene- 

 tration in the earth's crust. Several components of this complex are identical 

 with corpuscular and electromagnetic radiations studied in the laboratory. 

 Other components include particles of energies still not attained in the laboratory. 

 Cosmic rays seem to have little biological significance because their over-all 

 intensity is quite low. 



Table 1-1. Classification of Biologically Important Radiations 



Mesons include a variety of subatomic particles heavier than electrons but 

 lighter than i)rotons or neutrons. They arise from the collision of very-high- 

 energy radiations with atomic nuclei but have only a brief existence and pre- 

 sumably no biological significance. 



The neutrino is the constituent of a radiation which is presumed to arise from 

 the emission of j8 rays and from other subatomic processes (see Sect. 1-lb). A 

 certain amount of energy and momentum remains unaccounted for in these 

 processes. The circumstances indicate that the balance of energy and momen- 

 tum is removed by a still undetected but otherwise well-defined radiation whose 

 carrier element is electrically neutral and is called a "neutrino." The circum- 

 stances also indicate that this radiation ought to exert an exceedingly small action 

 on matter and thus explain why this action has not yet been detected. 



Molecular rays are beams of mono- or polyatomic molecules which escape from 

 an enclosure into a high vacuum, carrying little energy and no electric charge. 



