10:1 Z-. 



or zing '-'-'- '--'. \- : 



187 



lengths of the individual particles are widely varied. It is possible to 

 determine a maximum distance of penetration such that the remaining 

 energv is less than 1 per cent of the incident ener^r r one may deter- 

 mine the distance at which the radiation has decreased to less than 

 background . 



It is often of greater biological importance to express the ionization 

 than to detail the path length. The ionization is expressed in terms of 

 dosage, but there is no generally accepted set of units. Instead, various 

 dosage units are used. These are described in the following section. 



2. Dosage 



The effects of several different types of radiation can be described in 

 terms of the ionization they produce-- Historically, the oldest unit used 

 to measure dosage was denned in terms of the ionization produced in 

 air. This unit is called the roentgen and is abbreviated r. It is denned 

 for X ravs and y ravs as: ""The roentgen I r is the quantity of X or 

 gamma radiation . . . producing 1 esu of ions of either sign per Q.01293 

 gm of air." This mass of air. 0.01293 gm. occupies one ml at 0~C and 

 760 mm of Hg pressure. An alternate form is that lr is the quantity 

 of X or gamma radiation which loses 83.4 ergs gm of a: 



To extend this definition to other types of radiation the following units 

 have sometimes been used: 



1 rep = roentgen equivalent physical — quantity of radiation. 



of any tvpe. producing energy losses of 83.- - gm 



in water or tissue . 

 1 rem = roentgen equivalent man' — quantity of radiation, of 



anv tvpe. producing effects in man equivalent to lr 



of X or gamma radiation. This unit depends on the 



s "ecific effect in man used as the criterion. 

 1 reb = roentgen equivalent biologic a. — sam - 



anim als or plants may be used instead of man. 



Accordingly this is also an equivoc:.- definition. 

 1 rod = quantity of radiation of any type producir- 



losses of 100 ergs gm of absorbing material. For X 



and y rays, lr is between 1 rad and I 



1 The figure of 83 ergs gm can be found - - - : - 5 :: 



- i electron-volts per ion pair in br: a ^ bond. - appeals 



constant for all substances, although - 



most bonds. The extra energy app; - 



electronic excitation within the ions iorr. 



