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XLIII. Long-range Alpha Particles from Thorium. By 

 Sir Ernest Rutherford, F.R.S., and A. B. Wood, 

 M.Sc, Lecturer in Physics, University of Liverpool * . 



IN the course of an examination of a strong source of the 

 active deposit of thorium by the scintillation method, 

 one of us observed the presence of a small number of bright 

 scintillations which were able to penetrate through a thick- 

 ness of matter corresponding to 11*3 cm. of air at 760 mm. 

 and 15° C. These scintillations were undoubtedly due to 

 alpha particles and of greater velocity than any previously 

 observed; for the swiftest alpha particles hitherto known, 

 viz. those from thorium C, have a range in air of 8*6 cm. 

 The number of these long-range alpha particles is only a 

 small fraction of the total number emitted by the source. 

 The actual number of long-range particles decreased ex- 

 ponentially with time, falling to half value in 106 hours — 

 the normal period of decay of the active deposit of thorium. 

 Owing to the pressure of other work, the experiments were 

 kindly repeated and extended by Mr. A. B. Wood, who 

 examined in detail the variation of the number of scintil- 

 lations with thickness of matter traversed. There are still a 

 number of points that require further examination, but as 

 neither of the authors is likely to have time to continue the 

 experiments in the near future, it has been thought desirable 

 to give a brief account of the preliminary results. 



Experimental arrangements. 



The end of a brass rod, 1 mm. in diameter, was exposed 

 as negative electrode in a small vessel containing a strongly 

 emanating preparation, either of radio-thorium or meso- 

 thorium. By suitable adjustment of the electrodes, the active 

 deposit was concentrated almost entirely on the end of the 

 rod — a condition essential to the accurate determination of 

 the ranges. After two days' exposure to the electric field, 

 the wire was removed and placed end-on at 4 mm. distance 

 from a small screen of zinc sulphide viewed with a low- 

 power microscope. Care was taken that the axis of the 

 microscope passed through the centre of the rod. The screen 

 was permanently covered with a mica plate whose thickness 

 corresponded to 8*6 cm. of air — the maximum range of 

 the alpha particle from thorium C. All scintillations then 

 observed were due to alpha particles which had passed 

 through the mica plate and 4 mm. of air, i. e. a distance 

 * Communicated bv the Authors. 



