MEASUREMENT OF X-RAYS AND RADIUM 



73 



SECONDARY IONIZATION CHAMBERS 



For the measurement of X-rays for biological purposes it is of course 

 ob^dous that the large chambers discussed m the foregoing are imprac- 

 tical, principally because they lack mobility. This lack has led to the 

 development of the small so-called, thimble chamber. Such chambers 

 (Fig. 17) usually consist of an outer shell of some conducting material 

 which is earthed, and a central electrode which is connected to a suitable 

 current-measuring device and charged to a potential sufficient to insure 

 saturation between it and the surrounding shell. When placed in an 



Sulphur) ^ Amber / ''^'^°^" 



o I r- . Graphite 



Brass unless specrried 



Fig. 17. — Thimble ionization chambers. Upper radiographs by Glasser; lower, Friedrich 



chamber. 



X-ray beam, the air within the shell is ionized, causing the charge to 

 leak off the central electrode. The loss of charge Q being proportioned 

 to the X-ray quantity is given by the fall in potential of the charged 

 system as indicated on the electroscope by the relation Q = C(Fi — F2), 

 where C is the electrostatic capacity of the complete insulated system 

 and Fi and F2 are the potentials at the begmning and the end of the 

 exposure of the chamber to the X-rays. 



Wall Effects. — In general, the ionization per unit volume of air in a 

 thimble chamber will not be the same as that in an open-air standard, 

 nor even proportional to it, as the radiation quality changes. This is 

 due to the so-called "wall effect" of the thimble chamber. X-rays strik- 

 ing the walls and collector electrode of the chamber produce both soft 

 secondary X-rays and photoelectrons, both of which contribute to the 

 ionization within the chamber in a manner different from that in the 



