174 RADIATION BIOLOGY 



Inaccuracies due to Compton scattering may be expected for the case of 

 large objects; in this case, however, direct verification of the dose by 

 direct experimentation with ionization chambers in suitable mockups 

 presents no particular difficulty. In any event, the experimenter dealing 

 with multiple 7-ray sources may avoid considerable computational effort 

 by arranging his experimental conditions of irradiation to conform with 

 the geometrical arrangements described in the extensive literature cover- 

 ing the subject of 7-ray dosimetry as applied to radium therapy (Mayne- 

 ord, 1950; Wilson, 1945). 



PRACTICAL ASPECTS OF RADIATION DOSIMETRY 



From the preceding discussions it seems clear that, although the most 

 practical methods of measuring energy absorbed are based on ionization 

 in gases, any one ionization instrument is suited to do so only under a 

 somewhat idealized set of conditions. Obviously, these should be imple- 

 mented in practice as closely as possible, and much could be written on 

 the many techniques that have been applied to achieve that purpose. 

 Restrictions on the space available to this chapter, however, will limit our 

 presentation to the brief illustration of the most important principles 

 involved and reference to a few types of ionization chambers which 

 have proved useful for the direct measurement of dose under conditions 

 of irradiation of convenient attainment but otherwise unsuited to the 

 use of commercially available dosimeters. These questions will be dealt 

 with under two separate headings only, namely, charged particles and 

 photons. Neutrons will be omitted because of their as yet limited use 

 and of the relative lack of experimental background which covild be 

 described with profit to the reader. 



Particle Radiation. Dose measurements of radiation produced by elec- 

 trons, protons, or a particles of energy ranging up to a few mega electron 

 volts present particular difficulties in both theory and practice. This can 

 be traced to the facility with which electrons are scattered and the ease 

 with which the heavier particles are absorbed in matter. Calculations 

 are generally not useful in quantitative investigations, and recourse to 

 measurement is considered in general more realistic. From the latter 

 point of view, it is well to consider that, although the urge to regard a 

 point source *S and a "biological" point P as the only important entities 

 (Fig. 2-lOA) is really overwhelming, the fact remains that there must 

 exist also a support S' for the source, a support P' for the object, and an 

 enclosure E. It should be realized also that ionization at P will be caused 

 not only by the particles coming directly from S but also by other particles 

 traveling via the "first scatter routes" S8'P, SP'P, and SEP; moreover, 

 these paths should not be considered a priori as rectilinear.^"^ 



'2 The literature on the absohite determination of the disintegration rate of /3-ray- 



