Radioactivity resulting from a.-Ray Bombardment. 939 



been detected before now ; but if the time is of the order 

 of a small fraction of a second, it would not have been 

 differentiated from the direct effect by any experiments so 

 far performed. The present experiment was therefore 

 carried out to extend observations down to a time-interval 

 that would correspond to very rapid changes. 



There were two essentials governing the experimental 

 arrangement used to attack this problem. The first was a 

 very short time interval between the a-ray bombardment and 

 the observation. The second was the smallest possible space- 

 interval between the bombarded atoms and the observing 

 device in order to detect particles of short range. The scin- 

 tillation method was used, and the problem therefore 

 narrowed to the detection of particles of mass and velocity 

 capable of producing a scintillation comparable with the 

 H-scintillation. 



The small time-interval was obtained by the use of a 

 nickel steel disk of 8 inch diameter which could be rotated at 

 a very high speed. The time-interval between the bombard- 

 ment of a point on the wheel and the observation on that 

 point can then be made extremely small by making the dis- 

 tance between bombarding source and observing-screen very 

 small and the speed of the wheel very large. 



After a few minor difficulties, the final arrangement of 

 the source of a-ray s and of the observing-screen was 

 essentially as shown in the attached diagram. A lead 

 bracket having a half-inch hole was screwed firmly to the 

 framework holding the wheel so that its surface was about 1 

 to H mm. from the face of the steel disk. The metal 

 source was of 1'2 cm. diameter, and was arranged to be held 

 in the hole of the lead bracket with its active face about 

 2 mm. from the surface of the steel disk. It was found 

 necessary to protect the active material from the draught 

 caused by the disk when at high speed in order to prevent 

 contamination of the disk itself. A thin sheet of mica of 

 stopping power equal to *9 cm. of air was accordingly wnxed 

 over the opening in the lead bracket. It was unfortunate 

 that this was necessary, since it cut down the ran,i>e of the 

 a-particles to 6'06 cm. and consequently reduced their 

 effective energy. 



The distance between the source and screen was limited 

 by two factors ; a certain minimum thickness of lead was 

 required to cut down ^-radiation and so fluorescence of the 

 screen ; and it was necessary to separate source and screen 

 sufficiently to prevent the possibility of too many hydrogen 

 particles directly ejected by oblique a-particles from reaching 



