86 RADIOISOTOPES IN BIOLOGY AND AGRICULTURE 



of units of length. A common unit is the milhgram per square centimeter 

 (mg/cm-). The primary advantage is that the mass per unit area is more 

 directly related to the absorbing process than is the thickness. Further- 

 more the range expressed as mass per unit area will be independent of the 

 density for all practical purposes and will therefore be essentially the same 

 for various materials. For example, if the range of a given alpha particle 

 is 3.8 mg/cm^ of aluminum, it will also be 3.8 mg/cm- of air. However, 

 the particle will traverse only 0.014 mm aluminum as compared with 

 about 25 mm air. Alpha and beta rays have definite ranges; they can be 

 stopped completely by relatively small amounts of material. Gamma- 

 ray inten.sity, on the other hand, is decreased exponentially according to 

 the following equation: 



/ = he-'^' (3-1) 



where /o = intensity of incident radiation 



/ = intensity of radiation after passage through thickness t of 



absorbing material 

 k = constant for material which represents fractional change of 



intensity with thickness of absorber 

 Equation (3-1) will be recognized as identical with Eq. (1-15), which was 

 discussed in detail. The relationships presented in Chap. 1 hold also for 

 this case. Thus gamma rays may be characterized by the half-value 

 layer, which is the thickness or the mass per unit area required to reduce 

 the initial intensity to one-half its value. This will be recognized as 

 analogous to the half-life of a radioactive material or to the half-value 

 time of an exponential removal process. It should be noted that beta 

 rays, because of a combination of circumstances, follow Eq. (3-1) fairly 

 closely in the middle of the absorption curve. 



Table 3-1 presents a comparison of some of the characteristics of alpha, 

 beta, and gamma radiation. 



Units of Radioactivity. The amount of radioactive material is conven- 

 iently expressed in terms of a disintegration rate, since this is more closely 

 related to the properties of interest than is the mass of the radioisotope. 

 The curie (c), named in honor of the discoverers of radium, has been 

 adopted as the unit of radioactivity. The curie was originally defined in 

 terms of radium decay, and Evans (24) may be consulted for a discussion 

 of the difficulties in the use and misuse of the term. However, the follow- 

 ing definition has now been established (4) : .4 curie is the amount of any 

 radioactive material in which 3.7 X 10^° atoms disintegrate per second. 

 Smaller fractions of the curie are commonly used — millicurie (mc) = 10~^ c, 

 or microcurie (iic) = 10^*^ c. The magnitude of the curie unit in terms of 

 instrumental detection and biological response will be apparent in other 

 parts of the text. Another unit, the rutherford (rd), has been proposed 



