70 



BIOLOGICAL EFFECTS OF RADIATION 



collector requires greater total plate potentials than the parallel-plate 

 chamber, and while this offers no serious objections, care must always be 

 taken that the beam passes through the same portion of the chamber (36). 

 A field strength just sufficient to produce saturation at a given pomt 

 would be insufficient at a point farther from the collector. Field distor- 

 tion along the length of the chamber is essentially the same as in the 

 parallel-plate type. 



A third type known as the drumhead ionization chamber (Fig. 14) has 

 some desirable features (12), although it has not yet been extensively used 

 as a standard. In this the high-potential and collector electrodes are 

 parallel plates arranged as shown, the radiation now passing through, and 



not just between the plates. Where 

 the plates are very thin and made 

 of low-atomic-number material their 

 effect in producing secondary X-rays 

 and electrons is negligible. Suitable 

 plates at least for higher voltage 

 radiations have been made of silk 

 cloth, goldbeater skin, or a silk 

 thread mesh, each rendered suitably 

 conducting by light coatings of 

 India ink or "aquadag." 



The ionized volume is given by 

 the product of length of the path 

 between the two high-potential elec- 

 trodes H and the area of the dia- 

 phragm D. Except in extreme cases, field distortion is of no importance. 

 Such a chamber has the advantage of offering a relatively large volume of 

 ionized air, with consequent increase in ionization current. 



In the three types of chambers discussed in the foregoing it is neces- 

 sary, in general, that the electrode system be surrounded by a metal case 

 which serves the double purpose of providing electrostatic shielding and 

 shutting off any stray X-ray radiation. Such a shield may, if placed too 

 close to the electrodes, introduce undesirable field distortion. In the 

 case of the parallel-plate chamber a separation between the shield and 

 high-potential electrode (11) approximately equal to the plate spacing, is 

 sufficient to avoid field distortion. This means that for a 10-cm. collector 

 electrode the complete ionization chamber will have a minimum total 

 length of 65 cm., and about 40 cm. between the collector electrode C 

 and the limiting diaphragm D. 



The primary standard X-ray ionization chamber employed by the 

 National Bureau of Standards is a modification of the parallel-plate 

 type (47). The principal feature of this is the avoidance of field dis- 



FlQ 



14. — "Drumhead" 

 ber. 



ionization cham- 



