Hydrogen Nuclei from Swift c*. Particles, 903 



Cavendish Laboratory for suggesting the method finally 

 adopted for deducing the numbers of recoil atoms from the 

 experimental curves. My best thanks are due to Professor 

 Rutherford for his continued interest and advice during the 

 course of this research. 



Note. 



An Experiment to determine the relation between 

 the oc- and y-JRay activities of Thorium (\ 



The standard of 7-ray activity was the ionization produced 

 by 1 mg. of radium in equilibrium with its products, in an 

 electroscope with sides of lead 6 mm. thick. The 7-ray 

 activity of a source of thorium 0, which had been allowed 

 to come into equilibrium with thorium D, was measured on 

 the standard electroscope and then placed in an exhausted 

 glass tube. To the end of this was waxed a brass plate in 

 which was a mica window of about 2 mm. diameter. Outside 

 the tube was a zinc-sulphide screen on which the a particles, 

 passing through the window from the • source, produced 

 scintillations. These were observed and counted by means 

 of a scintillation microscope. Between window and screen 

 was mounted a wheel with a hole near the circumference, 

 which when rotated reduced the number of scintillations in 

 a known ratio. This wheel was kindly lent me by Mr. Chad- 

 wick, who has given a full description of its use in a recent 

 paper *. The stopping-power of the mica window was just 

 sufficient to prevent the <x particles of range 4'8 cm. from 

 reaching the screen. 



The number of long-range a. particles projected within a 

 definite solid angle by a thorium C source of known 7-ray 

 activity was deduced from the above experiment. Precisely 

 the same measurements were then made, usino" a source of 

 radium C in place of thorium C. The number of a particles 

 given off by the radium C in equilibrium with 1 mg. of radium 

 is known ; and the ratio of the numbers from thorium and 

 radium C sources of equal 7-ray activities can be deduced 

 from the foregoing. From these values the relation between 

 the a- and the 7-ray activity of thorium C can be obtained. 



In the present research the quantity actually required was 

 the ratio of the number of a particles given off by equal 7-ray 

 activities (as measured through 6 mm. of lead) of radium 

 active deposit and thorium C. Radium active deposit has a 

 slightly greater 7-ray activity than its radium C constituent 



* Chadwick, Phil. Mag. Dec. 1920, 



