CHAPTER III. 



REGIONS OF MAXIMUM IONIZATION, AND MISCELLANEOUS EXPERIMENTS. 

 REGIONS OF MAXIMUM IONIZATION DUE TO GAMMA RADIATION. 



32. Introductory. I have recently* standardized the fog chamber 

 by the aid of Thomson's electron. The method (as will be shown in the 

 following chapters) is not only expeditious, but leads by inversion, when 

 my old values of the nucleations of the coronas are inserted, to values of 

 e which agree with Thomson's and other estimates. This affords an 

 incidental check on the broader bearings of the work. Before describing 

 these experiments it will be expedient to refer to a number of incidental 

 results bearing on the work, among which the displacement of ioniza- 

 tion on rapid exhaustion is most important. This has been studied both 

 in long and in short fog chambers. 



33. Short fog chamber (see fig. 18, below). The experiments them- 

 selves run smoothly and take but a few minutes each; but there is an 

 inherent difficulty involved in the interpretation of the distributions of 

 ionization observed in the fog chamber. The radium (10 mg., ioo,oooX , 

 contained in a small, thin, sealed glass tube), is introduced into the inside 

 of a cylindrical fog chamber, 45 cm. long, by aid of an aluminum tube 

 (walls i mm. thick and about 0.25 inch in diameter), thrust axially from 

 one end to the other of the horizontal chamber. The inner end of the 

 aluminum tube is thoroughly sealed ; the other end lies quite outside the 

 fog chamber, is open, and serves for the introduction of the radium tube. 

 In this way the latter may be moved axially from the glass end of the 

 fog chamber on the right of the observer to the metal cap which closes 

 the fog chamber on the left. 



When the radium is placed successively at distances of about 1 1 cm. 

 apart, within the available 45 cm. of the length of the fog chamber, 

 the maximum nucleation (ionization) coincides with the position of 

 the radium when both are near the glass end of the chamber (12 cm. in 

 diameter). The nucleation then falls off rapidly and at first uniformly 

 from the glass end to the metal end, where the coronas are strikingly 

 smaller and the nucleation less than one-half of that observed at the glass 

 end. Considered alone this would appear like the natural effect of an 

 increasing distance from the source, except that the coronas near the 

 distant end approach a constant diameter. 



*Am. Jour. Sci., xxvi, pp. 87-90, 1908. 



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