IONIZATION MEASUREMENTS IN A WATER PHANTOM 47 



ber tube is then reinserted into its socket. This connects the partly 

 discharged graphite electrode to the electrometer, and the string drops 

 to a point on the r scale indicating directly the roentgens per minute 

 emitted by the tube at the point in space previously occupied by the 

 chamber tube. 



The ionization produced in the small thimble chamber by the x-radia- 

 tion neutralizes the charge given to the insulated graphite rod, and it is 

 this decrease in charge that is recorded by the electrometer, which has 

 been previously calibrated in r units by means of a standard ionization 

 chamber. Usually the capacitance of these instruments is small. The 

 loss in charge due to the ionization is Q = CV. The capacitance is 

 constant, but the potential of the graphite rod has dropped from V 2 , 

 its charged potential, to P 1} its partially discharged potential state, 

 during the 1-minute exposure to the x-radiation. Since Q = It = C 



I = 7 (V 2 - Pi) = I 



t t 



it follows that the roentgens per minute are proportional only to the 

 change in potential. 



In order that the above thimble chamber may effectively simulate 

 the standard open-air chamber, it is necessary to construct the walls of 

 this chamber from material of low atomic number so that the so-called 

 " wall effect " of this chamber is equivalent to a comparable mass of air. 

 The x-radiation incident on this chamber wall and internal collector 

 electrode produces, owing to absorption, secondary x-rays and photo- 

 electrons. These contribute to the ionization within the chamber in a 

 manner different from that in the standard open-air chamber. The 

 ionization contributed by these sources depends upon the atomic number 

 of the materials of which wall and electrode are constructed. The walls 

 of the chamber, therefore, must be made out of a material whose effective 

 ionization is that of the free air. Accordingly horn, celluloid, or Bake- 

 lite is used in the construction of these small ionization chambers. 



Ionization Measurements in a Water Phantom 



Small ionization chambers such as have been described above are 

 especially well adapted for intensity measurements within a " water 

 phantom." 



A water phantom consists merely of a water container, constructed 

 to simulate human tissue. A small water-tight ionization chamber 

 may be used to measure the effective penetration of an x-ray beam. 

 According to Kulenkampff [1926], the relative absorption of average 



