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LII. Long-range Particles from Thorium Active Deposit, 

 By A. B. Wood. D.Sc* 



§ 1. O 1916 Sir Ernest Rutherford and the author pub- 

 JL lished a paperf describing experiments which gave 

 evidence tiiat thorium active deposit emits a small number 

 of high-velocity particles, of range about 11*3 cm., in addition 

 to the main group of a-rays o£ ranges 5'0 cm. and 8 6 cm. 

 At the time the experiments were made there seemed no 

 reason to suppose that these long-range particles were other 

 than a-particles. More recently, however, Sir E. Rutherford 

 lias shown t that when a-particles are fired into a medium 

 containing light atoms, some of these atoms are enormously 

 accelerated by close collision of the a-particles with their 

 nuclei, and consequently attain very high velocities and 

 correspondingly long ranges. Such high-velocity particles 

 behave in a manner similar to that of the a-particles them- 

 selves in that they can produce intense ionization and 

 scintillation effects. It was calculated that oxygen atoms, 

 for example, could be accelerated by close collision with 

 a-particles of S'6 cm. range, so that they attained the high 

 velocity corresponding to a range of 11*1 cm. in air. 



In the investigation of the long-range particles from 

 thorium § a mica screen was employed to absorb the ordi- 

 nary a-particles of ranges 5-0 and 8'6 cm., the long-range 

 particles pentrating the mica and striking a zinc-sulphide 

 screen. The maximum range of such particles was found to 

 be 11"3 cm., a value differing only slightly from that 

 deduced by Sir E. Rutherford for the range of oxygen atoms 

 accelerated by 8'6 cm. a-particles. Since mica contains a 

 considerable amount of oxygen, it seemed not improbable that 

 the "long-range a-particles from thorium ' J might conceivably 

 be oxygen atoms originating in the mica absorbing screen. 

 A fraction of the long-range particles must undoubtedly have 

 originated in this manner, since Rutherford has shown this 

 to be so when RaC a-particles are fired through mica screens. 

 On the other hand, the number of long-range particles 

 observed in the case of thorium, about 1 in 10,000 of the 

 total number of a-particles, is about 10 times the number 

 obtained in the RaC experiments, viz. 1 in 100,000. 



It was considered desirable, therefore, to re-determine the 



* Communicated bv Professor Sir E. Rutherford, F.R.S. 

 t Rutherford & Wood, Phil. Mag, xxxi. April 1916. 

 X Rutherford, Phil. Mag-, xxxvii. June 1919. 

 § Rutherford and Wood, loc. cit. 



