RADIATION DOSE 



The absorljed energy is usually about equally divided between ionization 

 and excitation, but wc have as yet no precise information concerning the 

 relative magnitudes of these two forms of energy transfer in condensed 

 systems. In certain gases, rather accurate information is available, and it is 

 the fact that the average energy, \V, exiicndcd by ionizing particles per 

 ion-pair formed in gases is nearly independent of the speed of the particles, 

 within very wide limits, which has made ionization a useful measure of 

 absorbed energy. 



By the apjiHcation of the cavity theory of ionization, and the use of 

 appropriate physical constants^ the absorbed dose in a solid or liquid may 

 be inferred from a measurement of gas ionization in a small cavity with an 

 uncertainty which only slightly exceeds the uncertainty in our knowledge of 

 W. The value of W for air which is recommended by the International 

 Commission, is 34 eV for X- and y-ray dosimetry, and 35 eV for neutron 

 dosimetry. Table 1, compiled by Boag^, shows the considerable measure of 

 agreement between the recent determinations of W. 



Table 1. The average energy expended by electrons per ion-pair formed in air 



Author 



Radiation 



W air 

 eV 



Standard error 

 eV 



Emerya 

 Gross et al.^ 

 Bay et al.^ 

 Barberd 



Jesse and Sadauskise 



>> )) >> 



Weiss and Bernsteinf 

 Bernier et al.S 

 Skarsgard et al.^ 



a. Emery, E. W. Brit. J. Radiol. .\.S. 29 (1956) 370 



b. Gross, W., Wingate, C. and Failla, G. Radiation Res. 1 (1957) 570 



c. Bav, Z., Mann, W. B. Seliger, H. H. and Wyckoff, H. O. Radiation Res. 7 (1957) 558 



d. Barber, \V. C. Phvs. Rev. 97 (1955) 1071 



e. Jesse, W. P. and Sadauskis, J. Plivs. Rev. 97 (1955) 1668 



f. Weiss, J. and Bernstein, VV. Phys. Rev. 103 (1956) 1253 



g. Bemier, J. P., Skarsgard, L. D., Cormack, D. V. and Johns, H. E. Radiation Res. 5 (1956) 613 

 h. Skarsgard, L. D., Bernier, J P., Cormack, D. V. and Johns, H. E. Radiation Res. 7 (1957) 217 



Since the difference between the mean of these experimental values and 

 34 eV is still doubtfully significant, and the precise evaluation of W for parti- 

 cles of different speed is still under investigation in a number of laboratories, 

 the International Commission recommends that the value for IV of 34 eV 

 should still be used in the interests of uniformity. 



When we attempt to analyse dose response relationships in biological 

 materials account has to be taken of the fact that energy is not delivered 

 uniformly to matter, but discontinuously along the tracks of individual 

 ionizing particles. Nearly 50 years ago C. T. R. Wilson"*, who celebrates his 

 90th birthday this year, made his first cloud chamber expansion in the pre- 

 sence of X-rays, and wrote, 'with little expectation of success, and in making 

 an expansion of the proper magnitude for condensation on the ions while the 



