[douglas] 



/3-RAYS FROM RADIUM 



115 



o 



•^h 



■6 



Fig. 2 



•8 



h MM. 



must have an "effective range," i.e., if there be a given stream of 

 /3-rays of any one type there must be some definite thickness of any 

 absorbing material through which the /5-rays cannot be detected no 

 matter how great their original intensity. 



A very complete description of the experiments referred to above is 

 given in Rutherford's "Radioactive Substances and their Radiations". 



It was pointed out by J. A. Gray (Proc. R.S., 1912) that, if 

 )3-rays like those from Ra.E appear to be exponentially absorbed at 

 first, this can only be an approximation, and a stage must be reached 

 when the absorption increases more and more rapidly until finally 



100 /s, 

 the effective range is reached. The values of such terms as — - — 



etc. (referred to above), become less and less, the limit being zero 

 when the range is reached. The range can only depend on the fastest 

 /3-rays in the original beam, and hence this affords a method of measur- 

 ing the relative maximum speeds of /3-rays from different radioactive 

 substances. 



At the suggestion of Dr. Gray, the writer has carried out a series 

 of experiments following the lines indicated above with two main 

 objects: 



(1) To determine whether /j-rays lose velocity when scattered 

 through large angles. 



