PARALLEL-PLATE STANDARD IONIZATION CHAMBER 



43 



beam passing centrally between the plates, a spacing of 12 cm was satis- 

 factory. Since a guard ring GG surrounds the collector plate P 2 , and 

 since a perfectly parallel electric field across the whole width of the 

 collector electrode P\P 2 was desirable, the widths of the guard plates 

 were made from one and one half to two times the plate spacing. Thus 

 for a plate spacing of 12 cm the guards must be about 20 cm wide. The 

 standard chamber is rather large and unwieldy. In order to reduce 

 the guard-ring dimensions to 5 cm a guard wire system was added. 



X-radi 



Ground 



1 12 cm 

 T37cm hole 



To electrometer 



Ground 



Fig. 1-16. Diagrammatic section to scale of the National Bureau of Standards 

 guarded-field ionization chamber. For use with 50- to 200-kv x-radiation. Guard 

 wires a, b, c, etc., used to aid in creating a parallel field between electrodes P1P2G. 

 Pi aluminum collector electrode, shielded by guard ring G. 



The electric field between the plates is rendered parallel by placing 

 ten small aluminum guard wires (a, b, c, • • •) across the ends of the 

 chamber parallel to the electrodes about 1.1 cm apart, except for the 

 center pair which are spaced about 1.6 cm apart. The electrode system 

 is completely surrounded by a lead box. The diaphragm system used 

 with this design is held in place by a lead-lined brass tube which fastens 

 in front of the ionization chamber. The limiting diaphragm has a 

 diameter of 0.8 cm; the inner end is diaphragmed to 1.2 cm. At the 

 back of the chamber the beam passes out through a 3-cm hole covered 

 with a thin sheet of celluloid to eliminate air drafts. 



X-radiation, in passing through the diaphragms and then between 

 the plates P1P2, produces ions in this space. They are drawn to the 

 plates along the paths of the lines of force. The effective volume of air 

 ionized is that of a cylinder of cross section equal to the area A and 

 length equal to the effective length of the collector electode P 2 . The 

 electric field must also be of sufficient magnitude, as provided by the 

 battery B, so that all the ions are removed before any are lost by recom- 



