﻿48 



Mr. G. H. Henderson on the 



Fig. 2 can only be considered to give an approximate 

 idea of the .facts, as the points are not well distributed and 

 some may be seriously in error. It would seem, however, 

 that the straggling increases very slowly as the stopping 

 power of the gas is increased. It is unfortunate that the 

 dearth of suitable gases of high stopping power makes the 

 checking of this point difficult. 



Fig. 2. 



. 4 



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so, £ H * a 





O 2 J*-*—"" 



Air *'" cs d 



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Molecular S+o 



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Power. 



§ 4. T7i£ Straggling due to Solids. 



The great difficulty which at once arises in determining 

 the straggling due to solids is the uneven thickness of the 

 solid foils used, the effect of which may completely mask 

 the true straggling looked for. An attempt to avoid this 

 difficulty was made by using a large number of the thinnest 

 beaten foils of the solid obtainable ; with gold, for example, 

 as many as 128 thicknesses were used. Composite sheets of 

 gold and other metals were placed immediately over the 

 radioactive source (ThC) and the ionization curves deter- 

 mined in air with the same apparatus as before. Although 

 a rough calculation seemed to show that the irregularities in 

 the individual foils would be smoothed out enough to avoid 

 masking any true increase in straggling, this result was not 

 borne out by experiment. It was finally concluded that the 

 increase in straggling observed was mainly, if not entirely, 

 due to unevenness of the foils, and hence need not be gone 

 into in detail here. In mica the increase in straggling was 

 much the smallest, as was indeed to be expected. 



Fortunately, experiments on the straggling produced by 



