Retardation of a Particles by Metals. 



191 



equivalent required. The values of M and R/ can be taken 

 from the curve for the particular material used. 



From the curves given above we have determined the 

 mass per unit area per centimetre air-equivalent at different 

 parts of the range. The results are given in Table III., and 

 the values for the other materials for which we have not 

 determined the curves sufficiently accurately are given 

 between 5 and 6 cm. of air. The results are expressed 

 throughout at 15° C. and 76 cm. pressure. The limits of 

 air value are in each case reckoned from the end of the 

 range and not from the beginning. The reason for this is 

 that the end of the range is the only point common to a 

 particles of different radioactive products. Moreover, there 

 is some doubt as to the exact range of Ra C. This has 

 generally been assumed to be 7'06 cm. at 76 cm. and 

 20° C. Bragg, however, has recently given the value 

 7*14 cm.* In our experiments the end of the range at 

 which the zinc-sulphide screen was generally set corre- 

 sponded to 7'04 cm. from the source (76 cm. and 20° C), 

 although an occasional scintillation could be observed up to 

 7-10 cm. (76 cm. and 20° C). 



Table III. • 



Material. 



Atomic 

 weight 



Mass per unit area of foil equivalent 

 to 1 cm. air at 15° C. and 76 cm. 

 (=zwXl0 3 grms.). 



Bragg's results. 



0-1 cm. 



1-2 

 171 



3-28 



4-84 



2-3 



3-4 



4-5 



5-6 



6-7 



Stopping 

 power. 



mxl0 3 . 



Aluminium... 



Copper 



Silver 



271 

 636 

 107-9 

 1190 

 195-2 

 1972 



190 

 3805 



610 



165 

 3-10 



444 



164 

 301 



425 



163 

 2-93 



406 



162 



2-26 



2-86 



317 



4-4 



3-96 



1-43 



162 

 2-81 



391 



1-495 



2-46 



3-28 



3-56 



414 



422 



1-545 

 221 



2-80 

 2-85 

 4-02 

 398 



Tin 



Platinum ... 

 Gold 



Mica 





In column 10 of the above table we have entered Bragg's 

 values for the stopping powers of the materials considered. 

 They are taken from p. 44 of his book 'Studies in Radio- 

 activity,' and Prof. Bragg has kindly informed us that they 

 generally refer to an incident range of about 6 cm. and 



* Loc. cit. p. 21. 



