320 RADIATION BIOLOGY 



Fast Neutrons. Neutron dosimetry has been carried out by means of 

 small air-filled ionization chambers with walls of low atomic number 

 (e.g., carbon, " Aerion," or Bakelite). The relation between the observed 

 ionization in the air of the chamber and the energy absorption in tissues 

 varies with many factors, including the wall material. With certain 

 carbon-walled chambers Gray and Read (1939) calculated that an 

 exposure producing 1 esu/cc of ionization in air would transfer to unit 

 volume of tissue the same energy as 7 r of 7 rays. With the Aerion 

 chamber the corresponding value is about 2 (1.8 according to Zimmer, 

 1938; 2.1 according to Gray and Read, 1939). In the United States the 

 instrument almost universally used has been the Victoreen condenser 

 r meter, the chamber of which has Bakelite walls. This instrument is 

 calibrated in r for X rays, and the custom has been to use its 100-r 

 chamber for measurements of fast neutrons in n units, 1 n being the 

 amount of neutron radiation which produces the same scale reading as 

 1 r of X rays.* The number of r of X rays which produces the same 

 amount of tissue energy absorption as 1 n of neutrons has not yet been 

 established with precision. Aebersold, who has investigated this matter 

 in detail, states that his measurements indicate a value of 2 r, but that 

 certain sources of error which are difficult to evaluate might increase this 

 to 2.5 r (Aebersold and Lawrence, 1942). The latter figure has been used 

 in this paper in order that the effectiveness of fast neutrons relative to 

 other radiations may be set conservatively low.^ 



Alpha Particles. Two types of a-particle source have been used: 

 polonium deposited on a metal disc ; radon and its short-lived a-emitting 

 decay products (RaA, RaC') introduced into the culture medium. 



Polonium sources have been used only in investigations of very small 

 biological objects, usually single-celled forms, because of the low penetra- 

 tion of the polonium a particles (about 39 jx in water). To determine the 

 energy absorption in the cells, one first measures the number of a particles 

 emitted per unit time per unit surface of the metal disc. This is done 

 either by an ionization method (Lea et al., 1936), by a scintillation method 

 (Zirkle, 1940), or by means of a proportional counter. Then, from the 

 geometrical relations between source and cells, one calculates the number 

 of particles traversing unit area in unit time at the position of the cells. 

 This number is then multiplied by the exposure time and by the 

 average energy absorbed per unit length of track in the cell, the latter 

 quantity determined from the known LET in air and the ratio of the 



* There is considerable variation in the response of different 100-r chambers to the 

 same beam of fast neutrons — as much as 20 per cent in our experience. This necessi- 

 tates the use of some one chamber as a standard. 



^ Fast neutron dosimetry should become much more satisfactory with the develop- 

 ment of thimble chambers in which both wall and gas have approximately the atomic 

 composition of tissue (Failla and Rossi, 1950; Rossi and Failla, 1950). 



