104 RADIOACTIVITY; BIOLOGICAL TRACERS 



It is exactly this huge energy carried by the alpha or beta particle, or by 

 the gamma photon (packet of light), which is responsible for its detection 

 as well as the damage it does to the molecules of a tissue. Thus, as the 

 emanation is absorbed by molecules of a gas, say, its energy is gradually dis- 

 sipated by being passed over to the gas molecules; these in turn are at least 

 excited, and many are ionized, a process which requires only a few electron- 

 volts per molecule. 



The number of protons in the nucleus determines its positive charge, and 

 hence its position in the periodic table. Protons plus neutrons determine the 

 weight of the nucleus. There may be several numbers of neutrons which can 

 combine with a given number of protons, and thus there can be several 

 weights of the same element. These different weights of the same elements 

 are called "isotopes" (iso topos — in the same place in the periodic table). 

 Some isotopes are quite stable, some spontaneously disintegrate. For ex- 

 ample, carbon with 6 protons in the nucleus, may have 4 to 9 neutrons in 

 the nucleus, to form C 6 !0 , C 6 n , C 6 12 , C 6 13 , C 6 14 , C 6 15 . The isotope C 6 12 is 

 the basic carbon in nature, and is quite stable, whereas C 6 H is a long-lived 

 beta emitter also found in nature. The others are short-lived, and are made 

 artificially by bombardment of nuclei by the "bullets" listed in Table 5-1 . 



IONIZATION AND DETECTION 



Ionization 



Positive Ions 



The mechanism by which ionization takes place in the path of each 

 emanation is important to considerations of penetration. Each mechanism 

 is different from the others because the emanations differ so remarkably. 

 The alpha (He 2 4 ) ++ , the broton (H, 1 ) + , and the deuteron (H, 2 ) + are very 

 small, but dense; the alpha carries the positive charge of two protons. Upon 

 collision with electron clouds of a target material, it easily ionizes the atoms 

 by pulling the negative electrons after it, wasting a small fraction of its 

 kinetic energy in the process. Since it is likely to tear at least one electron 

 out of every atom through which it passes, it leaves a very dense wake of 

 ionization (Figure 5-1). The alpha of radium (Ra) has a kinetic energy of 

 4.8 x 10 6 electron volts, which means that it leaves a wake of about 140,000 

 ionized atoms. Thus in air it can travel a few inches; in metal it can pene- 

 trate only about 0.0001 cm; and in fact can be stopped by a piece of paper! 

 Although its path is short, the radiation damage or ionization along the path 

 is intense. Actually, theory shows that the energy transferred per centimeter 

 of path (called the linear energy transfer, LET) increases with increasing 

 charge, q, and decreases with increasing velocity, v, as follows: 



LET cc q 2/ v 2 



