XVI 

 BIOLOGICAL EFFECTS OF ALPHA PARTICLES 



R. E. ZlRKLEi 



Eldridge Reeves Johnson Foundation for Medical Physics, University of 



Pennsylvania, Philadelphia 



Advantageous properties of alpha particles. Suitable sources of alpha particles. 

 Effects upon macroscopic structures and systems. Effects upon single cells, parts of cells, 

 and small groups of cells. Summary. References. 



ADVANTAGEOUS PROPERTIES OF ALPHA PARTICLES2 



Alpha particles^ are an intensely ionizing type of radiation. Hence, 

 their most striking physical effect is the same as that of cathode, beta, 

 gamma, and X-rays. However, their penetration in matter is very 

 low. Those from polonium penetrate air about 3.9 cm. and water about 

 32 At (Michl, 14). Since their penetration is very nearly the same in 

 substances of equal density, 32 ijl is also a fair value for their penetration 

 in hving matter. This low penetration makes them useless for cancer 

 therapy, w^hich fact accounts for their relatively infrequent use in purely 

 biological experiments dealing with effects of ionizing radiations. 



However, alpha particles offer several advantages in such investi- 

 gations if small biological objects are used. Their paths in matter are 

 straight lines,^ and the lengths of the paths are nearly equal if only one 

 radioactive substance is used as the source. These properties, together 

 with the low penetration (short length of path), make it possible to attack 

 limited portions of single cells. The number of ions produced per linear 

 unit of an alpha-particle path varies in a characteristic fashion with the 

 distance from the end of the path (Fig. 1). This makes it possible to 

 place a suitably small biological test object at various distances from 

 the end of the path and thus study directly the effect of ion concentration 



1 National Research Fellow in the Biological Sciences. 



2 Unless otherwise indicated all statements of a physical nature are based upon the 

 book by Rutherford, Chadwiok, and Ellis (19). 



3 An alpha particle is a helium atom lacking two electrons and hence carrying 

 a double positive electronic charge. It is ejected from an atom of a radioactive 

 substance with an initial velocity of the order of one twentieth that of light. The 

 exact value of the initial velocity, and hence of the initial energy, is different for 

 particles from different radioactive substances but is the same for all particles emitted 

 by any one substance. The same is approximately true of the total penetration in 

 matter, for this quantity varies nearly as the cube of the initial velocity. 



* Significant deviations from straight lines, due to collisions with atomic nuclei, 

 are so rare as to be negligible in biological work. 



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