PRINCIPLES OF RADIOLOGICAL PHYSICS 



85 



ficial layers of the same material if the radiation source is external to the 

 material. If the source is embedded in the material, the portions distant 

 from the source are similarly protected. An immediate cjuestion is: 

 How far does a particular radiation penetrate within a material? More 

 specifically, how does the chance of activation of any atom depend on its 

 location within a material with respect to the radiation source? This 

 chance does not vary much between points at a short distance from one 

 another unless the structure of the material lacks uniformity. 



Our problem is, therefore, one of macroscopic distribution and concerns 

 really the average action of radiation over portions of matter containing 

 large numbers of atoms. The average amount of energy absorbed per 

 unit mass (or volume) by different portions of a material can well be taken 

 as an index of the chance of activation of any one atom within each 

 portion. 



Besides the attenuation of radiation due to its interaction with a 

 material, we must always consider the simple attenuation due to diver- 

 gence of the radiation flow away from its source. If the source is con- 

 centrated in a narrow space, this effect alone reduces the radiation 

 intensity in inverse ratio to the square of the distance from the source, as 

 shown in Fig. 1-50. Therefore this effect is commonly referred to as the 

 "1/rMaw." 



Fig. 1-50. Illustration of the l/r'^ law. The areas Si and <S2 intercepted by a cone 

 diverging from a source located at T' are proportional to the squares, al and Oj, of 

 their respective distances from V. If the same quantity of radiation flows through 

 Si and Si, the average intensity on these surfaces must be inversely proportional to 

 their areas, i.e., proportional to l/a^ and l/al, respectively. 



The interactions of radiation with a material oppose the penetration to 

 deep-lying portions of the material in two different ways, namely: 



(a) Progressive dissipation, or outright absorption, of radiation energy 

 in layers of the material nearer to the source of radiation. 



(b) Scattering actions which divert the flow of radiation from its 

 initial direction. Scattering actions complicate the pattern of flow of 

 radiation and make the study of penetration more difficult. 



Scattering causes some of the radiation energy to flow sideways with 

 respect to the direction of incidence. Nevertheless portions of a material 



