﻿Straggling of u Particles by Matter, 51 



increase over the straggling which would be produced by a 

 layer of air equivalent to the gold foil used. The ionization 

 must be measured in a gas such as air at a reasonable 

 pressure, and hence we have the complication of the straggling 

 due to the air between the solid and the ionization chamber, 

 and even in the chamber itself. This sets a limitation to 

 the amount of information to be obtained From ionization 

 data. Accordingly the ionization experiments were not 

 carried beyond the stage described, but it is hoped to push 

 further the attack on the problem by more suitable methods. 

 An isolated experiment with iron foils may be referred to 

 before closing this section. A sheet of iron of about 4 cm. 

 air equivalent was placed directly over the source and the 

 ionization curve measured when the iron was magnetized 

 parallel with and perpendicular to the direction of travel of 

 the a particles. The purpose of this experiment was to see 

 if there was any change in the straggling due possibly to 

 rearrangement or change of orientation of the electrons 

 in the iron, a point which has been discussed by Flamm *. 

 Alternate readings of the ionization with parallel and per- 

 pendicular fields of about 100 gauss were made at various 

 points along the range. No appreciable difference could be 

 detected- It should be added that the iron was very uneven 

 in thickness, and the consequent; straggling was so large 

 (about 20 mm.) that a small change in straggling might 

 well have been masked. 



§ 5. Summary and Conclusion. 



It has been shown in this paper that the straggling of 

 « particles, as deduced from both ionization and counting 

 experiments, is several times greater than that deduced from 

 theory based on our present views of the mechanism involved 

 in the passage of a particles through matter. It has been 

 shown in a previous number of this Magazine that the effect 

 of the calculated straggling can be adequately accounted for 

 by the tailing off of the ionization curve at the extreme end 

 of the range. The large additional straggling observed 

 behaves quite differently. Evidence has been given in this 

 paper to show that it increases very slowly with increase of 

 molecular stopping power, and furthermore, that it all takes 

 place within the last few millimetres of the range. Here we 

 seem to be confronted with a behaviour of the a part cle 

 which present theory is unable to explain. 



* Flamm, Wien. Ber. II a. cxxiv. p. 597 (1915). 

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