CHAPTER 2 



The Measurement of Ionizing Radiations 

 for Biological Purposes 



L. D. Marinelli 



Argonne National Laboratory, Lemont, Illinois 



AND L. S. Taylor 



National Bureau of Standards 



Radiation dosimetry: Energij absorbed as an expression of dose — Survey of dosimetric 

 methods — Measurement of dose by means of ionization in gases — Relation between dose 

 and exposure in roentgens. Experimental realization of the roentgen: Standard air- 

 ionization chamber— Air-ioall thimble chamber. Measurement of dose by means of 

 cavity-ionization chambers: Gamma rays — Neutrons — Ionizing particles— Status of W 

 and pm- Dose from radioelements deposited in biological systems: Beta-ray emitters — 

 Gamma-ray emitters. Practical aspects of radiation dosimetry: Particle radiation — 

 Photons — Calibration of thimble chambers in roentgens — Measurement of very-low-energy 

 X rays — So7ne details of practical interest in photon dosimetry — Photon-electron equilib- 

 rium. References. 



RADIATION DOSIMETRY 



Energy Absorbed as an Expression of Dose. The ultimate purpose of 

 dosimetry in radiobiology is to contribute to the eUicidation of the 

 mechanism of action of ionizing radiations. In principle the task could 

 be considered accompUshed if a statement concerning the total number of 

 ionizations and excitations produced in a biological or chemical system 

 could be made as a result of direct measurement, since these phenomena, 

 characteristic of ionizing radiation, may be regarded as the initial steps 

 of radiochemical and radiobiological action. Since measurements of this 

 sort are considered experimentally unfeasible, efforts have been made 

 instead to evaluate the energy absorbed by the irradiated system. This 

 approach has been adopted because of the fact that, whereas ionization 

 and excitation depend markedly on molecular structure, energy loss by 

 ionizing particles is essentially dependent on atomic composition only 

 and therefore is more easily estimated by indirect means. The choice of 

 energy as the independent variable retains the desirable qualities inherent 

 in any fundamental and unambiguous physical entity; it should not be 



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