THE STRUCTURE OF MATTER 



69 



on the hydrogen ion was equal and opposite to that on the electron (being 

 simply the absence of an electron), and hence the mass of the H + was deter- 

 mined to be 1820 times the mass of the electron, i.e., approximately 

 2 x 10" 24 g. 



In 1896 Becquerel reported that he had accidentally discovered a pene- 

 trating emanation from uranium salts. Thus, his photographic plates, kept 

 in a drawer, with a key in the drawer above, became exposed with the im- 

 print of the key in the presence of some phosphorescent minerals — notably 

 salts of uranium — lying on the top of the bench. These emanations were 

 also found to ionize gases. The Curies, in 1898, extracted a concentrate from 

 pitchblende which had high emissive power, and named it radium (hence the 

 terms "■radium-active" or "radioactive" elements, and "radioactive emana- 

 tion"). 



They measured the strength of the emission by means of an electroscope. 

 This instrument is essentially a vertical metal rod with a thin gold leaf at- 

 tached to it by one end. If the electroscope is charged, the free end of the 

 gold leaf is held out from the main shaft by repulsion of the like electro- 

 static charges. It falls to the shaft in the presence of ionizing radiation, at a 

 rate which increases with the strength of the emitter, because the electro- 

 static charge on the metal is neutralized by charged particles formed during 

 the absorption of radiation. Today ionization chambers based on this prin- 

 ciple have wide use: a burst of current due to ionizing radiation is ampli- 

 fied and recorded. One pulse of current occurs for each bundle of emanation 

 absorbed. Ionization chambers are discussed in Chapter 5. 



In an experiment whose origin is obscure but which was refined and ex- 

 panded by Rutherford (see Figure 4-1), three fractions emanating from a 

 radioactive source such as radium were separated, and called alpha (a), 

 beta (/3), and gamma (7) rays. 



It was found that alpha rays are positively charged and are much heavier 

 than the betas. They are completely stopped by thin paper or a few milli- 



shields 



rodioactive 

 source 



Figure 4-1 . Rutherford's Separation of Alpha, Beta, and Gamma Rays, by Means of 

 an Electric Field Applied Between the Deflecting Plates. Tube is evacuated. 



