The Measurement of Radiation 183 



of the wall and the central electrode, the wall thickness and 

 chamber size, it has proved possible to produce empirically 

 chambers having a sufficiently close response to that of the 

 "free-air" chamber, and the chambers can be calibrated to read 

 directly in rontgens. The precise quality of the very hetero- 

 geneous radiation within a given medium is not in general 

 calculable, or even easily measurable, and so it is of great prac- 

 tical importance that the "thimble" chamber to be used should 

 not require an appreciable quality correction. It is clearly also 

 of importance that the chamber should be as small as possible 

 in order to define closely the .precise location of the measure- 

 ment, and that it should be sufficiently transparent to the radia- 

 tion not to produce an appreciable "shadow." 



Doserneters 



"Thimble" chamber dosemeters may be used in the direct 

 measurement of dose rate or of dose. In the first case, the 

 actual ionization current is determined by measuring the voltage 

 drop across a high resistance. In the second case, the ionization 

 current is allowed to charge a condenser, the final voltage of 

 which is a measure of the total dose. In either case, a sensitive 

 voltmeter of the electrometer type is likely to be required. All 

 insulations must be of very high standards, for the currents 

 dealt with are very small, for example, the relatively high dose 

 rate of 1 rontgen per second produces in a chamber of 1 cubic 

 centimeter volume a current of only one three-thousandth of a 

 microampere. In some instruments, the ionization chamber, 

 electrometer system, and recording mechanism are permanently 

 connected, often with long cables, so that readings may be taken 

 at relatively long distances from the point of measurement. 



In the condenser-dosemeter, the ionization chamber is entirely 

 separate from the electrometer and measuring devices during 

 exposure to the radiation. The ionization current serves partially 

 to discharge the originally fully charged capacity formed by 

 the chamber itself, and any added condenser. The charge lost 

 is thus ^ measure of the dose, This type of chamber is particu- 



