Subject matter: the basic dosimetric quantities 
  Several quantities and units are needed in the field of 
       diagnostic radiology and related dosimetry
  Some can be measured directly while others can only 
                   be calculated
  Introduction
  Alarmed  by  increasing  number  of  radiation  injuries  reported,  
  therefore the scientists try to minimize the radiation exposure & 
             .use unit called it skin erethyma dose
  This unit was defined as dose of radiation that causes diffused  
          .redness over an area of skin after irradiation
  In 1937 ICRP has defined radiation unit which was Roentgen,  
     .although not accurately defined & has several limitation
  In 1980 ICRP adapted SI units (international system of units) for  
                 .use with ionizing radiation
     Effectiveness of Ionizing Radiation
  Heat and light are different forms of energy their effect            
  appear  as  increasing  in  temperature  which  can  be 
             .prevented by shielding or increasing the distance
  However  the  radiation  differ  in  that  the  increasing  in       
  temperature is very small even with lethal dose of Gamma 
  nuclear radiation the temperature will increase by 1/1000 
        .of 1 C which the skin is unable to sense so radiation
  Radiation may cause a damage to the living tissue through            
  a process of ionization ,which is a removal of an orbital 
  electron  from  an  atom  to  produce  ion-pair  as  result  of 
                                  .absorption of radiation energy
     Principles of Free Ionization Chamber
   Commonly the free electron will recombine with its atom         
   but it can be prevented from recombination and that can be 
   done  by  applying  potential  difference  between  two 
                                                     .electrodes
   so the ion-pair will be attracted to either side which forms    
           .the basis of measurement of the amount of radiation
   To obtain a precise measurement of radiation exposure in        
   medical radiography,  the total amount of ionization an x-
   r  ay beam produces in a known mass of air must be obtained
                                                              .