84 APPLIED RADIOACTIVITY 



the air of the standard chamber. The roentgen cannot be used as a 

 unit of tissue dose where scattering is a predominant phenomenon. 

 Since tissue dose is what the radiologist desires for his work, Failla sug- 

 gested the following definition: " The roentgen is the quantity of any- 

 ionizing radiation capable of producing 1.615 X 10 12 ion pairs per gram 

 of air at a given point in a given medium under the conditions in which 

 the radiation is to be utilized."* 



The unit thus defined can be used for measurements of radiation in 

 air or tissues. 



It has been suggested that, as a temporary expedient, the gamma-ray 

 quantity of 1 mg-hr measured at a distance of 1 cm from a point source 

 of radium filtered through 0.5 mm platinum be adopted as equivalent 

 to 8.3 " effective " roentgens. 



The Biological Roentgen 



Living organisms have been used as dosage indicators of both x-rays 

 and gamma rays. The reaction most often used is the effect of the 

 radiation upon the rate of cell growth and repair. 



Since the absorbed radiation can slow down the rate of cell division, 

 a sufficient dose can stop cell division. This results in the death of the 

 organism. 



A large variety of biological material has been used to test the lethal 

 action of radiant energy, among them bacteria, yeasts, and spores of 

 many kinds; algae and a great variety of seeds; protozoa; the eggs of 

 salamanders, insects, and frogs. 



In order to get quantitatively reproducible results, large colonies 

 of small organisms must be used so that the results may be handled 

 statistically. If small enough, the entire organism can be considered 

 as having been irradiated uniformly, and if the organism is properly 

 suspended scattered radiation can be disregarded. 



The technique involved in evaluating the gamma radiation in terms 

 of the roentgen is illustrated by some typical results obtained with 

 Drosophila eggs. The lethal effects of 120-kv x-rays filtered with 0.25 

 mm Cu plus 1 mm Al will be compared with gamma rays from radon 

 filtered with 0.5 mm Pt and 4 mm Bakelite. 



The criterion of the effect produced is the proportion of eggs, in a 

 standard sample colony, which survive and hatch as larvae. A typical 

 survival curve, as obtained by Packard [1936], is reproduced in 

 Fig. 11-11. This shows the effect of various doses, of the above-specified 



* For the provisional definition of the roentgen for gamma radiation see Chapter I. 



