116 RADIOACTIVITY; BIOLOGICAL TRACERS 



"Daughter Products": Products of Radioactive Decay 



Any radioactive source, before being administered for any good reason, 

 should be examined for the radioactivity and the chemical properties of its 

 disintegration products. Refer to Figure 5-7. Thus, loss of an alpha means 



a shift downward of two places in the periodic table (e.g., radium — * radon); 



and loss of a beta means a shift upward of one place (e.g., iodine I 131 — > 

 xenon 131 ), because these charged particles (electrons) are ejected from the 

 nucleus, and it is the charge on the nucleus which determines the position of 

 the element in the periodic table. Loss of a gamma results in no shift, but is 

 simply a loss of energy during a nuclear reorganization. 



The daughter products often are unstable and give rise to further disin- 

 tegration. Several steps may occur before a nucleus reaches a stable state. 

 One of the simplest disintegrations is that of Na 24 , used in determining the 

 role of sodium in a cell-membrane transfer. The scheme was seen depicted 

 in Figure 5-7. The isotope Na 24 gives off a 1.39 mev beta to become excited 

 Mg 24 (magnesium); but this in turn emits two hard gammas before reaching 

 a stable product. 



The Ra 88 226 nucleus and its daughters produce a total of eight alphas, eight 

 betas, and eight gammas before reaching the stable isotope Pb 82 206 (lead). Three 

 isotopes of polonium (Po 84 ), two of bismuth (Bi 83 ), one of thallium (Tl 81 ) 

 and three of lead take part in the disintegration scheme! Note that all the 

 daughters except radon are solid elements. Although all have short half- 

 lives, they take a fleeting part in the chemistry of the molecules in the vicin- 

 ity in which they are formed. 



By interesting contrast with radium (Ra), Po 210 is a pure alpha emitter, 

 and P 32 (phosphorus) is a pure beta emitter. I 131 and radio-gold, Au 198 , emit 

 both betas and gammas. Decay schemes for some of these are given in 

 Figure 5-7. 



PENETRATION OF THE RAYS INTO TISSUE 



It is preferable to discuss the penetration of the pure emanations and then 

 to infer the effects of the mixed emission of mother and daughters. 



The alpha (and also the proton and deuteron) penetrates in a straight line 

 until it is stopped (Figure 5-1), provided of course that it does not "hit" a 

 nucleus (Figure 4-2). Because both the a and the target nucleus are so 

 small, the likelihood of collision is small. Since alphas are monoenergetic 

 from a source, all penetrate to about the same depth. 



Both beta-scattering and gamma-absorption are governed more or less by 

 chance collisions in which energy is lost from the penetrating radiation. The 

 intensity decays more or less exponentially with distance in each case 

 (Figure 5-9). This is only true to a first approximation, however, because of 

 scattering which is related to the geometry of the system. 



