70 



ELECTROMAGNETIC RADIATIONS AND MATTER 



meters of air, and lose one half their intensity if directed through 0.005 mm 

 aluminum foil. By contrast, the beta rays are negatively charged, only 

 weakly ionize gases, can travel many centimeters through air, and lose one 

 half their intensity only if passed through 0.5 mm of aluminum sheet. The 

 gamma ray has no charge. It strongly ionizes gases and penetrates up to 

 4 in. of lead. 



Careful determination of e/m showed the beta rays to be fast electrons, 

 traveling at speeds up to 0.99 times the velocity of light (3 x 10 10 cm/sec). 

 Similar experiments, and actual collection of alpha rays in a lead box, 

 showed that the alphas are helium ions, He ++ . Experiments on penetration 

 and analogous properties indicated that the gammas are simply electromag- 

 netic waves like light, except of very short wavelength, shorter (or "harder") 

 and more energetic than X rays. 



Rutherford's famous scattering experiments, performed about 1911, dis- 

 closed the inner structure of the atom. Alpha rays were used as the bullets 

 and metal foil as the target (Figure 4-2). He surrounded the target with a 



photographic plate 







nucleus 



© 



Ni foil 



Figure 4-2. Scattering of Alpha Rays by Nickel Nuclei. Definite scattering angles and 



even back-scatter were observed. See text. 



cylindrical photographic plate, and observed, in addition to dark spots re- 

 sulting from direct penetration through the foil, dark spots at certain char- 

 acteristic angles of scatter. Most important, though, was the observation of 

 ia^-scattering, in which the incident radiation was reflected almost straight 

 back, like a ball bouncing off a wall. In his own words, in a lecture delivered 

 at Cambridge many years later, in 1936, Rutherford said: 



On consideration, I realized that this scattering backwards must be the result 

 of a single collision; and when I made calculations I saw it was impossible to get 

 anything of that order of magnitude unless one took a system in which the 

 greater part of the mass of the atom was concentrated in a minute nucleus .... 



The back-scatter requires such energy that the alphas must penetrate to 

 within 1/10,000 of the center of the positive charge in the atom; this means 

 that the positive charge is centered in a nucleus of diameter 1/10,000 that of 



