118 BIOLOGICAL EFFECTS OF RADIATION 



the true intensity of radiation at the point remains constant with respect 

 to time. The energy which passes through 1 cm.- of a surface per- 

 pendicular to the hne of motion of the photons at a certain point in a 

 given time is the product of the true intensity of radiation and the time. 

 This is known as the quantity of radiation. It should be noted particu- 

 larly that by this definition "quantity" refers only to 1 cm.^ of area 

 (unit area) and not to the entire cross section of the beam. Analogously, 

 the product of effective intensity and time may be called the effective 

 quantity of radiation. It represents the amount of energy spent by the 

 radiation in producing ions in 1 cm.^ of atmospheric air at a given point in 

 the beam in a given time, under the conditions stipulated by the definition 

 of the roentgen (page 62) . Since in practice one is dealing almost entirely 

 with effective intensity and quantity, the adjective is omitted. It should 

 always be borne in mind, however, that the two terms (intensity and 

 quantity) have this special meaning in radiology and that certain limita- 

 tions are imposed thereby. The factor which is expressed in roentgens 

 is the (effective) quantity of radiation. By the mathematical relation 

 between intensity and quantity it follows that (effective) intensity may be 

 expressed in roentgens per second or roentgens per minute. 



Now, if the scale of a measuring instrument is graduated in roentgens, 

 or roentgens per minute, its calibration must be made under the condi- 

 tions imposed by the definition of the unit. In instruments with closed 

 ionization chambers this is done usually by comparison with a standard 

 chamber and the calibration holds for a more or less restricted range of 

 wave-lengths. If the instrument is used to measure radiation outside of 

 this quality range, the readings are incorrect. The error may be small 

 or large, depending on circumstances. Furthermore, another experi- 

 menter using a different ionization chamber is apt to get a different read- 

 ing for the same beam of radiation. This makes it difficult for others to 

 duplicate the results of the first experimenter. 



In radiology it is often the practice to place a small ionization chamber 

 on the skin of the patient, with the idea of including in the measurement 

 the back-scattered radiation. If the biologist must irradiate a test object 

 of large dimensions, it is preferable to make the ionization measurement 

 in air at the cross section of the beam where the surface of the material 

 will be, and to state this factor in the report. On account of the marked 

 heterogeneity of the back-scattered radiation, the wall effect of the 

 chamber is apt to be larger in this case than for the primary beam of 

 radiation. Also the geometrical relation of the chamber on the surface 

 of the material will determine to a considerable extent what proportion 

 of the back-scattered radiation is included in the measurement. Accord- 

 ingly, the uncertainty in such measurements is apt to be greater than in 

 those made without the back-scattered radiation. 



