Chapter II 



APPLIED RADIOACTIVITY 



At the meeting of the Academy of Science in Paris, on the twenty- 

 fourth of February, 1896, Henri Becquerel read his epoch-making paper 

 in which he announced that compounds of uranium emitted radiations 

 that were able to affect a photographic plate through an envelope opaque 

 to light. 



Rontgen had just previously announced (1895) that x-rays appeared 

 to originate from those parts of his discharge tubes which fluoresced 

 intensely. Becquerel at this time was investigating the cause of phos- 

 phorescent emissions. He apparently reasoned that a direct connection 

 must exist between the cause of phosphorescence and the x-rays pro- 

 ducing the fluorescence in Rontgen's vacuum tubes, since both fogged a 

 photographic plate enclosed in a light-tight envelope. It was, however, 

 the accidental fogging of a photographic plate by means of a sample of 

 uranium mineral not previously activated to fluorescence by sunlight 

 that led him to the conclusion that fluorescence had nothing 

 to do with the fogging of a covered photographic plate. His conclusion 

 was that some active radiation was emitted spontaneously from the 

 uranium mineral. He coined the word " radioactive " to designate this 

 type of active radiation. 



After Becquerel 's discovery, numerous substances were examined 

 for similar properties, and as a result the radioactive properties of the 

 uranium-radium, actinium, and thorium families of elements were 

 established. Mme. Curie, for instance, succeeded in isolating minute 

 quantities of two highly radioactive substances from uranium minerals, 

 to which she gave the names polonium and radium. 



Rutherford, by 1899, through a series of brilliant investigations, 

 conclusively showed that the radiations continuously emitted by uran- 

 ium could be separated into two types. He called the first "alpha 

 rays." These were easily absorbed b}^ a few thin sheets of paper and 

 produced intense ionization in the air through which they passed. The 

 second, a more penetrating type, he called " beta rays." These beta 

 rays have speeds ranging from three to ten times the speed of alpha 

 rays and are able to penetrate several centimeters of air or even 1 mm 

 of aluminum. 



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