a Particles with L'ujlit Atoms. 579 



the total number of scintillations observed in air for a known 

 activity of the source. The number of scintillations per 

 minute due to N and atoms at a distance of 7*5 cm. in 

 air at 15° 0. was 2 2 on an area of the zinc sulphide screen 

 equal to 314 sq. mm. Referring to curve 1, it is seen that 

 the number corresponding to an absorption of 7 cm. should 

 be 26 and the number for 8 cm. absorption 15. 



All those atoms of range equal 1o or greater than 8 cm. 

 must be produced in the first 3*5 cm. of the path of the 

 a. rajs ; for the atoms produced by a. particles of range 

 3' 5 cm. cannot travel further than 8 cm. from the source, 

 and probably only a small fraction reach this distance owing 

 to scattering and straggling. 



For the purpose of calculation, suppose that the production 

 of swift atoms is uniform over the first 3'5 cm. of the range 

 and that p is the ratio of the number of swift atoms produced 

 per cm. of path to the number of a particles passing through 

 the gas. 



The number Q of recoil atoms falling per second on the 

 screen of area A after passing through I cm. of gas is given by 



Q=/>.V 



A/N 



4-7TV 



where N is the total number of a. particles emitted by the 

 source per second (3*7 x 10 7 per second per mg. Ra of 

 activity) and r is the distance of the source from the screen. 

 Putting 



1*5 

 Q= — , A = '0311 sq. cm., Z = 3*5 cm., r= 7*5 cm , 



then the average value of /o = 4'3/10 6 . 



When we take into- consideration the well-knov\n way in 

 which the « particles fall off near the end of the range 

 in consequence of scattering, it is obvious that the true value 

 of p is considerably greater than the above and is probably 

 about 7/10 6 . 



In the experiments with hydrogen, it was shown that 

 p = 1/10 5 about — a value not very different from that 

 observed in these experiments. We may consequently con- 

 clude that about the same number of swift atoms are 

 produced per centimetre of path by the passage of a particles 

 through air, oxygen, and hydrogen. As in the case of 

 hydrogen, it can be showm that all a. particles, shot within 

 a perpendicular distance jo = 2'4 x 10~ 13 cm. of the atomic 

 nucleus, give rise to swift atoms of nitrogen and oxygen. 



