Sec. 16.2] INTERNAL DOSIMETRY 413 



dose units as such. It can, however, be expressed as a dose unit (energy 

 absorbed per gram of tissue at a point) by writing it as a gram roentgen 

 per gram of tissue. From its definition, 1 gm r = 83 ergs, assuming that the 

 energy required to form one ion pair in air by a secondary electron is 32.5 ev. 

 Like the rep, the gram roentgen represents an amount of energy absorbed 

 independent of tissue composition and is not equal to the energy absorbed 

 per gram of tissue exposed to 1 r. On the other hand, the relation between 

 gram roentgen and rep is exactly 1 rep = 1 gm r per gram of tissue. 



d. Energy Unit {proposed by L. II. Gray [11]). A dose of one energy unit 

 means 



93 ergs absorbed per gm water (soft tissue) 



Definition: The energy unit is that dose delivered to tissue by ionizing 

 radiation such that the energy absorbed per gram of tissue is equal to the 

 energy absorbed per gram of water exposed to 1 r of gamma radiation. 



The energy unit is a unit of energy delivered to tissue by any kind of 

 radiation, and its magnitude is chosen so that the energy absorbed in soft 

 tissue is the same whether it is expressed in roentgens or energy units. This 

 equality is not valid however for hard tissue and other substances differing 

 from water (soft tissue) in atomic composition and density. 



The relations between eu, rep, and gm r per gm are approximately 



1 eu = 1.1 rep = 1.1 gm r per gm 



e. J Unit [10]. 



Definition: "One J has been received at any point in a medium when the 

 ionization which would have been observed in an infinitesimal cavity contain- 

 ing the point is 1.58 X 10 12 ion pairs per gram or air enclosed in the cavity." 



The J is a unit of dose proposed by the British Committee for Radiological 

 Units (1948) and intended to replace existing dose units. It defines dose 

 in terms of a quantity that is measured directly, namely, the ionization pro- 

 duced in the gas of a cavity ionization chamber (see Sec. 16.1). Because of 

 the properties of the cavity chamber, the J is applicable to all ionizing radia- 

 tions, electromagnetic and corpuscular, and is valid for all energies of the 

 primary radiation. The numerical value of the J in terms of ion pairs is 

 chosen to correspond to the number of ion pairs formed in water or soft tissue 

 exposed to 1 r. Consequently, 1 J (1.58 X 10 12 ion pairs per gram of air) 

 corresponds to 93 ergs absorbed per gram of air, or alternatively, air exposed 

 to 1 r (1.61 ion pairs per gram of air) of gamma rays not appreciably absorbed 

 photoelectrically corresponds to a dose of 1.02 J. 



/. Roentgen Equivalent Man {Mammal) {proposed by H. M.Parker [19]). A 

 dose of one roentgen equivalent man has approximately the following 

 significance: 



