Sec. 4.9] 



PROTONS, DEUTERONS, AND ALPHA PARTICLES 



89 



I« \z P / 



Rd ~ inr I -- J -^a -- const 

 ^ = 2.012i? c 



cm 



4.9. Specific Ionization. The number of ion pairs formed by a charged 

 particle per unit length of path is referred to as the specific ionization. Its 

 value depends upon the absorbing medium and on the velocity and charge of 

 the particle at any instant. The most intense ionization, as may be seen 

 from the rate of energy loss indicated by the stopping formula, is produced 

 near the end of the particle's range where the velocity is low. A maximum 



rV.X SOURCE 



THICK CCURCE 



Fig. 18. 



3 



RESIDUAL RANGE IN CENTIMETERS 

 Bragg curve of specific ionization by alpha particles in air. 



value is reached and then drops rapidly to zero as the particle is brought to 

 rest. At high energies the specific ionization remains relatively constant at 

 one-third to one-tenth of its maximum value near the end of the particle's 

 range. In air at normal temperature and pressure the specific ionization of 

 alpha particles, for example, remains essentially constant with a value of 

 roughly 2,000 ion pairs per centimeter for high energies. At low energies, it 

 increases rapidly to a maximum of ~ 6,000 ion pairs for an energy correspond- 

 ing to a residual range of approximately 0.4 cm, as shown in Fig. 18. Similar 

 curves hold for protons and deuterons. 



The average energy W spent by charged particles in forming one ion pair 

 has been subjected to extensive experimental and theoretical investigation. 

 It is found that for energies greater than about 2.0 mev the value of W for 

 protons and alpha particles remains essentially constant although there is a 

 very weak dependence on velocity. In an exhaustive survey of the results 

 obtained by various investigators, Gray [29] proposed the value of W = 36 

 ev per ion pair formed in air by protons and W = 31.5 ev per ion pair formed 

 by alpha particles (measured with alpha particles from RaC')- For hydrogen 



