CHAPTER 3 

 BETA PARTICLES 



3.1. Physical Properties. Negatrons (negative electrons) and positrons 

 (positive electrons) are identical in their interaction with matter and in their 

 physical properties with the single exception of the sign of the charge asso- 

 ciated with the particle. The following sections apply equally well to 

 negatrons and positrons, observing only the proper sign of the charge where 

 it occurs in formulas. 



The static and dynamic properties of the beta particle are given below. 



Rest mass [1]: m = (9.10660 ± 0.0032) X 10~ 28 gm 



Charge [1]: e = (1.602033 ± 0.00034) X lO" 20 abs emu 



= (4.80251 + 0.0010) X 10- 10 abs esu 



p 



Specific electronic charge [1]: — = (1.7502 + 0.0005) X 10 8 abs emu/gm 



^ i • • • m o 



Relativistic mass: m = 



4 



V 2 



1 --» 



c 2 



m v 1 



Momentum: p = — . = - -\/T 2 + 2m c 2 T 



^ c 



Kinetic energy: T = mc 2 — m c 2 



= m c~ 



'1 - 



7 2 



where v = velocity of beta particle, cm per sec 

 c = velocity of light, cm per sec 

 Energy is expressed usually in electron volts although units of Hp are 

 frequently used since this quantity can be measured directly with the beta 

 spectrograph and in the Wilson cloud chamber operated in a uniform mag- 

 netic field. The relation between Hp and energy is given by 



10 4 



Hp = -j- VE 2 + 1.02E 



where H = magnetic field strength, gauss 



p = radius of curvature, cm 



E — energy, mev 

 This relation is plotted in Fig. 9 for more convenient conversion. 



49 



