6i4 



NA TURE 



[May 



1922 



Artificial Disintegration of the Elements.^ 



By Sir Ernest Rutherford, F.R.S. 



range of the a-particles is reduced to 4-9 cm. of 

 air. The effect shown in curve D (Fig. 2) is due 

 almost entirely to the " natural " scintillations from 

 the source. When we remember that the decrease in 

 velocity corresponding with the reduction of the range 

 of an a-particle from 7 cm. 



Absorption Curves in Nitrogen and Aluminium". 



The variation in the number of scintillations as the 

 absorption in the path of the rays increases from 

 10 cm. is shown in Fig. i. The source of a-rays is in 



I caoivALCHT ens c 



Fig. I. 



all cases radium-C Curve A shows the effect in 

 nitrogen (air) where the maximum range is 40 cm. 

 Curve B is the corresponding absorption curve for a 

 mixture of hydrogen and carbon dioxide, about i 

 volume of hydrogen to 1-5 of carbon dioxide, which 

 gives the same stopping power as air for a-rays. The 

 number of scintillations due to hydrogen is very great 

 in this case for absorptions less than 20 cm., but falls 

 off rapidl)'-, and none could be dis- 

 tinguished beyond 30 cm. Curve C 

 gives the natural effect when the air 

 is replaced by dry oxygen. This is 

 small compared with that observed in 

 nitrogen. Curve D shows the effect 

 when an aluminium plate of 3*5 cm. 

 stopping power is placed over the source 

 and the air replaced by oxygen. Thus 

 the particles liberated from aluminium 

 are able to. penetrate a much greater 

 thickness than the particles from hydrogen 

 or nitrogen. 



It is a matter of great interest to find 

 how the absorption curves for these long- 

 range particles vary with the velocity of 

 the bombarding a-particles. This has 

 been examined for two typical elements, 

 nitrogen and aluminium, and the results 

 for the latter are shown in Fig. 2. It was 

 found that to a first approximation the 

 maximum range of the particles liberated " *° 



from an element was proportional to the 

 range of the bombarding particles. In 

 all cases, the number of scintillations falls off rapidly 

 as the velocity of the a-particles is decreased. The 

 effect of velocity is specially marked in aluminium, 

 and few, if any, particles are observed when the 



1 Continued from p. 586. 

 NO. 2741, VOL. 109] 



to 4-9 cm. is only ii per 

 cent., we see how rapidly 

 the number falls off with 

 lowering of the velocity. It 

 seems likely that no disin- 

 tegration can be effected in 

 the case of aluminium if the 

 velocity of the a-particle falls 

 below a certain critical value. 

 This is not easy to prove 

 conclusively, but, if correct, 

 it indicates that the a-particle 

 must have a certain critical 

 energy to release an H-atom 

 from the nucleus. 



A very striking result was 

 '* " "* observed in the case of 



aluminium. It is to be ex- 

 pected that the liberated 

 particles should for the most part be projected in the 

 direction of the bombarding a-particles. Actually, it 

 was found, however, that nearly as many were shot 

 in the backward as in the forward direction. No 

 evidence of such an effect was observed in the case of 

 the nitrogen particles. The other elements have not 

 yet been examined from this point of view, but we 

 should expect an element like phosphorus, which gives 



ABSOKPTiQN IN ens or UIB 

 Fig. 2. 



rise to long-range particles, to show a similar effect. 

 A possible explanation of this striking result will be given 

 later. 



Nature of the Expelled Particles. 

 It can be shown readily that the long-range particles 



