DISINTEGRATION (DECAY) 113 



If two radioactive elements have been concentrated chemically to the same 

 value of JV , the one with the shorter half-life decays faster, has greater "ac- 

 tivity" (higher dN/dt) at time zero, or delivers more emanations per second 

 to the tissue being irradiated. 



The unit of activity is the curie (c), that amount of radioactive material 

 which provides 37 billion (i.e., 3.7 x 10 10 ) disintegrations each second. 

 Thus 1 g of pure Ra 226 which gives off 4.8 Mev (average of 3) alphas, has 

 a total activity of about 1 c. Sr 38 90 , which gives off only a 0.6 Mev 

 beta, decays faster and is less dense than radium; 1 g of pure Sr 90 provides 

 an activity of 147 c. However, since a pure radioactive substance is always 

 contaminated by its daughter products, the activity per unit weight is deter- 

 mined by the concentration of radioactive substance. Clearly 1 millicurie (mc) 

 per gram might be usable in a medical application, whereas 1 mc per ton 

 should be quite impractical. Specific activity is defined as the number of mc/g. 



Figure 5-7 shows decay schemes for several radioactive isotopes of use as 

 tracers in diagnosis and as irradiation sources in therapy. 



Energy Distribution of the Emitted Rays 



Before we come to the question of depth of penetration and extent of 

 ionization of the rays from a radioactive source, we must consider two more 

 factors: the energy distribution (spectrum) of the rays from any given pure 

 source, and the number and kind of products of disintegration. 



Both alphas and gammas are the result of a particular kind of fracture 

 or rearrangement of unstable nuclei. One could consider the nuclei to be in 

 excited states (think of an undulating water droplet), existing as such from 

 the time of their formation (in the sun?) millions of years ago, and disinte- 

 grating at a rate which we can measure but which we are not able to vary. 

 Thus, although half the atoms of Ra 226 in a sample will undergo alpha decay 

 in a definite and reproducible time, we do not understand why the disinte- 

 gration of Ra 226 is always by loss of one alpha particle, a package of 2 pro- 

 tons + 2 neutrons; and the most striking fact of all is that these alphas al- 

 ways come off with the same velocity. The similarity of this quantum-like be- 

 havior to the quantized absorption and radiation of light by the electron 

 cloud of the atom, suggested to theoreticians that a Bohr-like model for the 

 nucleus should be useful. Development of theory has proceeded along these 

 lines, and has led at least to a quantitative description, if not an answer to 

 the question "why?". 



The alpha or the gamma radiation from a single elemental source occurs 

 at discrete energies — alphas of single velocity, gammas of single frequency 

 (Figure 5-8). However, with the beta is expelled a neutrino, a tiny neutral 

 particle of variable velocity; and therefore the beta radiation from a single 

 elemental source has a distribution of energies — low, corresponding to a 



