﻿Straggling of a. Particles by Matter. 



49 



solid foils for low velocities of the a particles gave results 

 which were not masked by irregularities of the foils. In 

 these experiments the foils were placed 3 mm. from the 

 middle of the ionization chamber (itself 1 mm. dee])) in air 

 at a pressure of roughly 17 cm. Reduced to air at normal 

 pressure the distance from foil to centre of ionization chamber 

 was therefore about "7 mm. Most of the foils used were 

 made up of a few T thicknesses of gold leaf. The air equivalent 

 of a single sheet was about *I5 mm. when placed directly 

 over the source ; when placed near the ionization chamber 

 the air equivalent was about '28 mm. The straggling of 

 the a particles after passing through these foils was deter- 

 mined in the same manner as before from the ionization 

 curve in air. 



The results are given in Table II. The straggling is in 

 millimetres, and the probable error is about 2 per cent. The 

 third column shows the straggling observed when the foils 

 are placed directly over the source, the steady increase with 

 increasing number of leaves being mainly due to unevenness 

 of foils. The fourth column shows the increase in straggling 

 at low velocity over that at high. This .increased straggling 

 is real and almost independent of the unevenness of the 

 foils. Results with aluminium (1*0 mm. air equivalent) 

 and mica (S'6 mm. air eq.) are also included in the Table, 

 but with these foils the change observed is scarcely more 

 than the experimental error. 



Table II. 



No. of Leaves ( ^™ 8 loHifa«on Stra ^ in S Foil 

 111 FolL Chamber. near Source ' 



Difference. 



2-88 



1 3-30 



4 ■ 3-65 



12 400 



2-88 

 2-92 

 301 

 3-28 

 3-01 

 306 



•38 

 •64 

 •72 

 •07 

 •10 



Aluminium ' 308 



Mica 3-16 



These results show quite clearly the rapid increase of 

 straggling near the end of the range. One gold leaf nearest 

 the ionization chamber causes nearly twice as much straggling 

 as three leaves immediately behind it. Eight more leaves 



Phil. Mag. S. 6. Vol. 44. No. 259. July 1922. E 



