MEASUREMENT OF IONIZING RADIATIONS 177 



where p'„ is the ratio of the stopping power of the biological medium to 

 that of the material of the chamber electrodes. For this expression to be 

 valid the biological specimen should cover an area equal to that of C and 

 should rest on material of identical atomic composition and thickness as 

 used in the lower electrode of the chamber. For many practical reasons 

 it is preferable always to extend this thickness beyond the range of the 

 most energetic particle available in the source. When both exposure and 

 measurement are conducted as outlined above, the chamber will permit 

 correct measurement of the dose irrespective of the geometrical dimen- 

 sions of the source. Errors, however, may arise as a consequence of 

 inadequate extrapolation in either electrode thickness or spacing. When- 

 ever "depth doses" in thicker biological specimens are desired, extrapola- 

 tion to zero spacing is carried out with thicknesses of A equivalent to the 

 depths at which the dose is desired. For the case of a radiation, special 

 attention should be paid to the complete collection of ions, since satura- 

 tion is more difficult to obtain whenever the ions are drawn in the direc- 

 tion of the particles; extrapolation in this case might have to be carried out 

 at air pressures lower than atmospheric in order to overcome the effects 

 of ion recombination. 



Photons. Under idealized conditions, the measurement of radiation 

 dose (ergs per gram) resulting from exposure to photons can proceed via 

 measurement of the roentgen or by means of Gray's principle. Experi- 

 ience has proved the first to be a unit of practical use in many instances 

 within the energy range 1 to 3000 kev, but the objectives of radiobiological 

 experimentation can be best attained by the second procedure. Since 

 ionization chambers available commercially are suitable only for the first 

 task, the remainder of this section will be devoted mostly to the measure- 

 ment of dose in roentgens. 



Calibration of Thimble Chambers in Roentgens. The standard ioniza- 

 tion chamber has found few enthusiastic users in radiobiology, principally 

 because it lacks mobility and because it is not suited to yield in practice 

 the corrections necessary to an accurate estimate of the dose (see pp. 

 179 ff). These shortcomings have led to the development of the air-wall 

 (thimble) chambers of much smaller dimensions. Such chambers usually 

 consist of an outer shell with an internal conducting material and a central 

 electrode, which is connected to a suitable current-measuring device. 

 Sufficient voltage is applied to them to ensure saturation. When the 

 radiation quality changes, the ionization per unit mass of air will not, in 

 general, be the same as that in an open-air standard even when the appro- 

 priate experimental conditions of calibration are rigorously obeyed. This 

 is due to the so-called "wall effect" of the thimble chamber. Photons 

 striking the walls and collector electrode produce both soft secondary 

 photons and electrons, both of which contribute to the ionization within 

 the chamber. The relative magnitude of the ionization from this source 



