148 RADIATION BIOLOGY 



are either too cumbersome or of questionable usefulness. To cite an 

 example, a chemical method that could distinguish the energy absorbed 

 from the components of a mixed beam of neutrons and gamma rays would 

 prove exceedingly useful both in practice and in research. 



Analogous comments could be made on the subject of radiation 

 dosimetry by means of scintillators (crystal and solutions), which have 

 recently commanded the attention of many experimental physicists 

 because of their sensitivity and their potentiality as rather crude but 

 very convenient spectrometers. The use of organic scintillators, e.g., 

 naphthalene, anthracene, stilbene, as secondary dosimeters appears 

 promising as a result of prehminary experiments (Smeltzer, 1950; Cassen 

 and Curtis, 1950; Taylor, 1951) with roentgen rays. Investigations Avith 

 different ionizing particles, however, seem to indicate that the light emis- 

 sion of organic crystals per unit of energy loss is not constant under all 

 conditions of irradiation, but that for at least one inorganic crystal (Nal) 

 it is nearly so, irrespective of the nature of the ionizing particle (Jentschke 

 et al., 1951). Other characteristics of scintillators pertinent to their use 

 as dosimeters are under study. 



Although ionization in dieletric liquids offers the decided advantage of 

 greater sensitivity of measurement, the method has not received wide 

 support because of the practical difficulties experienced in ion collection. 

 This is due to low ion mobilities, usually of the order of 10"^ cm/sec/volt/cm, 

 and to the high probability of recombination. An exception is pro- 

 vided by liquid argon in which saturation is obtained practically "with 

 electric fields of the order of 10 kev/cm as contrasted to CS2, for example, 

 in which only about 75 per cent saturation is reached with fields of the 

 order of 75 kev/cm (Davidson and Larsh, 1948; Hutchinson, 1948; 

 Mohler and Taylor, 1934). The use of liquid argon as a standard, how- 

 ever, is precluded not only by the inconvenience of the low temperatures 

 required but also by the fact that e^ would generally differ markedly from 

 that obtaining in biological substances. It must be concluded, therefore, 

 that the future of dielectric liquids as standard media for ionization 

 measurements is predicated upon the discovery of a liquid organic com- 

 pound exhibiting high ion mobihty at room temperatures and of atomic 

 composition representative of biological systems; for obvious practical 

 reasons, this liciuid should produce no electrochemical events at electrode 

 surfaces. 



Measuremeiit of Dose bij Means of Ionization in Gases. Ionization in 

 gases has proved to be much more adapted to the task mainly by virtue of 

 the ease with which ions can be collected and of the convenience and 

 reproducibility which characterize the measurements of ionization cur- 

 rents. This was recognized soon after the discovery of radioactivity and 

 X rays; Villard (1908) proposed a quantitative unit of X-ray intensity 

 based on the ionization produced in air, and Szilard (1915) made the 



